Merge branch 'master' into new-cli

54 files changed, 2044 insertions, 490 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index de945442224d..8ce2f3dfa6af 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -128,6 +128,9 @@ static void printValue(std::ostream & str, std::set<const Value *> & active, con
     case tExternal:
         str << *v.external;
         break;
+    case tFloat:
+        str << v.fpoint;
+        break;
     default:
         throw Error("invalid value");
     }
@@ -161,6 +164,7 @@ string showType(const Value & v)
         case tPrimOp: return "a built-in function";
         case tPrimOpApp: return "a partially applied built-in function";
         case tExternal: return v.external->showType();
+        case tFloat: return "a float";
     }
     abort();
 }
@@ -579,6 +583,12 @@ Value * ExprInt::maybeThunk(EvalState & state, Env & env)
     return &v;
 }
 
+Value * ExprFloat::maybeThunk(EvalState & state, Env & env)
+{
+    nrAvoided++;
+    return &v;
+}
+
 Value * ExprPath::maybeThunk(EvalState & state, Env & env)
 {
     nrAvoided++;
@@ -666,6 +676,11 @@ void ExprInt::eval(EvalState & state, Env & env, Value & v)
 }
 
 
+void ExprFloat::eval(EvalState & state, Env & env, Value & v)
+{
+    v = this->v;
+}
+
 void ExprString::eval(EvalState & state, Env & env, Value & v)
 {
     v = this->v;
@@ -1211,6 +1226,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
     PathSet context;
     std::ostringstream s;
     NixInt n = 0;
+    NixFloat nf = 0;
 
     bool first = !forceString;
     ValueType firstType = tString;
@@ -1229,15 +1245,30 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         }
 
         if (firstType == tInt) {
-            if (vTmp.type != tInt)
+            if (vTmp.type == tInt) {
+                n += vTmp.integer;
+            } else if (vTmp.type == tFloat) {
+                // Upgrade the type from int to float;
+                firstType = tFloat;
+                nf = n;
+                nf += vTmp.fpoint;
+            } else
                 throwEvalError("cannot add %1% to an integer, at %2%", showType(vTmp), pos);
-            n += vTmp.integer;
+        } else if (firstType == tFloat) {
+            if (vTmp.type == tInt) {
+                nf += vTmp.integer;
+            } else if (vTmp.type == tFloat) {
+                nf += vTmp.fpoint;
+            } else
+                throwEvalError("cannot add %1% to a float, at %2%", showType(vTmp), pos);
         } else
             s << state.coerceToString(pos, vTmp, context, false, firstType == tString);
     }
 
     if (firstType == tInt)
         mkInt(v, n);
+    else if (firstType == tFloat)
+        mkFloat(v, nf);
     else if (firstType == tPath) {
         if (!context.empty())
             throwEvalError("a string that refers to a store path cannot be appended to a path, at %1%", pos);
@@ -1295,6 +1326,17 @@ NixInt EvalState::forceInt(Value & v, const Pos & pos)
 }
 
 
+NixFloat EvalState::forceFloat(Value & v, const Pos & pos)
+{
+    forceValue(v, pos);
+    if (v.type == tInt)
+        return v.integer;
+    else if (v.type != tFloat)
+        throwTypeError("value is %1% while a float was expected, at %2%", v, pos);
+    return v.fpoint;
+}
+
+
 bool EvalState::forceBool(Value & v)
 {
     forceValue(v);
@@ -1413,6 +1455,7 @@ string EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
         if (v.type == tBool && v.boolean) return "1";
         if (v.type == tBool && !v.boolean) return "";
         if (v.type == tInt) return std::to_string(v.integer);
+        if (v.type == tFloat) return std::to_string(v.fpoint);
         if (v.type == tNull) return "";
 
         if (v.isList()) {
@@ -1474,6 +1517,13 @@ bool EvalState::eqValues(Value & v1, Value & v2)
        uniqList on a list of sets.)  Will remove this eventually. */
     if (&v1 == &v2) return true;
 
+    // Special case type-compatibility between float and int
+    if (v1.type == tInt && v2.type == tFloat)
+        return v1.integer == v2.fpoint;
+    if (v1.type == tFloat && v2.type == tInt)
+        return v1.fpoint == v2.integer;
+
+    // All other types are not compatible with each other.
     if (v1.type != v2.type) return false;
 
     switch (v1.type) {
@@ -1531,6 +1581,9 @@ bool EvalState::eqValues(Value & v1, Value & v2)
         case tExternal:
             return *v1.external == *v2.external;
 
+        case tFloat:
+            return v1.fpoint == v2.fpoint;
+
         default:
             throwEvalError("cannot compare %1% with %2%", showType(v1), showType(v2));
     }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index cf2d578644e1..40e05712bab1 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -147,6 +147,7 @@ public:
 
     /* Force `v', and then verify that it has the expected type. */
     NixInt forceInt(Value & v, const Pos & pos);
+    NixFloat forceFloat(Value & v, const Pos & pos);
     bool forceBool(Value & v);
     inline void forceAttrs(Value & v);
     inline void forceAttrs(Value & v, const Pos & pos);
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 1002ee6285af..996c2c5f4975 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -106,7 +106,8 @@ bool DrvInfo::checkMeta(Value & v)
             if (!checkMeta(*i.value)) return false;
         return true;
     }
-    else return v.type == tInt || v.type == tBool || v.type == tString;
+    else return v.type == tInt || v.type == tBool || v.type == tString ||
+                v.type == tFloat;
 }
 
 
@@ -127,7 +128,7 @@ string DrvInfo::queryMetaString(const string & name)
 }
 
 
-int DrvInfo::queryMetaInt(const string & name, int def)
+NixInt DrvInfo::queryMetaInt(const string & name, NixInt def)
 {
     Value * v = queryMeta(name);
     if (!v) return def;
@@ -135,12 +136,26 @@ int DrvInfo::queryMetaInt(const string & name, int def)
     if (v->type == tString) {
         /* Backwards compatibility with before we had support for
            integer meta fields. */
-        int n;
+        NixInt n;
         if (string2Int(v->string.s, n)) return n;
     }
     return def;
 }
 
+NixFloat DrvInfo::queryMetaFloat(const string & name, NixFloat def)
+{
+    Value * v = queryMeta(name);
+    if (!v) return def;
+    if (v->type == tFloat) return v->fpoint;
+    if (v->type == tString) {
+        /* Backwards compatibility with before we had support for
+           float meta fields. */
+        NixFloat n;
+        if (string2Float(v->string.s, n)) return n;
+    }
+    return def;
+}
+
 
 bool DrvInfo::queryMetaBool(const string & name, bool def)
 {
diff --git a/src/libexpr/get-drvs.hh b/src/libexpr/get-drvs.hh
index 98f762494aa5..365c66c8d710 100644
--- a/src/libexpr/get-drvs.hh
+++ b/src/libexpr/get-drvs.hh
@@ -47,7 +47,8 @@ public:
     StringSet queryMetaNames();
     Value * queryMeta(const string & name);
     string queryMetaString(const string & name);
-    int queryMetaInt(const string & name, int def);
+    NixInt queryMetaInt(const string & name, NixInt def);
+    NixFloat queryMetaFloat(const string & name, NixFloat def);
     bool queryMetaBool(const string & name, bool def);
     void setMeta(const string & name, Value * v);
 
diff --git a/src/libexpr/json-to-value.cc b/src/libexpr/json-to-value.cc
index 0898b560911b..1daf84600dca 100644
--- a/src/libexpr/json-to-value.cc
+++ b/src/libexpr/json-to-value.cc
@@ -105,17 +105,21 @@ static void parseJSON(EvalState & state, const char * & s, Value & v)
         mkString(v, parseJSONString(s));
     }
 
-    else if (isdigit(*s) || *s == '-') {
-        bool neg = false;
-        if (*s == '-') {
-            neg = true;
-            if (!*++s) throw JSONParseError("unexpected end of JSON number");
+    else if (isdigit(*s) || *s == '-' || *s == '.' ) {
+        // Buffer into a string first, then use built-in C++ conversions
+        std::string tmp_number;
+        ValueType number_type = tInt;
+
+        while (isdigit(*s) || *s == '-' || *s == '.' || *s == 'e' || *s == 'E') {
+            if (*s == '.' || *s == 'e' || *s == 'E')
+                number_type = tFloat;
+            tmp_number += *s++;
         }
-        NixInt n = 0;
-        // FIXME: detect overflow
-        while (isdigit(*s)) n = n * 10 + (*s++ - '0');
-        if (*s == '.' || *s == 'e') throw JSONParseError("floating point JSON numbers are not supported");
-        mkInt(v, neg ? -n : n);
+
+        if (number_type == tFloat)
+            mkFloat(v, stod(tmp_number));
+        else
+            mkInt(v, stoi(tmp_number));
     }
 
     else if (strncmp(s, "true", 4) == 0) {
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 478f4164106a..f3660ab43723 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -86,6 +86,7 @@ static Expr * unescapeStr(SymbolTable & symbols, const char * s)
 
 ID          [a-zA-Z\_][a-zA-Z0-9\_\'\-]*
 INT         [0-9]+
+FLOAT       (([1-9][0-9]*\.[0-9]*)|(0?\.[0-9]+))([Ee][+-]?[0-9]+)?
 PATH        [a-zA-Z0-9\.\_\-\+]*(\/[a-zA-Z0-9\.\_\-\+]+)+
 HPATH       \~(\/[a-zA-Z0-9\.\_\-\+]+)+
 SPATH       \<[a-zA-Z0-9\.\_\-\+]+(\/[a-zA-Z0-9\.\_\-\+]+)*\>
@@ -126,6 +127,12 @@ or          { return OR_KW; }
                   throw ParseError(format("invalid integer ‘%1%’") % yytext);
               return INT;
             }
+{FLOAT}     { errno = 0;
+              yylval->nf = strtod(yytext, 0);
+              if (errno != 0)
+                  throw ParseError(format("invalid float ‘%1%’") % yytext);
+              return FLOAT;
+            }
 
 \$\{        { PUSH_STATE(INSIDE_DOLLAR_CURLY); return DOLLAR_CURLY; }
 }
@@ -188,5 +195,7 @@ or          { return OR_KW; }
 
 }
 
+<<EOF>> { data->atEnd = true; return 0; }
+
 %%
 
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 5bc8e4202a49..b2c9f0528ca9 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -68,6 +68,11 @@ void ExprInt::show(std::ostream & str)
     str << n;
 }
 
+void ExprFloat::show(std::ostream & str)
+{
+    str << nf;
+}
+
 void ExprString::show(std::ostream & str)
 {
     showString(str, s);
@@ -226,6 +231,10 @@ void ExprInt::bindVars(const StaticEnv & env)
 {
 }
 
+void ExprFloat::bindVars(const StaticEnv & env)
+{
+}
+
 void ExprString::bindVars(const StaticEnv & env)
 {
 }
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index ef07d4557fe8..d2ca09b3a5bb 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -11,6 +11,7 @@ namespace nix {
 
 MakeError(EvalError, Error)
 MakeError(ParseError, Error)
+MakeError(IncompleteParseError, ParseError)
 MakeError(AssertionError, EvalError)
 MakeError(ThrownError, AssertionError)
 MakeError(Abort, EvalError)
@@ -98,6 +99,15 @@ struct ExprInt : Expr
     Value * maybeThunk(EvalState & state, Env & env);
 };
 
+struct ExprFloat : Expr
+{
+    NixFloat nf;
+    Value v;
+    ExprFloat(NixFloat nf) : nf(nf) { mkFloat(v, nf); };
+    COMMON_METHODS
+    Value * maybeThunk(EvalState & state, Env & env);
+};
+
 struct ExprString : Expr
 {
     Symbol s;
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 3e6c6d917d24..80ecd44c5981 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -31,10 +31,12 @@ namespace nix {
         Path basePath;
         Symbol path;
         string error;
+        bool atEnd;
         Symbol sLetBody;
         ParseData(EvalState & state)
             : state(state)
             , symbols(state.symbols)
+            , atEnd(false)
             , sLetBody(symbols.create("<let-body>"))
             { };
     };
@@ -244,6 +246,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * err
   nix::Formals * formals;
   nix::Formal * formal;
   nix::NixInt n;
+  nix::NixFloat nf;
   const char * id; // !!! -> Symbol
   char * path;
   char * uri;
@@ -264,6 +267,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * err
 %token <id> ID ATTRPATH
 %token <e> STR IND_STR
 %token <n> INT
+%token <nf> FLOAT
 %token <path> PATH HPATH SPATH
 %token <uri> URI
 %token IF THEN ELSE ASSERT WITH LET IN REC INHERIT EQ NEQ AND OR IMPL OR_KW
@@ -366,6 +370,7 @@ expr_simple
           $$ = new ExprVar(CUR_POS, data->symbols.create($1));
   }
   | INT { $$ = new ExprInt($1); }
+  | FLOAT { $$ = new ExprFloat($1); }
   | '"' string_parts '"' { $$ = $2; }
   | IND_STRING_OPEN ind_string_parts IND_STRING_CLOSE {
       $$ = stripIndentation(CUR_POS, data->symbols, *$2);
@@ -536,7 +541,12 @@ Expr * EvalState::parse(const char * text,
     int res = yyparse(scanner, &data);
     yylex_destroy(scanner);
 
-    if (res) throw ParseError(data.error);
+    if (res) {
+      if (data.atEnd)
+        throw IncompleteParseError(data.error);
+      else
+        throw ParseError(data.error);
+    }
 
     data.result->bindVars(staticEnv);
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 2371ae1748a6..3c899d769253 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -195,6 +195,7 @@ static void prim_typeOf(EvalState & state, const Pos & pos, Value * * args, Valu
         case tExternal:
             t = args[0]->external->typeOf();
             break;
+        case tFloat: t = "float"; break;
         default: abort();
     }
     mkString(v, state.symbols.create(t));
@@ -224,6 +225,12 @@ static void prim_isInt(EvalState & state, const Pos & pos, Value * * args, Value
     mkBool(v, args[0]->type == tInt);
 }
 
+/* Determine whether the argument is a float. */
+static void prim_isFloat(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceValue(*args[0]);
+    mkBool(v, args[0]->type == tFloat);
+}
 
 /* Determine whether the argument is a string. */
 static void prim_isString(EvalState & state, const Pos & pos, Value * * args, Value & v)
@@ -245,11 +252,17 @@ struct CompareValues
 {
     bool operator () (const Value * v1, const Value * v2) const
     {
+        if (v1->type == tFloat && v2->type == tInt)
+            return v1->fpoint < v2->integer;
+        if (v1->type == tInt && v2->type == tFloat)
+            return v1->integer < v2->fpoint;
         if (v1->type != v2->type)
             throw EvalError(format("cannot compare %1% with %2%") % showType(*v1) % showType(*v2));
         switch (v1->type) {
             case tInt:
                 return v1->integer < v2->integer;
+            case tFloat:
+                return v1->fpoint < v2->fpoint;
             case tString:
                 return strcmp(v1->string.s, v2->string.s) < 0;
             case tPath:
@@ -1423,27 +1436,40 @@ static void prim_sort(EvalState & state, const Pos & pos, Value * * args, Value
 
 static void prim_add(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    mkInt(v, state.forceInt(*args[0], pos) + state.forceInt(*args[1], pos));
+    if (args[0]->type == tFloat || args[1]->type == tFloat)
+        mkFloat(v, state.forceFloat(*args[0], pos) + state.forceFloat(*args[1], pos));
+    else
+        mkInt(v, state.forceInt(*args[0], pos) + state.forceInt(*args[1], pos));
 }
 
 
 static void prim_sub(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    mkInt(v, state.forceInt(*args[0], pos) - state.forceInt(*args[1], pos));
+    if (args[0]->type == tFloat || args[1]->type == tFloat)
+        mkFloat(v, state.forceFloat(*args[0], pos) - state.forceFloat(*args[1], pos));
+    else
+        mkInt(v, state.forceInt(*args[0], pos) - state.forceInt(*args[1], pos));
 }
 
 
 static void prim_mul(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    mkInt(v, state.forceInt(*args[0], pos) * state.forceInt(*args[1], pos));
+    if (args[0]->type == tFloat || args[1]->type == tFloat)
+        mkFloat(v, state.forceFloat(*args[0], pos) * state.forceFloat(*args[1], pos));
+    else
+        mkInt(v, state.forceInt(*args[0], pos) * state.forceInt(*args[1], pos));
 }
 
 
 static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    NixInt i2 = state.forceInt(*args[1], pos);
-    if (i2 == 0) throw EvalError(format("division by zero, at %1%") % pos);
-    mkInt(v, state.forceInt(*args[0], pos) / i2);
+    NixFloat f2 = state.forceFloat(*args[1], pos);
+    if (f2 == 0) throw EvalError(format("division by zero, at %1%") % pos);
+
+    if (args[0]->type == tFloat || args[1]->type == tFloat)
+        mkFloat(v, state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
+    else
+        mkInt(v, state.forceInt(*args[0], pos) / state.forceInt(*args[1], pos));
 }
 
 
@@ -1735,7 +1761,7 @@ void EvalState::createBaseEnv()
        language feature gets added.  It's not necessary to increase it
        when primops get added, because you can just use `builtins ?
        primOp' to check. */
-    mkInt(v, 3);
+    mkInt(v, 4);
     addConstant("__langVersion", v);
 
     // Miscellaneous
@@ -1752,6 +1778,7 @@ void EvalState::createBaseEnv()
     addPrimOp("__isFunction", 1, prim_isFunction);
     addPrimOp("__isString", 1, prim_isString);
     addPrimOp("__isInt", 1, prim_isInt);
+    addPrimOp("__isFloat", 1, prim_isFloat);
     addPrimOp("__isBool", 1, prim_isBool);
     addPrimOp("__genericClosure", 1, prim_genericClosure);
     addPrimOp("abort", 1, prim_abort);
diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index b0cf85e21f18..47ee324a6e4f 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -84,6 +84,10 @@ void printValueAsJSON(EvalState & state, bool strict,
             v.external->printValueAsJSON(state, strict, str, context);
             break;
 
+        case tFloat:
+            str << v.fpoint;
+            break;
+
         default:
             throw TypeError(format("cannot convert %1% to JSON") % showType(v));
     }
diff --git a/src/libexpr/value-to-json.hh b/src/libexpr/value-to-json.hh
index f6796f2053e9..c59caf5641bc 100644
--- a/src/libexpr/value-to-json.hh
+++ b/src/libexpr/value-to-json.hh
@@ -36,7 +36,18 @@ struct JSONObject
         attr(s);
         escapeJSON(str, t);
     }
-    void attr(const string & s, int n)
+    void attr(const string & s, const char * t)
+    {
+        attr(s);
+        escapeJSON(str, t);
+    }
+    void attr(const string & s, bool b)
+    {
+        attr(s);
+        str << (b ? "true" : "false");
+    }
+    template<typename T>
+    void attr(const string & s, const T & n)
     {
         attr(s);
         str << n;
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index cb52ce1e67bd..00b1918a82aa 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -148,6 +148,10 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
             v.external->printValueAsXML(state, strict, location, doc, context, drvsSeen);
             break;
 
+        case tFloat:
+            doc.writeEmptyElement("float", singletonAttrs("value", (format("%1%") % v.fpoint).str()));
+            break;
+
         default:
             doc.writeEmptyElement("unevaluated");
     }
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index e6d1502cb607..62bdd9281f08 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -22,6 +22,7 @@ typedef enum {
     tPrimOp,
     tPrimOpApp,
     tExternal,
+    tFloat
 } ValueType;
 
 
@@ -38,6 +39,7 @@ class XMLWriter;
 
 
 typedef long NixInt;
+typedef float NixFloat;
 
 /* External values must descend from ExternalValueBase, so that
  * type-agnostic nix functions (e.g. showType) can be implemented
@@ -141,6 +143,7 @@ struct Value
             Value * left, * right;
         } primOpApp;
         ExternalValueBase * external;
+        NixFloat fpoint;
     };
 
     bool isList() const
@@ -181,6 +184,14 @@ static inline void mkInt(Value & v, NixInt n)
 }
 
 
+static inline void mkFloat(Value & v, NixFloat n)
+{
+    clearValue(v);
+    v.type = tFloat;
+    v.fpoint = n;
+}
+
+
 static inline void mkBool(Value & v, bool b)
 {
     clearValue(v);
diff --git a/src/libmain/local.mk b/src/libmain/local.mk
index 16dbf752823d..f1fd3eb72424 100644
--- a/src/libmain/local.mk
+++ b/src/libmain/local.mk
@@ -6,6 +6,8 @@ libmain_DIR := $(d)
 
 libmain_SOURCES := $(wildcard $(d)/*.cc)
 
+libmain_LDFLAGS = $(OPENSSL_LIBS)
+
 libmain_LIBS = libstore libutil libformat
 
 libmain_ALLOW_UNDEFINED = 1
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index bf6bf5b712e7..e883967b71a1 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -6,10 +6,11 @@
 #include "store-api.hh"
 #include "util.hh"
 
-#include <iostream>
+#include <algorithm>
 #include <cctype>
 #include <exception>
-#include <algorithm>
+#include <iostream>
+#include <mutex>
 
 #include <cstdlib>
 #include <sys/time.h>
@@ -17,7 +18,7 @@
 #include <unistd.h>
 #include <signal.h>
 
-extern char * * environ;
+#include <openssl/crypto.h>
 
 
 namespace nix {
@@ -94,7 +95,18 @@ string getArg(const string & opt,
 }
 
 
-void detectStackOverflow();
+/* OpenSSL is not thread-safe by default - it will randomly crash
+   unless the user supplies a mutex locking function. So let's do
+   that. */
+static std::vector<std::mutex> opensslLocks;
+
+static void opensslLockCallback(int mode, int type, const char * file, int line)
+{
+    if (mode & CRYPTO_LOCK)
+        opensslLocks[type].lock();
+    else
+        opensslLocks[type].unlock();
+}
 
 
 void initNix()
@@ -105,11 +117,16 @@ void initNix()
     std::cerr.rdbuf()->pubsetbuf(buf, sizeof(buf));
 #endif
 
+    // FIXME: do we need this? It's not thread-safe.
     std::ios::sync_with_stdio(false);
 
     if (getEnv("IN_SYSTEMD") == "1")
         logType = ltSystemd;
 
+    /* Initialise OpenSSL locking. */
+    opensslLocks = std::vector<std::mutex>(CRYPTO_num_locks());
+    CRYPTO_set_locking_callback(opensslLockCallback);
+
     settings.processEnvironment();
     settings.loadConfFile();
 
diff --git a/src/libmain/shared.hh b/src/libmain/shared.hh
index 20744be0ab9b..6d94a22f788e 100644
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@@ -65,6 +65,7 @@ template<class N> N getIntArg(const string & opt,
     return n * multiplier;
 }
 
+
 /* Show the manual page for the specified program. */
 void showManPage(const string & name);
 
@@ -99,4 +100,8 @@ struct PrintFreed
 };
 
 
+/* Install a SIGSEGV handler to detect stack overflows. */
+void detectStackOverflow();
+
+
 }
diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
new file mode 100644
index 000000000000..dc086fe9cf4e
--- /dev/null
+++ b/src/libstore/binary-cache-store.cc
@@ -0,0 +1,347 @@
+#include "binary-cache-store.hh"
+#include "sync.hh"
+
+#include "archive.hh"
+#include "compression.hh"
+#include "derivations.hh"
+#include "globals.hh"
+#include "nar-info.hh"
+#include "worker-protocol.hh"
+
+#include <chrono>
+
+namespace nix {
+
+BinaryCacheStore::BinaryCacheStore(std::shared_ptr<Store> localStore,
+    const Path & secretKeyFile, const Path & publicKeyFile)
+    : localStore(localStore)
+{
+    if (secretKeyFile != "")
+        secretKey = std::unique_ptr<SecretKey>(new SecretKey(readFile(secretKeyFile)));
+
+    if (publicKeyFile != "") {
+        publicKeys = std::unique_ptr<PublicKeys>(new PublicKeys);
+        auto key = PublicKey(readFile(publicKeyFile));
+        publicKeys->emplace(key.name, key);
+    }
+
+    StringSink sink;
+    sink << narVersionMagic1;
+    narMagic = sink.s;
+}
+
+void BinaryCacheStore::init()
+{
+    std::string cacheInfoFile = "nix-cache-info";
+    if (!fileExists(cacheInfoFile))
+        upsertFile(cacheInfoFile, "StoreDir: " + settings.nixStore + "\n");
+}
+
+void BinaryCacheStore::notImpl()
+{
+    throw Error("operation not implemented for binary cache stores");
+}
+
+const BinaryCacheStore::Stats & BinaryCacheStore::getStats()
+{
+    return stats;
+}
+
+Path BinaryCacheStore::narInfoFileFor(const Path & storePath)
+{
+    assertStorePath(storePath);
+    return storePathToHash(storePath) + ".narinfo";
+}
+
+void BinaryCacheStore::addToCache(const ValidPathInfo & info,
+    const string & nar)
+{
+    auto narInfoFile = narInfoFileFor(info.path);
+    if (fileExists(narInfoFile)) return;
+
+    assert(nar.compare(0, narMagic.size(), narMagic) == 0);
+
+    auto narInfo = make_ref<NarInfo>(info);
+
+    narInfo->narSize = nar.size();
+    narInfo->narHash = hashString(htSHA256, nar);
+
+    if (info.narHash.type != htUnknown && info.narHash != narInfo->narHash)
+        throw Error(format("refusing to copy corrupted path ‘%1%’ to binary cache") % info.path);
+
+    /* Compress the NAR. */
+    narInfo->compression = "xz";
+    auto now1 = std::chrono::steady_clock::now();
+    string narXz = compressXZ(nar);
+    auto now2 = std::chrono::steady_clock::now();
+    narInfo->fileHash = hashString(htSHA256, narXz);
+    narInfo->fileSize = narXz.size();
+
+    auto duration = std::chrono::duration_cast<std::chrono::milliseconds>(now2 - now1).count();
+    printMsg(lvlTalkative, format("copying path ‘%1%’ (%2% bytes, compressed %3$.1f%% in %4% ms) to binary cache")
+        % narInfo->path % narInfo->narSize
+        % ((1.0 - (double) narXz.size() / nar.size()) * 100.0)
+        % duration);
+
+    /* Atomically write the NAR file. */
+    narInfo->url = "nar/" + printHash32(narInfo->fileHash) + ".nar.xz";
+    if (!fileExists(narInfo->url)) {
+        stats.narWrite++;
+        upsertFile(narInfo->url, narXz);
+    } else
+        stats.narWriteAverted++;
+
+    stats.narWriteBytes += nar.size();
+    stats.narWriteCompressedBytes += narXz.size();
+    stats.narWriteCompressionTimeMs += duration;
+
+    /* Atomically write the NAR info file.*/
+    if (secretKey) narInfo->sign(*secretKey);
+
+    upsertFile(narInfoFile, narInfo->to_string());
+
+    {
+        auto state_(state.lock());
+        state_->narInfoCache.upsert(narInfo->path, narInfo);
+        stats.narInfoCacheSize = state_->narInfoCache.size();
+    }
+
+    stats.narInfoWrite++;
+}
+
+NarInfo BinaryCacheStore::readNarInfo(const Path & storePath)
+{
+    {
+        auto state_(state.lock());
+        auto res = state_->narInfoCache.get(storePath);
+        if (res) {
+            stats.narInfoReadAverted++;
+            return **res;
+        }
+    }
+
+    auto narInfoFile = narInfoFileFor(storePath);
+    auto narInfo = make_ref<NarInfo>(getFile(narInfoFile), narInfoFile);
+    assert(narInfo->path == storePath);
+
+    stats.narInfoRead++;
+
+    if (publicKeys) {
+        if (!narInfo->checkSignature(*publicKeys))
+            throw Error(format("invalid signature on NAR info file ‘%1%’") % narInfoFile);
+    }
+
+    {
+        auto state_(state.lock());
+        state_->narInfoCache.upsert(storePath, narInfo);
+        stats.narInfoCacheSize = state_->narInfoCache.size();
+    }
+
+    return *narInfo;
+}
+
+bool BinaryCacheStore::isValidPath(const Path & storePath)
+{
+    return fileExists(narInfoFileFor(storePath));
+}
+
+void BinaryCacheStore::exportPath(const Path & storePath, bool sign, Sink & sink)
+{
+    assert(!sign);
+
+    auto res = readNarInfo(storePath);
+
+    auto nar = getFile(res.url);
+
+    stats.narRead++;
+    stats.narReadCompressedBytes += nar.size();
+
+    /* Decompress the NAR. FIXME: would be nice to have the remote
+       side do this. */
+    if (res.compression == "none")
+        ;
+    else if (res.compression == "xz")
+        nar = decompressXZ(nar);
+    else
+        throw Error(format("unknown NAR compression type ‘%1%’") % nar);
+
+    stats.narReadBytes += nar.size();
+
+    printMsg(lvlTalkative, format("exporting path ‘%1%’ (%2% bytes)") % storePath % nar.size());
+
+    assert(nar.size() % 8 == 0);
+
+    sink((unsigned char *) nar.c_str(), nar.size());
+
+    // FIXME: check integrity of NAR.
+
+    sink << exportMagic << storePath << res.references << res.deriver << 0;
+}
+
+Paths BinaryCacheStore::importPaths(bool requireSignature, Source & source)
+{
+    assert(!requireSignature);
+    Paths res;
+    while (true) {
+        unsigned long long n = readLongLong(source);
+        if (n == 0) break;
+        if (n != 1) throw Error("input doesn't look like something created by ‘nix-store --export’");
+        res.push_back(importPath(source));
+    }
+    return res;
+}
+
+struct TeeSource : Source
+{
+    Source & readSource;
+    std::string data;
+    TeeSource(Source & readSource) : readSource(readSource)
+    {
+    }
+    size_t read(unsigned char * data, size_t len)
+    {
+        size_t n = readSource.read(data, len);
+        this->data.append((char *) data, n);
+        return n;
+    }
+};
+
+struct NopSink : ParseSink
+{
+};
+
+Path BinaryCacheStore::importPath(Source & source)
+{
+    /* FIXME: some cut&paste of LocalStore::importPath(). */
+
+    /* Extract the NAR from the source. */
+    TeeSource tee(source);
+    NopSink sink;
+    parseDump(sink, tee);
+
+    uint32_t magic = readInt(source);
+    if (magic != exportMagic)
+        throw Error("Nix archive cannot be imported; wrong format");
+
+    ValidPathInfo info;
+    info.path = readStorePath(source);
+
+    info.references = readStorePaths<PathSet>(source);
+
+    readString(source); // deriver, don't care
+
+    bool haveSignature = readInt(source) == 1;
+    assert(!haveSignature);
+
+    addToCache(info, tee.data);
+
+    return info.path;
+}
+
+ValidPathInfo BinaryCacheStore::queryPathInfo(const Path & storePath)
+{
+    return ValidPathInfo(readNarInfo(storePath));
+}
+
+void BinaryCacheStore::querySubstitutablePathInfos(const PathSet & paths,
+    SubstitutablePathInfos & infos)
+{
+    PathSet left;
+
+    if (!localStore) return;
+
+    for (auto & storePath : paths) {
+        if (!localStore->isValidPath(storePath)) {
+            left.insert(storePath);
+            continue;
+        }
+        ValidPathInfo info = localStore->queryPathInfo(storePath);
+        SubstitutablePathInfo sub;
+        sub.references = info.references;
+        sub.downloadSize = 0;
+        sub.narSize = info.narSize;
+        infos.emplace(storePath, sub);
+    }
+
+    if (settings.useSubstitutes)
+        localStore->querySubstitutablePathInfos(left, infos);
+}
+
+Path BinaryCacheStore::addToStore(const string & name, const Path & srcPath,
+    bool recursive, HashType hashAlgo, PathFilter & filter, bool repair)
+{
+    // FIXME: some cut&paste from LocalStore::addToStore().
+
+    /* Read the whole path into memory. This is not a very scalable
+       method for very large paths, but `copyPath' is mainly used for
+       small files. */
+    StringSink sink;
+    Hash h;
+    if (recursive) {
+        dumpPath(srcPath, sink, filter);
+        h = hashString(hashAlgo, sink.s);
+    } else {
+        auto s = readFile(srcPath);
+        dumpString(s, sink);
+        h = hashString(hashAlgo, s);
+    }
+
+    ValidPathInfo info;
+    info.path = makeFixedOutputPath(recursive, hashAlgo, h, name);
+
+    if (repair || !isValidPath(info.path))
+        addToCache(info, sink.s);
+
+    return info.path;
+}
+
+Path BinaryCacheStore::addTextToStore(const string & name, const string & s,
+    const PathSet & references, bool repair)
+{
+    ValidPathInfo info;
+    info.path = computeStorePathForText(name, s, references);
+    info.references = references;
+
+    if (repair || !isValidPath(info.path)) {
+        StringSink sink;
+        dumpString(s, sink);
+        addToCache(info, sink.s);
+    }
+
+    return info.path;
+}
+
+void BinaryCacheStore::buildPaths(const PathSet & paths, BuildMode buildMode)
+{
+    for (auto & storePath : paths) {
+        assert(!isDerivation(storePath));
+
+        if (isValidPath(storePath)) continue;
+
+        if (!localStore)
+            throw Error(format("don't know how to realise path ‘%1%’ in a binary cache") % storePath);
+
+        localStore->addTempRoot(storePath);
+
+        if (!localStore->isValidPath(storePath))
+            localStore->ensurePath(storePath);
+
+        ValidPathInfo info = localStore->queryPathInfo(storePath);
+
+        for (auto & ref : info.references)
+            if (ref != storePath)
+                ensurePath(ref);
+
+        StringSink sink;
+        dumpPath(storePath, sink);
+
+        addToCache(info, sink.s);
+    }
+}
+
+void BinaryCacheStore::ensurePath(const Path & path)
+{
+    buildPaths({path});
+}
+
+}
diff --git a/src/libstore/binary-cache-store.hh b/src/libstore/binary-cache-store.hh
new file mode 100644
index 000000000000..2235d6d67268
--- /dev/null
+++ b/src/libstore/binary-cache-store.hh
@@ -0,0 +1,172 @@
+#pragma once
+
+#include "crypto.hh"
+#include "store-api.hh"
+
+#include "lru-cache.hh"
+#include "sync.hh"
+#include "pool.hh"
+
+#include <atomic>
+
+namespace nix {
+
+struct NarInfo;
+
+class BinaryCacheStore : public Store
+{
+private:
+
+    std::unique_ptr<SecretKey> secretKey;
+    std::unique_ptr<PublicKeys> publicKeys;
+
+    std::shared_ptr<Store> localStore;
+
+    struct State
+    {
+        LRUCache<Path, ref<NarInfo>> narInfoCache{32 * 1024};
+    };
+
+    Sync<State> state;
+
+protected:
+
+    BinaryCacheStore(std::shared_ptr<Store> localStore,
+        const Path & secretKeyFile, const Path & publicKeyFile);
+
+    [[noreturn]] void notImpl();
+
+    virtual bool fileExists(const std::string & path) = 0;
+
+    virtual void upsertFile(const std::string & path, const std::string & data) = 0;
+
+    virtual std::string getFile(const std::string & path) = 0;
+
+public:
+
+    virtual void init();
+
+    struct Stats
+    {
+        std::atomic<uint64_t> narInfoRead{0};
+        std::atomic<uint64_t> narInfoReadAverted{0};
+        std::atomic<uint64_t> narInfoWrite{0};
+        std::atomic<uint64_t> narInfoCacheSize{0};
+        std::atomic<uint64_t> narRead{0};
+        std::atomic<uint64_t> narReadBytes{0};
+        std::atomic<uint64_t> narReadCompressedBytes{0};
+        std::atomic<uint64_t> narWrite{0};
+        std::atomic<uint64_t> narWriteAverted{0};
+        std::atomic<uint64_t> narWriteBytes{0};
+        std::atomic<uint64_t> narWriteCompressedBytes{0};
+        std::atomic<uint64_t> narWriteCompressionTimeMs{0};
+    };
+
+    const Stats & getStats();
+
+private:
+
+    Stats stats;
+
+    std::string narMagic;
+
+    std::string narInfoFileFor(const Path & storePath);
+
+    void addToCache(const ValidPathInfo & info, const string & nar);
+
+protected:
+
+    NarInfo readNarInfo(const Path & storePath);
+
+public:
+
+    bool isValidPath(const Path & path) override;
+
+    PathSet queryValidPaths(const PathSet & paths) override
+    { notImpl(); }
+
+    PathSet queryAllValidPaths() override
+    { notImpl(); }
+
+    ValidPathInfo queryPathInfo(const Path & path) override;
+
+    Hash queryPathHash(const Path & path) override
+    { notImpl(); }
+
+    void queryReferrers(const Path & path,
+        PathSet & referrers) override
+    { notImpl(); }
+
+    Path queryDeriver(const Path & path) override
+    { return ""; }
+
+    PathSet queryValidDerivers(const Path & path) override
+    { return {}; }
+
+    PathSet queryDerivationOutputs(const Path & path) override
+    { notImpl(); }
+
+    StringSet queryDerivationOutputNames(const Path & path) override
+    { notImpl(); }
+
+    Path queryPathFromHashPart(const string & hashPart) override
+    { notImpl(); }
+
+    PathSet querySubstitutablePaths(const PathSet & paths) override
+    { return {}; }
+
+    void querySubstitutablePathInfos(const PathSet & paths,
+        SubstitutablePathInfos & infos) override;
+
+    Path addToStore(const string & name, const Path & srcPath,
+        bool recursive = true, HashType hashAlgo = htSHA256,
+        PathFilter & filter = defaultPathFilter, bool repair = false) override;
+
+    Path addTextToStore(const string & name, const string & s,
+        const PathSet & references, bool repair = false) override;
+
+    void exportPath(const Path & path, bool sign,
+        Sink & sink) override;
+
+    Paths importPaths(bool requireSignature, Source & source) override;
+
+    Path importPath(Source & source);
+
+    void buildPaths(const PathSet & paths, BuildMode buildMode = bmNormal) override;
+
+    BuildResult buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+        BuildMode buildMode = bmNormal) override
+    { notImpl(); }
+
+    void ensurePath(const Path & path) override;
+
+    void addTempRoot(const Path & path) override
+    { notImpl(); }
+
+    void addIndirectRoot(const Path & path) override
+    { notImpl(); }
+
+    void syncWithGC() override
+    { }
+
+    Roots findRoots() override
+    { notImpl(); }
+
+    void collectGarbage(const GCOptions & options, GCResults & results) override
+    { notImpl(); }
+
+    PathSet queryFailedPaths() override
+    { return {}; }
+
+    void clearFailedPaths(const PathSet & paths) override
+    { }
+
+    void optimiseStore() override
+    { }
+
+    bool verifyStore(bool checkContents, bool repair) override
+    { return true; }
+
+};
+
+}
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 180a558dc2e9..547981e5bb01 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1310,7 +1310,6 @@ void DerivationGoal::tryToBuild()
     for (auto & i : drv->outputs) {
         Path path = i.second.path;
         if (worker.store.isValidPath(path)) continue;
-        if (!pathExists(path)) continue;
         debug(format("removing invalid path ‘%1%’") % path);
         deletePath(path);
     }
@@ -1390,8 +1389,7 @@ void replaceValidPath(const Path & storePath, const Path tmpPath)
         rename(storePath.c_str(), oldPath.c_str());
     if (rename(tmpPath.c_str(), storePath.c_str()) == -1)
         throw SysError(format("moving ‘%1%’ to ‘%2%’") % tmpPath % storePath);
-    if (pathExists(oldPath))
-        deletePath(oldPath);
+    deletePath(oldPath);
 }
 
 
@@ -1490,7 +1488,7 @@ void DerivationGoal::buildDone()
 
         /* Delete unused redirected outputs (when doing hash rewriting). */
         for (auto & i : redirectedOutputs)
-            if (pathExists(i.second)) deletePath(i.second);
+            deletePath(i.second);
 
         /* Delete the chroot (if we were using one). */
         autoDelChroot.reset(); /* this runs the destructor */
@@ -1939,7 +1937,7 @@ void DerivationGoal::startBuilder()
            to ensure that we can create hard-links to non-directory
            inputs in the fake Nix store in the chroot (see below). */
         chrootRootDir = drvPath + ".chroot";
-        if (pathExists(chrootRootDir)) deletePath(chrootRootDir);
+        deletePath(chrootRootDir);
 
         /* Clean up the chroot directory automatically. */
         autoDelChroot = std::make_shared<AutoDelete>(chrootRootDir);
@@ -2514,7 +2512,7 @@ void DerivationGoal::runChild()
             debug(sandboxProfile);
 
             Path sandboxFile = drvPath + ".sb";
-            if (pathExists(sandboxFile)) deletePath(sandboxFile);
+            deletePath(sandboxFile);
             autoDelSandbox.reset(sandboxFile, false);
 
             writeFile(sandboxFile, sandboxProfile);
@@ -2706,8 +2704,7 @@ void DerivationGoal::registerOutputs()
                     return;
                 if (actualPath != dest) {
                     PathLocks outputLocks({dest});
-                    if (pathExists(dest))
-                        deletePath(dest);
+                    deletePath(dest);
                     if (rename(actualPath.c_str(), dest.c_str()) == -1)
                         throw SysError(format("moving ‘%1%’ to ‘%2%’") % actualPath % dest);
                 }
@@ -2735,10 +2732,10 @@ void DerivationGoal::registerOutputs()
         if (buildMode == bmCheck) {
             if (!worker.store.isValidPath(path)) continue;
             ValidPathInfo info = worker.store.queryPathInfo(path);
-            if (hash.first != info.hash) {
+            if (hash.first != info.narHash) {
                 if (settings.keepFailed) {
                     Path dst = path + checkSuffix;
-                    if (pathExists(dst)) deletePath(dst);
+                    deletePath(dst);
                     if (rename(actualPath.c_str(), dst.c_str()))
                         throw SysError(format("renaming ‘%1%’ to ‘%2%’") % actualPath % dst);
                     throw Error(format("derivation ‘%1%’ may not be deterministic: output ‘%2%’ differs from ‘%3%’")
@@ -2799,7 +2796,7 @@ void DerivationGoal::registerOutputs()
 
         ValidPathInfo info;
         info.path = path;
-        info.hash = hash.first;
+        info.narHash = hash.first;
         info.narSize = hash.second;
         info.references = references;
         info.deriver = drvPath;
@@ -2830,7 +2827,7 @@ void DerivationGoal::registerOutputs()
     if (settings.keepFailed) {
         for (auto & i : drv->outputs) {
             Path prev = i.second.path + checkSuffix;
-            if (pathExists(prev)) deletePath(prev);
+            deletePath(prev);
             if (curRound < nrRounds) {
                 Path dst = i.second.path + checkSuffix;
                 if (rename(i.second.path.c_str(), dst.c_str()))
@@ -2998,7 +2995,7 @@ Path DerivationGoal::addHashRewrite(const Path & path)
     string h1 = string(path, settings.nixStore.size() + 1, 32);
     string h2 = string(printHash32(hashString(htSHA256, "rewrite:" + drvPath + ":" + path)), 0, 32);
     Path p = settings.nixStore + "/" + h2 + string(path, settings.nixStore.size() + 33);
-    if (pathExists(p)) deletePath(p);
+    deletePath(p);
     assert(path.size() == p.size());
     rewritesToTmp[h1] = h2;
     rewritesFromTmp[h2] = h1;
@@ -3259,8 +3256,7 @@ void SubstitutionGoal::tryToRun()
     destPath = repair ? storePath + ".tmp" : storePath;
 
     /* Remove the (stale) output path if it exists. */
-    if (pathExists(destPath))
-        deletePath(destPath);
+    deletePath(destPath);
 
     worker.store.setSubstituterEnv();
 
@@ -3369,7 +3365,7 @@ void SubstitutionGoal::finished()
 
     ValidPathInfo info2;
     info2.path = storePath;
-    info2.hash = hash.first;
+    info2.narHash = hash.first;
     info2.narSize = hash.second;
     info2.references = info.references;
     info2.deriver = info.deriver;
diff --git a/src/libstore/crypto.cc b/src/libstore/crypto.cc
new file mode 100644
index 000000000000..c1b57e51d9b4
--- /dev/null
+++ b/src/libstore/crypto.cc
@@ -0,0 +1,88 @@
+#include "crypto.hh"
+#include "util.hh"
+
+#if HAVE_SODIUM
+#include <sodium.h>
+#endif
+
+namespace nix {
+
+static std::pair<std::string, std::string> split(const string & s)
+{
+    size_t colon = s.find(':');
+    if (colon == std::string::npos || colon == 0)
+        return {"", ""};
+    return {std::string(s, 0, colon), std::string(s, colon + 1)};
+}
+
+Key::Key(const string & s)
+{
+    auto ss = split(s);
+
+    name = ss.first;
+    key = ss.second;
+
+    if (name == "" || key == "")
+        throw Error("secret key is corrupt");
+
+    key = base64Decode(key);
+}
+
+SecretKey::SecretKey(const string & s)
+    : Key(s)
+{
+#if HAVE_SODIUM
+    if (key.size() != crypto_sign_SECRETKEYBYTES)
+        throw Error("secret key is not valid");
+#endif
+}
+
+[[noreturn]] static void noSodium()
+{
+    throw Error("Nix was not compiled with libsodium, required for signed binary cache support");
+}
+
+std::string SecretKey::signDetached(const std::string & data) const
+{
+#if HAVE_SODIUM
+    unsigned char sig[crypto_sign_BYTES];
+    unsigned long long sigLen;
+    crypto_sign_detached(sig, &sigLen, (unsigned char *) data.data(), data.size(),
+        (unsigned char *) key.data());
+    return name + ":" + base64Encode(std::string((char *) sig, sigLen));
+#else
+    noSodium();
+#endif
+}
+
+PublicKey::PublicKey(const string & s)
+    : Key(s)
+{
+#if HAVE_SODIUM
+    if (key.size() != crypto_sign_PUBLICKEYBYTES)
+        throw Error("public key is not valid");
+#endif
+}
+
+bool verifyDetached(const std::string & data, const std::string & sig,
+    const PublicKeys & publicKeys)
+{
+#if HAVE_SODIUM
+    auto ss = split(sig);
+
+    auto key = publicKeys.find(ss.first);
+    if (key == publicKeys.end()) return false;
+
+    auto sig2 = base64Decode(ss.second);
+    if (sig2.size() != crypto_sign_BYTES)
+        throw Error("signature is not valid");
+
+    return crypto_sign_verify_detached((unsigned char *) sig2.data(),
+        (unsigned char *) data.data(), data.size(),
+        (unsigned char *) key->second.key.data()) == 0;
+#else
+    noSodium();
+#endif
+}
+
+}
diff --git a/src/libstore/crypto.hh b/src/libstore/crypto.hh
new file mode 100644
index 000000000000..a1489e753649
--- /dev/null
+++ b/src/libstore/crypto.hh
@@ -0,0 +1,40 @@
+#pragma once
+
+#include "types.hh"
+
+#include <map>
+
+namespace nix {
+
+struct Key
+{
+    std::string name;
+    std::string key;
+
+    /* Construct Key from a string in the format
+       ‘<name>:<key-in-base64>’. */
+    Key(const std::string & s);
+
+};
+
+struct SecretKey : Key
+{
+    SecretKey(const std::string & s);
+
+    /* Return a detached signature of the given string. */
+    std::string signDetached(const std::string & s) const;
+};
+
+struct PublicKey : Key
+{
+    PublicKey(const std::string & data);
+};
+
+typedef std::map<std::string, PublicKey> PublicKeys;
+
+/* Return true iff ‘sig’ is a correct signature over ‘data’ using one
+   of the given public keys. */
+bool verifyDetached(const std::string & data, const std::string & sig,
+    const PublicKeys & publicKeys);
+
+}
diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index 01ce1ea2fd4c..e754e82fb27f 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -114,6 +114,8 @@ struct Curl
         curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, progressCallback_);
         curl_easy_setopt(curl, CURLOPT_PROGRESSDATA, (void *) &curl);
         curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0);
+
+        curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1);
     }
 
     ~Curl()
diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index 48fb2167a649..e082f67143a3 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -83,7 +83,7 @@ void LocalStore::addIndirectRoot(const Path & path)
 }
 
 
-Path addPermRoot(ref<Store> store, const Path & _storePath,
+Path Store::addPermRoot(const Path & _storePath,
     const Path & _gcRoot, bool indirect, bool allowOutsideRootsDir)
 {
     Path storePath(canonPath(_storePath));
@@ -101,7 +101,7 @@ Path addPermRoot(ref<Store> store, const Path & _storePath,
         if (pathExists(gcRoot) && (!isLink(gcRoot) || !isInStore(readLink(gcRoot))))
             throw Error(format("cannot create symlink ‘%1%’; already exists") % gcRoot);
         makeSymlink(gcRoot, storePath);
-        store->addIndirectRoot(gcRoot);
+        addIndirectRoot(gcRoot);
     }
 
     else {
@@ -127,7 +127,7 @@ Path addPermRoot(ref<Store> store, const Path & _storePath,
        check if the root is in a directory in or linked from the
        gcroots directory. */
     if (settings.checkRootReachability) {
-        Roots roots = store->findRoots();
+        Roots roots = findRoots();
         if (roots.find(gcRoot) == roots.end())
             printMsg(lvlError,
                 format(
@@ -139,7 +139,7 @@ Path addPermRoot(ref<Store> store, const Path & _storePath,
     /* Grab the global GC root, causing us to block while a GC is in
        progress.  This prevents the set of permanent roots from
        increasing while a GC is in progress. */
-    store->syncWithGC();
+    syncWithGC();
 
     return gcRoot;
 }
@@ -608,6 +608,9 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
 
     state.shouldDelete = options.action == GCOptions::gcDeleteDead || options.action == GCOptions::gcDeleteSpecific;
 
+    if (state.shouldDelete)
+        deletePath(reservedPath);
+
     /* Acquire the global GC root.  This prevents
        a) New roots from being added.
        b) Processes from creating new temporary root files. */
diff --git a/src/libstore/local-binary-cache-store.cc b/src/libstore/local-binary-cache-store.cc
new file mode 100644
index 000000000000..5714688e02df
--- /dev/null
+++ b/src/libstore/local-binary-cache-store.cc
@@ -0,0 +1,44 @@
+#include "local-binary-cache-store.hh"
+
+namespace nix {
+
+LocalBinaryCacheStore::LocalBinaryCacheStore(std::shared_ptr<Store> localStore,
+    const Path & secretKeyFile, const Path & publicKeyFile,
+    const Path & binaryCacheDir)
+    : BinaryCacheStore(localStore, secretKeyFile, publicKeyFile)
+    , binaryCacheDir(binaryCacheDir)
+{
+}
+
+void LocalBinaryCacheStore::init()
+{
+    createDirs(binaryCacheDir + "/nar");
+    BinaryCacheStore::init();
+}
+
+static void atomicWrite(const Path & path, const std::string & s)
+{
+    Path tmp = path + ".tmp." + std::to_string(getpid());
+    AutoDelete del(tmp, false);
+    writeFile(tmp, s);
+    if (rename(tmp.c_str(), path.c_str()))
+        throw SysError(format("renaming ‘%1%’ to ‘%2%’") % tmp % path);
+    del.cancel();
+}
+
+bool LocalBinaryCacheStore::fileExists(const std::string & path)
+{
+    return pathExists(binaryCacheDir + "/" + path);
+}
+
+void LocalBinaryCacheStore::upsertFile(const std::string & path, const std::string & data)
+{
+    atomicWrite(binaryCacheDir + "/" + path, data);
+}
+
+std::string LocalBinaryCacheStore::getFile(const std::string & path)
+{
+    return readFile(binaryCacheDir + "/" + path);
+}
+
+}
diff --git a/src/libstore/local-binary-cache-store.hh b/src/libstore/local-binary-cache-store.hh
new file mode 100644
index 000000000000..0303ebe734a6
--- /dev/null
+++ b/src/libstore/local-binary-cache-store.hh
@@ -0,0 +1,31 @@
+#pragma once
+
+#include "binary-cache-store.hh"
+
+namespace nix {
+
+class LocalBinaryCacheStore : public BinaryCacheStore
+{
+private:
+
+    Path binaryCacheDir;
+
+public:
+
+    LocalBinaryCacheStore(std::shared_ptr<Store> localStore,
+        const Path & secretKeyFile, const Path & publicKeyFile,
+        const Path & binaryCacheDir);
+
+    void init() override;
+
+protected:
+
+    bool fileExists(const std::string & path) override;
+
+    void upsertFile(const std::string & path, const std::string & data) override;
+
+    std::string getFile(const std::string & path) override;
+
+};
+
+}
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 13179459f1c1..33f2569128aa 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -40,10 +40,7 @@ MakeError(SQLiteError, Error);
 MakeError(SQLiteBusy, SQLiteError);
 
 
-static void throwSQLiteError(sqlite3 * db, const format & f)
-    __attribute__ ((noreturn));
-
-static void throwSQLiteError(sqlite3 * db, const format & f)
+[[noreturn]] static void throwSQLiteError(sqlite3 * db, const format & f)
 {
     int err = sqlite3_errcode(db);
     if (err == SQLITE_BUSY || err == SQLITE_PROTOCOL) {
@@ -219,8 +216,9 @@ void checkStoreNotSymlink()
 }
 
 
-LocalStore::LocalStore(bool reserveSpace)
-    : didSetSubstituterEnv(false)
+LocalStore::LocalStore()
+    : reservedPath(settings.nixDBPath + "/reserved")
+    , didSetSubstituterEnv(false)
 {
     schemaPath = settings.nixDBPath + "/schema";
 
@@ -279,25 +277,20 @@ LocalStore::LocalStore(bool reserveSpace)
        needed, we reserve some dummy space that we can free just
        before doing a garbage collection. */
     try {
-        Path reservedPath = settings.nixDBPath + "/reserved";
-        if (reserveSpace) {
-            struct stat st;
-            if (stat(reservedPath.c_str(), &st) == -1 ||
-                st.st_size != settings.reservedSize)
-            {
-                AutoCloseFD fd = open(reservedPath.c_str(), O_WRONLY | O_CREAT, 0600);
-                int res = -1;
+        struct stat st;
+        if (stat(reservedPath.c_str(), &st) == -1 ||
+            st.st_size != settings.reservedSize)
+        {
+            AutoCloseFD fd = open(reservedPath.c_str(), O_WRONLY | O_CREAT, 0600);
+            int res = -1;
 #if HAVE_POSIX_FALLOCATE
-                res = posix_fallocate(fd, 0, settings.reservedSize);
+            res = posix_fallocate(fd, 0, settings.reservedSize);
 #endif
-                if (res == -1) {
-                    writeFull(fd, string(settings.reservedSize, 'X'));
-                    ftruncate(fd, settings.reservedSize);
-                }
+            if (res == -1) {
+                writeFull(fd, string(settings.reservedSize, 'X'));
+                ftruncate(fd, settings.reservedSize);
             }
         }
-        else
-            deletePath(reservedPath);
     } catch (SysError & e) { /* don't care about errors */
     }
 
@@ -694,7 +687,7 @@ unsigned long long LocalStore::addValidPath(const ValidPathInfo & info, bool che
 {
     SQLiteStmtUse use(stmtRegisterValidPath);
     stmtRegisterValidPath.bind(info.path);
-    stmtRegisterValidPath.bind("sha256:" + printHash(info.hash));
+    stmtRegisterValidPath.bind("sha256:" + printHash(info.narHash));
     stmtRegisterValidPath.bind(info.registrationTime == 0 ? time(0) : info.registrationTime);
     if (info.deriver != "")
         stmtRegisterValidPath.bind(info.deriver);
@@ -845,7 +838,7 @@ ValidPathInfo LocalStore::queryPathInfo(const Path & path)
 
         const char * s = (const char *) sqlite3_column_text(stmtQueryPathInfo, 1);
         assert(s);
-        info.hash = parseHashField(path, s);
+        info.narHash = parseHashField(path, s);
 
         info.registrationTime = sqlite3_column_int(stmtQueryPathInfo, 2);
 
@@ -883,7 +876,7 @@ void LocalStore::updatePathInfo(const ValidPathInfo & info)
         stmtUpdatePathInfo.bind64(info.narSize);
     else
         stmtUpdatePathInfo.bind(); // null
-    stmtUpdatePathInfo.bind("sha256:" + printHash(info.hash));
+    stmtUpdatePathInfo.bind("sha256:" + printHash(info.narHash));
     stmtUpdatePathInfo.bind(info.path);
     if (sqlite3_step(stmtUpdatePathInfo) != SQLITE_DONE)
         throwSQLiteError(db, format("updating info of path ‘%1%’ in database") % info.path);
@@ -953,14 +946,6 @@ PathSet LocalStore::queryAllValidPaths()
 }
 
 
-void LocalStore::queryReferences(const Path & path,
-    PathSet & references)
-{
-    ValidPathInfo info = queryPathInfo(path);
-    references.insert(info.references.begin(), info.references.end());
-}
-
-
 void LocalStore::queryReferrers_(const Path & path, PathSet & referrers)
 {
     SQLiteStmtUse use(stmtQueryReferrers);
@@ -1064,7 +1049,7 @@ StringSet LocalStore::queryDerivationOutputNames(const Path & path)
 
 Path LocalStore::queryPathFromHashPart(const string & hashPart)
 {
-    if (hashPart.size() != 32) throw Error("invalid hash part");
+    if (hashPart.size() != storePathHashLen) throw Error("invalid hash part");
 
     Path prefix = settings.nixStore + "/" + hashPart;
 
@@ -1282,7 +1267,7 @@ void LocalStore::querySubstitutablePathInfos(const PathSet & paths,
 
 Hash LocalStore::queryPathHash(const Path & path)
 {
-    return queryPathInfo(path).hash;
+    return queryPathInfo(path).narHash;
 }
 
 
@@ -1306,7 +1291,7 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
         PathSet paths;
 
         for (auto & i : infos) {
-            assert(i.hash.type == htSHA256);
+            assert(i.narHash.type == htSHA256);
             if (isValidPath_(i.path))
                 updatePathInfo(i);
             else
@@ -1380,7 +1365,7 @@ Path LocalStore::addToStoreFromDump(const string & dump, const string & name,
 
         if (repair || !isValidPath(dstPath)) {
 
-            if (pathExists(dstPath)) deletePath(dstPath);
+            deletePath(dstPath);
 
             if (recursive) {
                 StringSource source(dump);
@@ -1405,7 +1390,7 @@ Path LocalStore::addToStoreFromDump(const string & dump, const string & name,
 
             ValidPathInfo info;
             info.path = dstPath;
-            info.hash = hash.first;
+            info.narHash = hash.first;
             info.narSize = hash.second;
             registerValidPath(info);
         }
@@ -1449,20 +1434,22 @@ Path LocalStore::addTextToStore(const string & name, const string & s,
 
         if (repair || !isValidPath(dstPath)) {
 
-            if (pathExists(dstPath)) deletePath(dstPath);
+            deletePath(dstPath);
 
             writeFile(dstPath, s);
 
             canonicalisePathMetaData(dstPath, -1);
 
-            HashResult hash = hashPath(htSHA256, dstPath);
+            StringSink sink;
+            dumpString(s, sink);
+            auto hash = hashString(htSHA256, sink.s);
 
             optimisePath(dstPath);
 
             ValidPathInfo info;
             info.path = dstPath;
-            info.hash = hash.first;
-            info.narSize = hash.second;
+            info.narHash = hash;
+            info.narSize = sink.s.size();
             info.references = references;
             registerValidPath(info);
         }
@@ -1493,9 +1480,6 @@ struct HashAndWriteSink : Sink
 };
 
 
-#define EXPORT_MAGIC 0x4558494e
-
-
 static void checkSecrecy(const Path & path)
 {
     struct stat st;
@@ -1532,7 +1516,7 @@ void LocalStore::exportPath(const Path & path, bool sign,
     PathSet references;
     queryReferences(path, references);
 
-    hashAndWriteSink << EXPORT_MAGIC << path << references << queryDeriver(path);
+    hashAndWriteSink << exportMagic << path << references << queryDeriver(path);
 
     if (sign) {
         Hash hash = hashAndWriteSink.currentHash();
@@ -1608,8 +1592,8 @@ Path LocalStore::importPath(bool requireSignature, Source & source)
 
     restorePath(unpacked, hashAndReadSource);
 
-    unsigned int magic = readInt(hashAndReadSource);
-    if (magic != EXPORT_MAGIC)
+    uint32_t magic = readInt(hashAndReadSource);
+    if (magic != exportMagic)
         throw Error("Nix archive cannot be imported; wrong format");
 
     Path dstPath = readStorePath(hashAndReadSource);
@@ -1675,7 +1659,7 @@ Path LocalStore::importPath(bool requireSignature, Source & source)
 
         if (!isValidPath(dstPath)) {
 
-            if (pathExists(dstPath)) deletePath(dstPath);
+            deletePath(dstPath);
 
             if (rename(unpacked.c_str(), dstPath.c_str()) == -1)
                 throw SysError(format("cannot move ‘%1%’ to ‘%2%’")
@@ -1691,7 +1675,7 @@ Path LocalStore::importPath(bool requireSignature, Source & source)
 
             ValidPathInfo info;
             info.path = dstPath;
-            info.hash = hash.first;
+            info.narHash = hash.first;
             info.narSize = hash.second;
             info.references = references;
             info.deriver = deriver != "" && isValidPath(deriver) ? deriver : "";
@@ -1775,21 +1759,21 @@ bool LocalStore::verifyStore(bool checkContents, bool repair)
 
                 /* Check the content hash (optionally - slow). */
                 printMsg(lvlTalkative, format("checking contents of ‘%1%’") % i);
-                HashResult current = hashPath(info.hash.type, i);
+                HashResult current = hashPath(info.narHash.type, i);
 
-                if (info.hash != nullHash && info.hash != current.first) {
+                if (info.narHash != nullHash && info.narHash != current.first) {
                     printMsg(lvlError, format("path ‘%1%’ was modified! "
                             "expected hash ‘%2%’, got ‘%3%’")
-                        % i % printHash(info.hash) % printHash(current.first));
+                        % i % printHash(info.narHash) % printHash(current.first));
                     if (repair) repairPath(i); else errors = true;
                 } else {
 
                     bool update = false;
 
                     /* Fill in missing hashes. */
-                    if (info.hash == nullHash) {
+                    if (info.narHash == nullHash) {
                         printMsg(lvlError, format("fixing missing hash on ‘%1%’") % i);
-                        info.hash = current.first;
+                        info.narHash = current.first;
                         update = true;
                     }
 
@@ -1878,9 +1862,9 @@ bool LocalStore::pathContentsGood(const Path & path)
     if (!pathExists(path))
         res = false;
     else {
-        HashResult current = hashPath(info.hash.type, path);
+        HashResult current = hashPath(info.narHash.type, path);
         Hash nullHash(htSHA256);
-        res = info.hash == nullHash || info.hash == current.first;
+        res = info.narHash == nullHash || info.narHash == current.first;
     }
     pathContentsGoodCache[path] = res;
     if (!res) printMsg(lvlError, format("path ‘%1%’ is corrupted or missing!") % path);
@@ -1932,7 +1916,7 @@ ValidPathInfo LocalStore::queryPathInfoOld(const Path & path)
         } else if (name == "Deriver") {
             res.deriver = value;
         } else if (name == "Hash") {
-            res.hash = parseHashField(path, value);
+            res.narHash = parseHashField(path, value);
         } else if (name == "Registered-At") {
             int n = 0;
             string2Int(value, n);
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index b6d39d345c94..5582acd0f72f 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -88,11 +88,13 @@ private:
 
     Path linksDir;
 
+    Path reservedPath;
+
 public:
 
     /* Initialise the local store, upgrading the schema if
        necessary. */
-    LocalStore(bool reserveSpace = true);
+    LocalStore();
 
     ~LocalStore();
 
@@ -108,8 +110,6 @@ public:
 
     Hash queryPathHash(const Path & path) override;
 
-    void queryReferences(const Path & path, PathSet & references) override;
-
     void queryReferrers(const Path & path, PathSet & referrers) override;
 
     Path queryDeriver(const Path & path) override;
diff --git a/src/libstore/local.mk b/src/libstore/local.mk
index e78f47949ad3..9a01596c36be 100644
--- a/src/libstore/local.mk
+++ b/src/libstore/local.mk
@@ -8,7 +8,7 @@ libstore_SOURCES := $(wildcard $(d)/*.cc)
 
 libstore_LIBS = libutil libformat
 
-libstore_LDFLAGS = $(SQLITE3_LIBS) -lbz2 $(LIBCURL_LIBS)
+libstore_LDFLAGS = $(SQLITE3_LIBS) -lbz2 $(LIBCURL_LIBS) $(SODIUM_LIBS)
 
 ifeq ($(OS), SunOS)
 	libstore_LDFLAGS += -lsocket
diff --git a/src/libstore/nar-info.cc b/src/libstore/nar-info.cc
new file mode 100644
index 000000000000..e9260a09bf5a
--- /dev/null
+++ b/src/libstore/nar-info.cc
@@ -0,0 +1,134 @@
+#include "crypto.hh"
+#include "globals.hh"
+#include "nar-info.hh"
+
+namespace nix {
+
+NarInfo::NarInfo(const std::string & s, const std::string & whence)
+{
+    auto corrupt = [&]() {
+        throw Error("NAR info file ‘%1%’ is corrupt");
+    };
+
+    auto parseHashField = [&](const string & s) {
+        string::size_type colon = s.find(':');
+        if (colon == string::npos) corrupt();
+        HashType ht = parseHashType(string(s, 0, colon));
+        if (ht == htUnknown) corrupt();
+        return parseHash16or32(ht, string(s, colon + 1));
+    };
+
+    size_t pos = 0;
+    while (pos < s.size()) {
+
+        size_t colon = s.find(':', pos);
+        if (colon == std::string::npos) corrupt();
+
+        std::string name(s, pos, colon - pos);
+
+        size_t eol = s.find('\n', colon + 2);
+        if (eol == std::string::npos) corrupt();
+
+        std::string value(s, colon + 2, eol - colon - 2);
+
+        if (name == "StorePath") {
+            if (!isStorePath(value)) corrupt();
+            path = value;
+        }
+        else if (name == "URL")
+            url = value;
+        else if (name == "Compression")
+            compression = value;
+        else if (name == "FileHash")
+            fileHash = parseHashField(value);
+        else if (name == "FileSize") {
+            if (!string2Int(value, fileSize)) corrupt();
+        }
+        else if (name == "NarHash")
+            narHash = parseHashField(value);
+        else if (name == "NarSize") {
+            if (!string2Int(value, narSize)) corrupt();
+        }
+        else if (name == "References") {
+            auto refs = tokenizeString<Strings>(value, " ");
+            if (!references.empty()) corrupt();
+            for (auto & r : refs) {
+                auto r2 = settings.nixStore + "/" + r;
+                if (!isStorePath(r2)) corrupt();
+                references.insert(r2);
+            }
+        }
+        else if (name == "Deriver") {
+            auto p = settings.nixStore + "/" + value;
+            if (!isStorePath(p)) corrupt();
+            deriver = p;
+        }
+        else if (name == "System")
+            system = value;
+        else if (name == "Sig")
+            sig = value;
+
+        pos = eol + 1;
+    }
+
+    if (compression == "") compression = "bzip2";
+
+    if (path.empty() || url.empty()) corrupt();
+}
+
+std::string NarInfo::to_string() const
+{
+    std::string res;
+    res += "StorePath: " + path + "\n";
+    res += "URL: " + url + "\n";
+    assert(compression != "");
+    res += "Compression: " + compression + "\n";
+    assert(fileHash.type == htSHA256);
+    res += "FileHash: sha256:" + printHash32(fileHash) + "\n";
+    res += "FileSize: " + std::to_string(fileSize) + "\n";
+    assert(narHash.type == htSHA256);
+    res += "NarHash: sha256:" + printHash32(narHash) + "\n";
+    res += "NarSize: " + std::to_string(narSize) + "\n";
+
+    res += "References: " + concatStringsSep(" ", shortRefs()) + "\n";
+
+    if (!deriver.empty())
+        res += "Deriver: " + baseNameOf(deriver) + "\n";
+
+    if (!system.empty())
+        res += "System: " + system + "\n";
+
+    if (!sig.empty())
+        res += "Sig: " + sig + "\n";
+
+    return res;
+}
+
+std::string NarInfo::fingerprint() const
+{
+    return
+        "1;" + path + ";"
+        + printHashType(narHash.type) + ":" + printHash32(narHash) + ";"
+        + std::to_string(narSize) + ";"
+        + concatStringsSep(",", references);
+}
+
+Strings NarInfo::shortRefs() const
+{
+    Strings refs;
+    for (auto & r : references)
+        refs.push_back(baseNameOf(r));
+    return refs;
+}
+
+void NarInfo::sign(const SecretKey & secretKey)
+{
+    sig = secretKey.signDetached(fingerprint());
+}
+
+bool NarInfo::checkSignature(const PublicKeys & publicKeys) const
+{
+    return sig != "" && verifyDetached(fingerprint(), sig, publicKeys);
+}
+
+}
diff --git a/src/libstore/nar-info.hh b/src/libstore/nar-info.hh
new file mode 100644
index 000000000000..22e27cb42ebf
--- /dev/null
+++ b/src/libstore/nar-info.hh
@@ -0,0 +1,43 @@
+#pragma once
+
+#include "types.hh"
+#include "hash.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+struct NarInfo : ValidPathInfo
+{
+    std::string url;
+    std::string compression;
+    Hash fileHash;
+    uint64_t fileSize = 0;
+    std::string system;
+    std::string sig; // FIXME: support multiple signatures
+
+    NarInfo() { }
+    NarInfo(const ValidPathInfo & info) : ValidPathInfo(info) { }
+    NarInfo(const std::string & s, const std::string & whence);
+
+    std::string to_string() const;
+
+    /*  Return a fingerprint of the store path to be used in binary
+        cache signatures. It contains the store path, the base-32
+        SHA-256 hash of the NAR serialisation of the path, the size of
+        the NAR, and the sorted references. The size field is strictly
+        speaking superfluous, but might prevent endless/excessive data
+        attacks. */
+    std::string fingerprint() const;
+
+    void sign(const SecretKey & secretKey);
+
+    /* Return true iff this .narinfo is signed by one of the specified
+       keys. */
+    bool checkSignature(const PublicKeys & publicKeys) const;
+
+private:
+
+    Strings shortRefs() const;
+};
+
+}
diff --git a/src/libstore/profiles.cc b/src/libstore/profiles.cc
index 8ab23cd317cf..cc83a838eddc 100644
--- a/src/libstore/profiles.cc
+++ b/src/libstore/profiles.cc
@@ -108,7 +108,7 @@ Path createGeneration(ref<Store> store, Path profile, Path outPath)
        user environment etc. we've just built. */
     Path generation;
     makeName(profile, num + 1, generation);
-    addPermRoot(store, outPath, generation, false, true);
+    store->addPermRoot(outPath, generation, false, true);
 
     return generation;
 }
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 679210d4cb16..a1ac169c2b9e 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -6,6 +6,7 @@
 #include "affinity.hh"
 #include "globals.hh"
 #include "derivations.hh"
+#include "pool.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -13,9 +14,8 @@
 #include <sys/un.h>
 #include <errno.h>
 #include <fcntl.h>
-
-#include <iostream>
 #include <unistd.h>
+
 #include <cstring>
 
 namespace nix {
@@ -39,62 +39,25 @@ template<class T> T readStorePaths(Source & from)
 template PathSet readStorePaths(Source & from);
 
 
-RemoteStore::RemoteStore()
+RemoteStore::RemoteStore(size_t maxConnections)
+    : connections(make_ref<Pool<Connection>>(
+            maxConnections,
+            [this]() { return openConnection(); },
+            [](const ref<Connection> & r) { return r->to.good() && r->from.good(); }
+            ))
 {
-    initialised = false;
 }
 
 
-void RemoteStore::openConnection(bool reserveSpace)
+ref<RemoteStore::Connection> RemoteStore::openConnection()
 {
-    if (initialised) return;
-    initialised = true;
+    auto conn = make_ref<Connection>();
 
     /* Connect to a daemon that does the privileged work for us. */
-    connectToDaemon();
-
-    from.fd = fdSocket;
-    to.fd = fdSocket;
-
-    /* Send the magic greeting, check for the reply. */
-    try {
-        to << WORKER_MAGIC_1;
-        to.flush();
-        unsigned int magic = readInt(from);
-        if (magic != WORKER_MAGIC_2) throw Error("protocol mismatch");
-
-        daemonVersion = readInt(from);
-        if (GET_PROTOCOL_MAJOR(daemonVersion) != GET_PROTOCOL_MAJOR(PROTOCOL_VERSION))
-            throw Error("Nix daemon protocol version not supported");
-        to << PROTOCOL_VERSION;
-
-        if (GET_PROTOCOL_MINOR(daemonVersion) >= 14) {
-            int cpu = settings.lockCPU ? lockToCurrentCPU() : -1;
-            if (cpu != -1)
-                to << 1 << cpu;
-            else
-                to << 0;
-        }
-
-        if (GET_PROTOCOL_MINOR(daemonVersion) >= 11)
-            to << reserveSpace;
-
-        processStderr();
-    }
-    catch (Error & e) {
-        throw Error(format("cannot start daemon worker: %1%") % e.msg());
-    }
-
-    setOptions();
-}
-
-
-void RemoteStore::connectToDaemon()
-{
-    fdSocket = socket(PF_UNIX, SOCK_STREAM, 0);
-    if (fdSocket == -1)
+    conn->fd = socket(PF_UNIX, SOCK_STREAM, 0);
+    if (conn->fd == -1)
         throw SysError("cannot create Unix domain socket");
-    closeOnExec(fdSocket);
+    closeOnExec(conn->fd);
 
     string socketPath = settings.nixDaemonSocketFile;
 
@@ -111,111 +74,135 @@ void RemoteStore::connectToDaemon()
     addr.sun_family = AF_UNIX;
     if (socketPathRel.size() >= sizeof(addr.sun_path))
         throw Error(format("socket path ‘%1%’ is too long") % socketPathRel);
-    using namespace std;
     strcpy(addr.sun_path, socketPathRel.c_str());
 
-    if (connect(fdSocket, (struct sockaddr *) &addr, sizeof(addr)) == -1)
+    if (connect(conn->fd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
         throw SysError(format("cannot connect to daemon at ‘%1%’") % socketPath);
 
     if (fchdir(fdPrevDir) == -1)
         throw SysError("couldn't change back to previous directory");
-}
 
+    conn->from.fd = conn->fd;
+    conn->to.fd = conn->fd;
 
-RemoteStore::~RemoteStore()
-{
+    /* Send the magic greeting, check for the reply. */
     try {
-        to.flush();
-        fdSocket.close();
-    } catch (...) {
-        ignoreException();
+        conn->to << WORKER_MAGIC_1;
+        conn->to.flush();
+        unsigned int magic = readInt(conn->from);
+        if (magic != WORKER_MAGIC_2) throw Error("protocol mismatch");
+
+        conn->daemonVersion = readInt(conn->from);
+        if (GET_PROTOCOL_MAJOR(conn->daemonVersion) != GET_PROTOCOL_MAJOR(PROTOCOL_VERSION))
+            throw Error("Nix daemon protocol version not supported");
+        conn->to << PROTOCOL_VERSION;
+
+        if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 14) {
+            int cpu = settings.lockCPU ? lockToCurrentCPU() : -1;
+            if (cpu != -1)
+                conn->to << 1 << cpu;
+            else
+                conn->to << 0;
+        }
+
+        if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 11)
+            conn->to << false;
+
+        conn->processStderr();
     }
+    catch (Error & e) {
+        throw Error(format("cannot start daemon worker: %1%") % e.msg());
+    }
+
+    setOptions(conn);
+
+    return conn;
 }
 
 
-void RemoteStore::setOptions()
+void RemoteStore::setOptions(ref<Connection> conn)
 {
-    to << wopSetOptions
+    conn->to << wopSetOptions
        << settings.keepFailed
        << settings.keepGoing
        << settings.tryFallback
        << verbosity
        << settings.maxBuildJobs
        << settings.maxSilentTime;
-    if (GET_PROTOCOL_MINOR(daemonVersion) >= 2)
-        to << settings.useBuildHook;
-    if (GET_PROTOCOL_MINOR(daemonVersion) >= 4)
-        to << settings.buildVerbosity
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 2)
+        conn->to << settings.useBuildHook;
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 4)
+        conn->to << settings.buildVerbosity
            << logType
            << settings.printBuildTrace;
-    if (GET_PROTOCOL_MINOR(daemonVersion) >= 6)
-        to << settings.buildCores;
-    if (GET_PROTOCOL_MINOR(daemonVersion) >= 10)
-        to << settings.useSubstitutes;
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 6)
+        conn->to << settings.buildCores;
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 10)
+        conn->to << settings.useSubstitutes;
 
-    if (GET_PROTOCOL_MINOR(daemonVersion) >= 12) {
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 12) {
         Settings::SettingsMap overrides = settings.getOverrides();
         if (overrides["ssh-auth-sock"] == "")
             overrides["ssh-auth-sock"] = getEnv("SSH_AUTH_SOCK");
-        to << overrides.size();
+        conn->to << overrides.size();
         for (auto & i : overrides)
-            to << i.first << i.second;
+            conn->to << i.first << i.second;
     }
 
-    processStderr();
+    conn->processStderr();
 }
 
 
 bool RemoteStore::isValidPath(const Path & path)
 {
-    openConnection();
-    to << wopIsValidPath << path;
-    processStderr();
-    unsigned int reply = readInt(from);
+    auto conn(connections->get());
+    conn->to << wopIsValidPath << path;
+    conn->processStderr();
+    unsigned int reply = readInt(conn->from);
     return reply != 0;
 }
 
 
 PathSet RemoteStore::queryValidPaths(const PathSet & paths)
 {
-    openConnection();
-    if (GET_PROTOCOL_MINOR(daemonVersion) < 12) {
+    auto conn(connections->get());
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 12) {
         PathSet res;
         for (auto & i : paths)
             if (isValidPath(i)) res.insert(i);
         return res;
     } else {
-        to << wopQueryValidPaths << paths;
-        processStderr();
-        return readStorePaths<PathSet>(from);
+        conn->to << wopQueryValidPaths << paths;
+        conn->processStderr();
+        return readStorePaths<PathSet>(conn->from);
     }
 }
 
 
 PathSet RemoteStore::queryAllValidPaths()
 {
-    openConnection();
-    to << wopQueryAllValidPaths;
-    processStderr();
-    return readStorePaths<PathSet>(from);
+    auto conn(connections->get());
+    conn->to << wopQueryAllValidPaths;
+    conn->processStderr();
+    return readStorePaths<PathSet>(conn->from);
 }
 
 
 PathSet RemoteStore::querySubstitutablePaths(const PathSet & paths)
 {
-    openConnection();
-    if (GET_PROTOCOL_MINOR(daemonVersion) < 12) {
+    auto conn(connections->get());
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 12) {
         PathSet res;
         for (auto & i : paths) {
-            to << wopHasSubstitutes << i;
-            processStderr();
-            if (readInt(from)) res.insert(i);
+            conn->to << wopHasSubstitutes << i;
+            conn->processStderr();
+            if (readInt(conn->from)) res.insert(i);
         }
         return res;
     } else {
-        to << wopQuerySubstitutablePaths << paths;
-        processStderr();
-        return readStorePaths<PathSet>(from);
+        conn->to << wopQuerySubstitutablePaths << paths;
+        conn->processStderr();
+        return readStorePaths<PathSet>(conn->from);
     }
 }
 
@@ -225,39 +212,39 @@ void RemoteStore::querySubstitutablePathInfos(const PathSet & paths,
 {
     if (paths.empty()) return;
 
-    openConnection();
+    auto conn(connections->get());
 
-    if (GET_PROTOCOL_MINOR(daemonVersion) < 3) return;
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 3) return;
 
-    if (GET_PROTOCOL_MINOR(daemonVersion) < 12) {
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) < 12) {
 
         for (auto & i : paths) {
             SubstitutablePathInfo info;
-            to << wopQuerySubstitutablePathInfo << i;
-            processStderr();
-            unsigned int reply = readInt(from);
+            conn->to << wopQuerySubstitutablePathInfo << i;
+            conn->processStderr();
+            unsigned int reply = readInt(conn->from);
             if (reply == 0) continue;
-            info.deriver = readString(from);
+            info.deriver = readString(conn->from);
             if (info.deriver != "") assertStorePath(info.deriver);
-            info.references = readStorePaths<PathSet>(from);
-            info.downloadSize = readLongLong(from);
-            info.narSize = GET_PROTOCOL_MINOR(daemonVersion) >= 7 ? readLongLong(from) : 0;
+            info.references = readStorePaths<PathSet>(conn->from);
+            info.downloadSize = readLongLong(conn->from);
+            info.narSize = GET_PROTOCOL_MINOR(conn->daemonVersion) >= 7 ? readLongLong(conn->from) : 0;
             infos[i] = info;
         }
 
     } else {
 
-        to << wopQuerySubstitutablePathInfos << paths;
-        processStderr();
-        unsigned int count = readInt(from);
+        conn->to << wopQuerySubstitutablePathInfos << paths;
+        conn->processStderr();
+        unsigned int count = readInt(conn->from);
         for (unsigned int n = 0; n < count; n++) {
-            Path path = readStorePath(from);
+            Path path = readStorePath(conn->from);
             SubstitutablePathInfo & info(infos[path]);
-            info.deriver = readString(from);
+            info.deriver = readString(conn->from);
             if (info.deriver != "") assertStorePath(info.deriver);
-            info.references = readStorePaths<PathSet>(from);
-            info.downloadSize = readLongLong(from);
-            info.narSize = readLongLong(from);
+            info.references = readStorePaths<PathSet>(conn->from);
+            info.downloadSize = readLongLong(conn->from);
+            info.narSize = readLongLong(conn->from);
         }
 
     }
@@ -266,27 +253,27 @@ void RemoteStore::querySubstitutablePathInfos(const PathSet & paths,
 
 ValidPathInfo RemoteStore::queryPathInfo(const Path & path)
 {
-    openConnection();
-    to << wopQueryPathInfo << path;
-    processStderr();
+    auto conn(connections->get());
+    conn->to << wopQueryPathInfo << path;
+    conn->processStderr();
     ValidPathInfo info;
     info.path = path;
-    info.deriver = readString(from);
+    info.deriver = readString(conn->from);
     if (info.deriver != "") assertStorePath(info.deriver);
-    info.hash = parseHash(htSHA256, readString(from));
-    info.references = readStorePaths<PathSet>(from);
-    info.registrationTime = readInt(from);
-    info.narSize = readLongLong(from);
+    info.narHash = parseHash(htSHA256, readString(conn->from));
+    info.references = readStorePaths<PathSet>(conn->from);
+    info.registrationTime = readInt(conn->from);
+    info.narSize = readLongLong(conn->from);
     return info;
 }
 
 
 Hash RemoteStore::queryPathHash(const Path & path)
 {
-    openConnection();
-    to << wopQueryPathHash << path;
-    processStderr();
-    string hash = readString(from);
+    auto conn(connections->get());
+    conn->to << wopQueryPathHash << path;
+    conn->processStderr();
+    string hash = readString(conn->from);
     return parseHash(htSHA256, hash);
 }
 
@@ -294,10 +281,10 @@ Hash RemoteStore::queryPathHash(const Path & path)
 void RemoteStore::queryReferences(const Path & path,
     PathSet & references)
 {
-    openConnection();
-    to << wopQueryReferences << path;
-    processStderr();
-    PathSet references2 = readStorePaths<PathSet>(from);
+    auto conn(connections->get());
+    conn->to << wopQueryReferences << path;
+    conn->processStderr();
+    PathSet references2 = readStorePaths<PathSet>(conn->from);
     references.insert(references2.begin(), references2.end());
 }
 
@@ -305,20 +292,20 @@ void RemoteStore::queryReferences(const Path & path,
 void RemoteStore::queryReferrers(const Path & path,
     PathSet & referrers)
 {
-    openConnection();
-    to << wopQueryReferrers << path;
-    processStderr();
-    PathSet referrers2 = readStorePaths<PathSet>(from);
+    auto conn(connections->get());
+    conn->to << wopQueryReferrers << path;
+    conn->processStderr();
+    PathSet referrers2 = readStorePaths<PathSet>(conn->from);
     referrers.insert(referrers2.begin(), referrers2.end());
 }
 
 
 Path RemoteStore::queryDeriver(const Path & path)
 {
-    openConnection();
-    to << wopQueryDeriver << path;
-    processStderr();
-    Path drvPath = readString(from);
+    auto conn(connections->get());
+    conn->to << wopQueryDeriver << path;
+    conn->processStderr();
+    Path drvPath = readString(conn->from);
     if (drvPath != "") assertStorePath(drvPath);
     return drvPath;
 }
@@ -326,37 +313,37 @@ Path RemoteStore::queryDeriver(const Path & path)
 
 PathSet RemoteStore::queryValidDerivers(const Path & path)
 {
-    openConnection();
-    to << wopQueryValidDerivers << path;
-    processStderr();
-    return readStorePaths<PathSet>(from);
+    auto conn(connections->get());
+    conn->to << wopQueryValidDerivers << path;
+    conn->processStderr();
+    return readStorePaths<PathSet>(conn->from);
 }
 
 
 PathSet RemoteStore::queryDerivationOutputs(const Path & path)
 {
-    openConnection();
-    to << wopQueryDerivationOutputs << path;
-    processStderr();
-    return readStorePaths<PathSet>(from);
+    auto conn(connections->get());
+    conn->to << wopQueryDerivationOutputs << path;
+    conn->processStderr();
+    return readStorePaths<PathSet>(conn->from);
 }
 
 
 PathSet RemoteStore::queryDerivationOutputNames(const Path & path)
 {
-    openConnection();
-    to << wopQueryDerivationOutputNames << path;
-    processStderr();
-    return readStrings<PathSet>(from);
+    auto conn(connections->get());
+    conn->to << wopQueryDerivationOutputNames << path;
+    conn->processStderr();
+    return readStrings<PathSet>(conn->from);
 }
 
 
 Path RemoteStore::queryPathFromHashPart(const string & hashPart)
 {
-    openConnection();
-    to << wopQueryPathFromHashPart << hashPart;
-    processStderr();
-    Path path = readString(from);
+    auto conn(connections->get());
+    conn->to << wopQueryPathFromHashPart << hashPart;
+    conn->processStderr();
+    Path path = readString(conn->from);
     if (!path.empty()) assertStorePath(path);
     return path;
 }
@@ -367,32 +354,32 @@ Path RemoteStore::addToStore(const string & name, const Path & _srcPath,
 {
     if (repair) throw Error("repairing is not supported when building through the Nix daemon");
 
-    openConnection();
+    auto conn(connections->get());
 
     Path srcPath(absPath(_srcPath));
 
-    to << wopAddToStore << name
+    conn->to << wopAddToStore << name
        << ((hashAlgo == htSHA256 && recursive) ? 0 : 1) /* backwards compatibility hack */
        << (recursive ? 1 : 0)
        << printHashType(hashAlgo);
 
     try {
-        to.written = 0;
-        to.warn = true;
-        dumpPath(srcPath, to, filter);
-        to.warn = false;
-        processStderr();
+        conn->to.written = 0;
+        conn->to.warn = true;
+        dumpPath(srcPath, conn->to, filter);
+        conn->to.warn = false;
+        conn->processStderr();
     } catch (SysError & e) {
         /* Daemon closed while we were sending the path. Probably OOM
            or I/O error. */
         if (e.errNo == EPIPE)
             try {
-                processStderr();
+                conn->processStderr();
             } catch (EndOfFile & e) { }
         throw;
     }
 
-    return readStorePath(from);
+    return readStorePath(conn->from);
 }
 
 
@@ -401,43 +388,43 @@ Path RemoteStore::addTextToStore(const string & name, const string & s,
 {
     if (repair) throw Error("repairing is not supported when building through the Nix daemon");
 
-    openConnection();
-    to << wopAddTextToStore << name << s << references;
+    auto conn(connections->get());
+    conn->to << wopAddTextToStore << name << s << references;
 
-    processStderr();
-    return readStorePath(from);
+    conn->processStderr();
+    return readStorePath(conn->from);
 }
 
 
 void RemoteStore::exportPath(const Path & path, bool sign,
     Sink & sink)
 {
-    openConnection();
-    to << wopExportPath << path << (sign ? 1 : 0);
-    processStderr(&sink); /* sink receives the actual data */
-    readInt(from);
+    auto conn(connections->get());
+    conn->to << wopExportPath << path << (sign ? 1 : 0);
+    conn->processStderr(&sink); /* sink receives the actual data */
+    readInt(conn->from);
 }
 
 
 Paths RemoteStore::importPaths(bool requireSignature, Source & source)
 {
-    openConnection();
-    to << wopImportPaths;
+    auto conn(connections->get());
+    conn->to << wopImportPaths;
     /* We ignore requireSignature, since the worker forces it to true
        anyway. */
-    processStderr(0, &source);
-    return readStorePaths<Paths>(from);
+    conn->processStderr(0, &source);
+    return readStorePaths<Paths>(conn->from);
 }
 
 
 void RemoteStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
 {
-    openConnection();
-    to << wopBuildPaths;
-    if (GET_PROTOCOL_MINOR(daemonVersion) >= 13) {
-        to << drvPaths;
-        if (GET_PROTOCOL_MINOR(daemonVersion) >= 15)
-            to << buildMode;
+    auto conn(connections->get());
+    conn->to << wopBuildPaths;
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 13) {
+        conn->to << drvPaths;
+        if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 15)
+            conn->to << buildMode;
         else
             /* Old daemons did not take a 'buildMode' parameter, so we
                need to validate it here on the client side.  */
@@ -449,22 +436,22 @@ void RemoteStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
         PathSet drvPaths2;
         for (auto & i : drvPaths)
             drvPaths2.insert(string(i, 0, i.find('!')));
-        to << drvPaths2;
+        conn->to << drvPaths2;
     }
-    processStderr();
-    readInt(from);
+    conn->processStderr();
+    readInt(conn->from);
 }
 
 
 BuildResult RemoteStore::buildDerivation(const Path & drvPath, const BasicDerivation & drv,
     BuildMode buildMode)
 {
-    openConnection();
-    to << wopBuildDerivation << drvPath << drv << buildMode;
-    processStderr();
+    auto conn(connections->get());
+    conn->to << wopBuildDerivation << drvPath << drv << buildMode;
+    conn->processStderr();
     BuildResult res;
     unsigned int status;
-    from >> status >> res.errorMsg;
+    conn->from >> status >> res.errorMsg;
     res.status = (BuildResult::Status) status;
     return res;
 }
@@ -472,50 +459,50 @@ BuildResult RemoteStore::buildDerivation(const Path & drvPath, const BasicDeriva
 
 void RemoteStore::ensurePath(const Path & path)
 {
-    openConnection();
-    to << wopEnsurePath << path;
-    processStderr();
-    readInt(from);
+    auto conn(connections->get());
+    conn->to << wopEnsurePath << path;
+    conn->processStderr();
+    readInt(conn->from);
 }
 
 
 void RemoteStore::addTempRoot(const Path & path)
 {
-    openConnection();
-    to << wopAddTempRoot << path;
-    processStderr();
-    readInt(from);
+    auto conn(connections->get());
+    conn->to << wopAddTempRoot << path;
+    conn->processStderr();
+    readInt(conn->from);
 }
 
 
 void RemoteStore::addIndirectRoot(const Path & path)
 {
-    openConnection();
-    to << wopAddIndirectRoot << path;
-    processStderr();
-    readInt(from);
+    auto conn(connections->get());
+    conn->to << wopAddIndirectRoot << path;
+    conn->processStderr();
+    readInt(conn->from);
 }
 
 
 void RemoteStore::syncWithGC()
 {
-    openConnection();
-    to << wopSyncWithGC;
-    processStderr();
-    readInt(from);
+    auto conn(connections->get());
+    conn->to << wopSyncWithGC;
+    conn->processStderr();
+    readInt(conn->from);
 }
 
 
 Roots RemoteStore::findRoots()
 {
-    openConnection();
-    to << wopFindRoots;
-    processStderr();
-    unsigned int count = readInt(from);
+    auto conn(connections->get());
+    conn->to << wopFindRoots;
+    conn->processStderr();
+    unsigned int count = readInt(conn->from);
     Roots result;
     while (count--) {
-        Path link = readString(from);
-        Path target = readStorePath(from);
+        Path link = readString(conn->from);
+        Path target = readStorePath(conn->from);
         result[link] = target;
     }
     return result;
@@ -524,56 +511,68 @@ Roots RemoteStore::findRoots()
 
 void RemoteStore::collectGarbage(const GCOptions & options, GCResults & results)
 {
-    openConnection(false);
+    auto conn(connections->get());
 
-    to << wopCollectGarbage << options.action << options.pathsToDelete << options.ignoreLiveness
+    conn->to << wopCollectGarbage << options.action << options.pathsToDelete << options.ignoreLiveness
        << options.maxFreed << 0;
-    if (GET_PROTOCOL_MINOR(daemonVersion) >= 5)
+    if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 5)
         /* removed options */
-        to << 0 << 0;
+        conn->to << 0 << 0;
 
-    processStderr();
+    conn->processStderr();
 
-    results.paths = readStrings<PathSet>(from);
-    results.bytesFreed = readLongLong(from);
-    readLongLong(from); // obsolete
+    results.paths = readStrings<PathSet>(conn->from);
+    results.bytesFreed = readLongLong(conn->from);
+    readLongLong(conn->from); // obsolete
 }
 
 
 PathSet RemoteStore::queryFailedPaths()
 {
-    openConnection();
-    to << wopQueryFailedPaths;
-    processStderr();
-    return readStorePaths<PathSet>(from);
+    auto conn(connections->get());
+    conn->to << wopQueryFailedPaths;
+    conn->processStderr();
+    return readStorePaths<PathSet>(conn->from);
 }
 
 
 void RemoteStore::clearFailedPaths(const PathSet & paths)
 {
-    openConnection();
-    to << wopClearFailedPaths << paths;
-    processStderr();
-    readInt(from);
+    auto conn(connections->get());
+    conn->to << wopClearFailedPaths << paths;
+    conn->processStderr();
+    readInt(conn->from);
 }
 
 void RemoteStore::optimiseStore()
 {
-    openConnection();
-    to << wopOptimiseStore;
-    processStderr();
-    readInt(from);
+    auto conn(connections->get());
+    conn->to << wopOptimiseStore;
+    conn->processStderr();
+    readInt(conn->from);
 }
 
 bool RemoteStore::verifyStore(bool checkContents, bool repair)
 {
-    openConnection();
-    to << wopVerifyStore << checkContents << repair;
-    processStderr();
-    return readInt(from) != 0;
+    auto conn(connections->get());
+    conn->to << wopVerifyStore << checkContents << repair;
+    conn->processStderr();
+    return readInt(conn->from) != 0;
+}
+
+
+RemoteStore::Connection::~Connection()
+{
+    try {
+        to.flush();
+        fd.close();
+    } catch (...) {
+        ignoreException();
+    }
 }
 
-void RemoteStore::processStderr(Sink * sink, Source * source)
+
+void RemoteStore::Connection::processStderr(Sink * sink, Source * source)
 {
     to.flush();
     unsigned int msg;
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index f15182285e8c..ddfb70a669a0 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <limits>
 #include <string>
 
 #include "store-api.hh"
@@ -12,15 +13,14 @@ class Pipe;
 class Pid;
 struct FdSink;
 struct FdSource;
+template<typename T> class Pool;
 
 
 class RemoteStore : public Store
 {
 public:
 
-    RemoteStore();
-
-    ~RemoteStore();
+    RemoteStore(size_t maxConnections = std::numeric_limits<size_t>::max());
 
     /* Implementations of abstract store API methods. */
 
@@ -91,19 +91,24 @@ public:
     bool verifyStore(bool checkContents, bool repair) override;
 
 private:
-    AutoCloseFD fdSocket;
-    FdSink to;
-    FdSource from;
-    unsigned int daemonVersion;
-    bool initialised;
 
-    void openConnection(bool reserveSpace = true);
+    struct Connection
+    {
+        AutoCloseFD fd;
+        FdSink to;
+        FdSource from;
+        unsigned int daemonVersion;
+
+        ~Connection();
+
+        void processStderr(Sink * sink = 0, Source * source = 0);
+    };
 
-    void processStderr(Sink * sink = 0, Source * source = 0);
+    ref<Pool<Connection>> connections;
 
-    void connectToDaemon();
+    ref<Connection> openConnection();
 
-    void setOptions();
+    void setOptions(ref<Connection> conn);
 };
 
 
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index e205fd0e8fe1..7058249f06da 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -61,7 +61,14 @@ Path followLinksToStorePath(const Path & path)
 string storePathToName(const Path & path)
 {
     assertStorePath(path);
-    return string(path, settings.nixStore.size() + 34);
+    return string(path, settings.nixStore.size() + storePathHashLen + 2);
+}
+
+
+string storePathToHash(const Path & path)
+{
+    assertStorePath(path);
+    return string(path, settings.nixStore.size() + 1, storePathHashLen);
 }
 
 
@@ -217,6 +224,13 @@ Path computeStorePathForText(const string & name, const string & s,
 }
 
 
+void Store::queryReferences(const Path & path, PathSet & references)
+{
+    ValidPathInfo info = queryPathInfo(path);
+    references.insert(info.references.begin(), info.references.end());
+}
+
+
 /* Return a string accepted by decodeValidPathInfo() that
    registers the specified paths as valid.  Note: it's the
    responsibility of the caller to provide a closure. */
@@ -231,7 +245,7 @@ string Store::makeValidityRegistration(const PathSet & paths,
         ValidPathInfo info = queryPathInfo(i);
 
         if (showHash) {
-            s += printHash(info.hash) + "\n";
+            s += printHash(info.narHash) + "\n";
             s += (format("%1%\n") % info.narSize).str();
         }
 
@@ -256,7 +270,7 @@ ValidPathInfo decodeValidPathInfo(std::istream & str, bool hashGiven)
     if (hashGiven) {
         string s;
         getline(str, s);
-        info.hash = parseHash(htSHA256, s);
+        info.narHash = parseHash(htSHA256, s);
         getline(str, s);
         if (!string2Int(s, info.narSize)) throw Error("number expected");
     }
@@ -299,18 +313,26 @@ void Store::exportPaths(const Paths & paths,
 
 
 #include "local-store.hh"
-#include "serialise.hh"
 #include "remote-store.hh"
+#include "local-binary-cache-store.hh"
 
 
 namespace nix {
 
 
-ref<Store> openStore(bool reserveSpace)
+ref<Store> openStoreAt(const std::string & uri)
 {
+    if (std::string(uri, 0, 7) == "file://") {
+        auto store = make_ref<LocalBinaryCacheStore>(std::shared_ptr<Store>(0),
+            "", "", // FIXME: allow the signing key to be set
+            std::string(uri, 7));
+        store->init();
+        return store;
+    }
+
     enum { mDaemon, mLocal, mAuto } mode;
 
-    mode = getEnv("NIX_REMOTE") == "daemon" ? mDaemon : mAuto;
+    mode = uri == "daemon" ? mDaemon : mAuto;
 
     if (mode == mAuto) {
         if (LocalStore::haveWriteAccess())
@@ -322,8 +344,14 @@ ref<Store> openStore(bool reserveSpace)
     }
 
     return mode == mDaemon
-        ? make_ref<Store, RemoteStore>()
-        : make_ref<Store, LocalStore>(reserveSpace);
+        ? (ref<Store>) make_ref<RemoteStore>()
+        : (ref<Store>) make_ref<LocalStore>();
+}
+
+
+ref<Store> openStore()
+{
+    return openStoreAt(getEnv("NIX_REMOTE"));
 }
 
 
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 1e43064394df..84ede157e87a 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -12,6 +12,13 @@
 namespace nix {
 
 
+/* Size of the hash part of store paths, in base-32 characters. */
+const size_t storePathHashLen = 32; // i.e. 160 bits
+
+/* Magic header of exportPath() output. */
+const uint32_t exportMagic = 0x4558494e;
+
+
 typedef std::map<Path, Path> Roots;
 
 
@@ -85,7 +92,7 @@ struct ValidPathInfo
 {
     Path path;
     Path deriver;
-    Hash hash;
+    Hash narHash;
     PathSet references;
     time_t registrationTime = 0;
     unsigned long long narSize = 0; // 0 = unknown
@@ -95,7 +102,7 @@ struct ValidPathInfo
     {
         return
             path == i.path
-            && hash == i.hash
+            && narHash == i.narHash
             && references == i.references;
     }
 };
@@ -156,10 +163,9 @@ public:
     /* Query the hash of a valid path. */
     virtual Hash queryPathHash(const Path & path) = 0;
 
-    /* Query the set of outgoing FS references for a store path.  The
+    /* Query the set of outgoing FS references for a store path. The
        result is not cleared. */
-    virtual void queryReferences(const Path & path,
-        PathSet & references) = 0;
+    virtual void queryReferences(const Path & path, PathSet & references);
 
     /* Queries the set of incoming FS references for a store path.
        The result is not cleared. */
@@ -255,6 +261,10 @@ public:
        `path' has disappeared. */
     virtual void addIndirectRoot(const Path & path) = 0;
 
+    /* Register a permanent GC root. */
+    Path addPermRoot(const Path & storePath,
+        const Path & gcRoot, bool indirect, bool allowOutsideRootsDir = false);
+
     /* Acquire the global GC lock, then immediately release it.  This
        function must be called after registering a new permanent root,
        but before exiting.  Otherwise, it is possible that a running
@@ -346,6 +356,9 @@ bool isStorePath(const Path & path);
 /* Extract the name part of the given store path. */
 string storePathToName(const Path & path);
 
+/* Extract the hash part of the given store path. */
+string storePathToHash(const Path & path);
+
 void checkStoreName(const string & name);
 
 
@@ -406,14 +419,25 @@ Path computeStorePathForText(const string & name, const string & s,
 void removeTempRoots();
 
 
-/* Register a permanent GC root. */
-Path addPermRoot(ref<Store> store, const Path & storePath,
-    const Path & gcRoot, bool indirect, bool allowOutsideRootsDir = false);
+/* Return a Store object to access the Nix store denoted by
+   ‘uri’ (slight misnomer...). Supported values are:
+
+   * ‘direct’: The Nix store in /nix/store and database in
+     /nix/var/nix/db, accessed directly.
+
+   * ‘daemon’: The Nix store accessed via a Unix domain socket
+     connection to nix-daemon.
+
+   * ‘file://<path>’: A binary cache stored in <path>.
+
+   If ‘uri’ is empty, it defaults to ‘direct’ or ‘daemon’ depending on
+   whether the user has write access to the local Nix store/database.
+   set to true *unless* you're going to collect garbage. */
+ref<Store> openStoreAt(const std::string & uri);
 
 
-/* Factory method: open the Nix database, either through the local or
-   remote implementation. */
-ref<Store> openStore(bool reserveSpace = true);
+/* Open the store indicated by the ‘NIX_REMOTE’ environment variable. */
+ref<Store> openStore();
 
 
 /* Display a set of paths in human-readable form (i.e., between quotes
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 6ee7981432b6..5363496c272e 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -29,7 +29,7 @@ bool useCaseHack =
     false;
 #endif
 
-static string archiveVersion1 = "nix-archive-1";
+const std::string narVersionMagic1 = "nix-archive-1";
 
 static string caseHackSuffix = "~nix~case~hack~";
 
@@ -113,11 +113,17 @@ static void dump(const Path & path, Sink & sink, PathFilter & filter)
 
 void dumpPath(const Path & path, Sink & sink, PathFilter & filter)
 {
-    sink << archiveVersion1;
+    sink << narVersionMagic1;
     dump(path, sink, filter);
 }
 
 
+void dumpString(const std::string & s, Sink & sink)
+{
+    sink << narVersionMagic1 << "(" << "type" << "regular" << "contents" << s << ")";
+}
+
+
 static SerialisationError badArchive(string s)
 {
     return SerialisationError("bad archive: " + s);
@@ -214,7 +220,8 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
         }
 
         else if (s == "executable" && type == tpRegular) {
-            readString(source);
+            auto s = readString(source);
+            if (s != "") throw badArchive("executable marker has non-empty value");
             sink.isExecutable();
         }
 
@@ -275,7 +282,7 @@ void parseDump(ParseSink & sink, Source & source)
         /* This generally means the integer at the start couldn't be
            decoded.  Ignore and throw the exception below. */
     }
-    if (version != archiveVersion1)
+    if (version != narVersionMagic1)
         throw badArchive("input doesn't look like a Nix archive");
     parse(sink, source, "");
 }
diff --git a/src/libutil/archive.hh b/src/libutil/archive.hh
index c216e9768fd1..90117f5ff168 100644
--- a/src/libutil/archive.hh
+++ b/src/libutil/archive.hh
@@ -55,6 +55,8 @@ extern PathFilter defaultPathFilter;
 void dumpPath(const Path & path, Sink & sink,
     PathFilter & filter = defaultPathFilter);
 
+void dumpString(const std::string & s, Sink & sink);
+
 struct ParseSink
 {
     virtual void createDirectory(const Path & path) { };
@@ -76,4 +78,7 @@ void restorePath(const Path & path, Source & source);
 extern bool useCaseHack;
 
 
+extern const std::string narVersionMagic1;
+
+
 }
diff --git a/src/libutil/compression.cc b/src/libutil/compression.cc
index fb4160669a29..a3fa0dab737b 100644
--- a/src/libutil/compression.cc
+++ b/src/libutil/compression.cc
@@ -6,34 +6,83 @@
 
 namespace nix {
 
+/* RAII wrapper around lzma_stream. */
+struct LzmaStream
+{
+    lzma_stream strm;
+    LzmaStream() : strm(LZMA_STREAM_INIT) { };
+    ~LzmaStream() { lzma_end(&strm); };
+    lzma_stream & operator()() { return strm; }
+};
+
+std::string compressXZ(const std::string & in)
+{
+    LzmaStream strm;
+
+    // FIXME: apply the x86 BCJ filter?
+
+    lzma_ret ret = lzma_easy_encoder(
+        &strm(), 6, LZMA_CHECK_CRC64);
+    if (ret != LZMA_OK)
+        throw Error("unable to initialise lzma encoder");
+
+    lzma_action action = LZMA_RUN;
+    uint8_t outbuf[BUFSIZ];
+    string res;
+    strm().next_in = (uint8_t *) in.c_str();
+    strm().avail_in = in.size();
+    strm().next_out = outbuf;
+    strm().avail_out = sizeof(outbuf);
+
+    while (true) {
+
+        if (strm().avail_in == 0)
+            action = LZMA_FINISH;
+
+        lzma_ret ret = lzma_code(&strm(), action);
+
+        if (strm().avail_out == 0 || ret == LZMA_STREAM_END) {
+            res.append((char *) outbuf, sizeof(outbuf) - strm().avail_out);
+            strm().next_out = outbuf;
+            strm().avail_out = sizeof(outbuf);
+        }
+
+        if (ret == LZMA_STREAM_END)
+            return res;
+
+        if (ret != LZMA_OK)
+            throw Error("error while decompressing xz file");
+    }
+}
+
 std::string decompressXZ(const std::string & in)
 {
-    lzma_stream strm = LZMA_STREAM_INIT;
+    LzmaStream strm;
 
     lzma_ret ret = lzma_stream_decoder(
-        &strm, UINT64_MAX, LZMA_CONCATENATED);
+        &strm(), UINT64_MAX, LZMA_CONCATENATED);
     if (ret != LZMA_OK)
         throw Error("unable to initialise lzma decoder");
 
     lzma_action action = LZMA_RUN;
     uint8_t outbuf[BUFSIZ];
     string res;
-    strm.next_in = (uint8_t *) in.c_str();
-    strm.avail_in = in.size();
-    strm.next_out = outbuf;
-    strm.avail_out = sizeof(outbuf);
+    strm().next_in = (uint8_t *) in.c_str();
+    strm().avail_in = in.size();
+    strm().next_out = outbuf;
+    strm().avail_out = sizeof(outbuf);
 
     while (true) {
 
-        if (strm.avail_in == 0)
+        if (strm().avail_in == 0)
             action = LZMA_FINISH;
 
-        lzma_ret ret = lzma_code(&strm, action);
+        lzma_ret ret = lzma_code(&strm(), action);
 
-        if (strm.avail_out == 0 || ret == LZMA_STREAM_END) {
-            res.append((char *) outbuf, sizeof(outbuf) - strm.avail_out);
-            strm.next_out = outbuf;
-            strm.avail_out = sizeof(outbuf);
+        if (strm().avail_out == 0 || ret == LZMA_STREAM_END) {
+            res.append((char *) outbuf, sizeof(outbuf) - strm().avail_out);
+            strm().next_out = outbuf;
+            strm().avail_out = sizeof(outbuf);
         }
 
         if (ret == LZMA_STREAM_END)
diff --git a/src/libutil/compression.hh b/src/libutil/compression.hh
index 962ce5ac7767..eb1697fc4aa4 100644
--- a/src/libutil/compression.hh
+++ b/src/libutil/compression.hh
@@ -4,6 +4,8 @@
 
 namespace nix {
 
+std::string compressXZ(const std::string & in);
+
 std::string decompressXZ(const std::string & in);
 
 }
diff --git a/src/libutil/lru-cache.hh b/src/libutil/lru-cache.hh
new file mode 100644
index 000000000000..4344d6601bc8
--- /dev/null
+++ b/src/libutil/lru-cache.hh
@@ -0,0 +1,84 @@
+#pragma once
+
+#include <map>
+#include <list>
+
+namespace nix {
+
+/* A simple least-recently used cache. Not thread-safe. */
+template<typename Key, typename Value>
+class LRUCache
+{
+private:
+
+    size_t maxSize;
+
+    // Stupid wrapper to get around circular dependency between Data
+    // and LRU.
+    struct LRUIterator;
+
+    using Data = std::map<Key, std::pair<LRUIterator, Value>>;
+    using LRU = std::list<typename Data::iterator>;
+
+    struct LRUIterator { typename LRU::iterator it; };
+
+    Data data;
+    LRU lru;
+
+public:
+
+    LRUCache(size_t maxSize) : maxSize(maxSize) { }
+
+    /* Insert or upsert an item in the cache. */
+    void upsert(const Key & key, const Value & value)
+    {
+        erase(key);
+
+        if (data.size() >= maxSize) {
+            /* Retire the oldest item. */
+            auto oldest = lru.begin();
+            data.erase(*oldest);
+            lru.erase(oldest);
+        }
+
+        auto res = data.emplace(key, std::make_pair(LRUIterator(), value));
+        assert(res.second);
+        auto & i(res.first);
+
+        auto j = lru.insert(lru.end(), i);
+
+        i->second.first.it = j;
+    }
+
+    bool erase(const Key & key)
+    {
+        auto i = data.find(key);
+        if (i == data.end()) return false;
+        lru.erase(i->second.first.it);
+        data.erase(i);
+        return true;
+    }
+
+    /* Look up an item in the cache. If it exists, it becomes the most
+       recently used item. */
+    // FIXME: use boost::optional?
+    Value * get(const Key & key)
+    {
+        auto i = data.find(key);
+        if (i == data.end()) return 0;
+
+        /* Move this item to the back of the LRU list. */
+        lru.erase(i->second.first.it);
+        auto j = lru.insert(lru.end(), i);
+        i->second.first.it = j;
+
+        return &i->second.second;
+    }
+
+    size_t size()
+    {
+        return data.size();
+    }
+};
+
+}
diff --git a/src/libutil/pool.hh b/src/libutil/pool.hh
new file mode 100644
index 000000000000..f291cd578388
--- /dev/null
+++ b/src/libutil/pool.hh
@@ -0,0 +1,151 @@
+#pragma once
+
+#include <functional>
+#include <limits>
+#include <list>
+#include <memory>
+#include <cassert>
+
+#include "sync.hh"
+#include "ref.hh"
+
+namespace nix {
+
+/* This template class implements a simple pool manager of resources
+   of some type R, such as database connections. It is used as
+   follows:
+
+     class Connection { ... };
+
+     Pool<Connection> pool;
+
+     {
+       auto conn(pool.get());
+       conn->exec("select ...");
+     }
+
+   Here, the Connection object referenced by ‘conn’ is automatically
+   returned to the pool when ‘conn’ goes out of scope.
+*/
+
+template <class R>
+class Pool
+{
+public:
+
+    /* A function that produces new instances of R on demand. */
+    typedef std::function<ref<R>()> Factory;
+
+    /* A function that checks whether an instance of R is still
+       usable. Unusable instances are removed from the pool. */
+    typedef std::function<bool(const ref<R> &)> Validator;
+
+private:
+
+    Factory factory;
+    Validator validator;
+
+    struct State
+    {
+        size_t inUse = 0;
+        size_t max;
+        std::vector<ref<R>> idle;
+    };
+
+    Sync<State> state;
+
+    std::condition_variable wakeup;
+
+public:
+
+    Pool(size_t max = std::numeric_limits<size_t>::max(),
+        const Factory & factory = []() { return make_ref<R>(); },
+        const Validator & validator = [](ref<R> r) { return true; })
+        : factory(factory)
+        , validator(validator)
+    {
+        auto state_(state.lock());
+        state_->max = max;
+    }
+
+    ~Pool()
+    {
+        auto state_(state.lock());
+        assert(!state_->inUse);
+        state_->max = 0;
+        state_->idle.clear();
+    }
+
+    class Handle
+    {
+    private:
+        Pool & pool;
+        std::shared_ptr<R> r;
+
+        friend Pool;
+
+        Handle(Pool & pool, std::shared_ptr<R> r) : pool(pool), r(r) { }
+
+    public:
+        Handle(Handle && h) : pool(h.pool), r(h.r) { h.r.reset(); }
+
+        Handle(const Handle & l) = delete;
+
+        ~Handle()
+        {
+            if (!r) return;
+            {
+                auto state_(pool.state.lock());
+                state_->idle.push_back(ref<R>(r));
+                assert(state_->inUse);
+                state_->inUse--;
+            }
+            pool.wakeup.notify_one();
+        }
+
+        R * operator -> () { return &*r; }
+        R & operator * () { return *r; }
+    };
+
+    Handle get()
+    {
+        {
+            auto state_(state.lock());
+
+            /* If we're over the maximum number of instance, we need
+               to wait until a slot becomes available. */
+            while (state_->idle.empty() && state_->inUse >= state_->max)
+                state_.wait(wakeup);
+
+            while (!state_->idle.empty()) {
+                auto p = state_->idle.back();
+                state_->idle.pop_back();
+                if (validator(p)) {
+                    state_->inUse++;
+                    return Handle(*this, p);
+                }
+            }
+
+            state_->inUse++;
+        }
+
+        /* We need to create a new instance. Because that might take a
+           while, we don't hold the lock in the meantime. */
+        try {
+            Handle h(*this, factory());
+            return h;
+        } catch (...) {
+            auto state_(state.lock());
+            state_->inUse--;
+            throw;
+        }
+    }
+
+    unsigned int count()
+    {
+        auto state_(state.lock());
+        return state_->idle.size() + state_->inUse;
+    }
+};
+
+}
diff --git a/src/libutil/ref.hh b/src/libutil/ref.hh
new file mode 100644
index 000000000000..a6d338d79622
--- /dev/null
+++ b/src/libutil/ref.hh
@@ -0,0 +1,67 @@
+#pragma once
+
+#include <memory>
+#include <exception>
+
+namespace nix {
+
+/* A simple non-nullable reference-counted pointer. Actually a wrapper
+   around std::shared_ptr that prevents non-null constructions. */
+template<typename T>
+class ref
+{
+private:
+
+    std::shared_ptr<T> p;
+
+public:
+
+    ref<T>(const ref<T> & r)
+        : p(r.p)
+    { }
+
+    explicit ref<T>(const std::shared_ptr<T> & p)
+        : p(p)
+    {
+        if (!p)
+            throw std::invalid_argument("null pointer cast to ref");
+    }
+
+    T* operator ->() const
+    {
+        return &*p;
+    }
+
+    T& operator *() const
+    {
+        return *p;
+    }
+
+    operator std::shared_ptr<T> ()
+    {
+        return p;
+    }
+
+    template<typename T2>
+    operator ref<T2> ()
+    {
+        return ref<T2>((std::shared_ptr<T2>) p);
+    }
+
+private:
+
+    template<typename T2, typename... Args>
+    friend ref<T2>
+    make_ref(Args&&... args);
+
+};
+
+template<typename T, typename... Args>
+inline ref<T>
+make_ref(Args&&... args)
+{
+    auto p = std::make_shared<T>(std::forward<Args>(args)...);
+    return ref<T>(p);
+}
+
+}
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index f136a13248ba..c9620e2bf32a 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -72,7 +72,17 @@ void FdSink::write(const unsigned char * data, size_t len)
             warned = true;
         }
     }
-    writeFull(fd, data, len);
+    try {
+        writeFull(fd, data, len);
+    } catch (SysError & e) {
+        _good = true;
+    }
+}
+
+
+bool FdSink::good()
+{
+    return _good;
 }
 
 
@@ -119,12 +129,18 @@ size_t FdSource::readUnbuffered(unsigned char * data, size_t len)
         checkInterrupt();
         n = ::read(fd, (char *) data, bufSize);
     } while (n == -1 && errno == EINTR);
-    if (n == -1) throw SysError("reading from file");
-    if (n == 0) throw EndOfFile("unexpected end-of-file");
+    if (n == -1) { _good = false; throw SysError("reading from file"); }
+    if (n == 0) { _good = false; throw EndOfFile("unexpected end-of-file"); }
     return n;
 }
 
 
+bool FdSource::good()
+{
+    return _good;
+}
+
+
 size_t StringSource::read(unsigned char * data, size_t len)
 {
     if (pos == s.size()) throw EndOfFile("end of string reached");
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 979ff849fcaf..9e269f3923ea 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -12,6 +12,7 @@ struct Sink
 {
     virtual ~Sink() { }
     virtual void operator () (const unsigned char * data, size_t len) = 0;
+    virtual bool good() { return true; }
 };
 
 
@@ -25,7 +26,7 @@ struct BufferedSink : Sink
         : bufSize(bufSize), bufPos(0), buffer(0) { }
     ~BufferedSink();
 
-    void operator () (const unsigned char * data, size_t len);
+    void operator () (const unsigned char * data, size_t len) override;
 
     void flush();
 
@@ -47,6 +48,8 @@ struct Source
        return the number of bytes stored.  If blocks until at least
        one byte is available. */
     virtual size_t read(unsigned char * data, size_t len) = 0;
+
+    virtual bool good() { return true; }
 };
 
 
@@ -60,7 +63,7 @@ struct BufferedSource : Source
         : bufSize(bufSize), bufPosIn(0), bufPosOut(0), buffer(0) { }
     ~BufferedSource();
 
-    size_t read(unsigned char * data, size_t len);
+    size_t read(unsigned char * data, size_t len) override;
 
     /* Underlying read call, to be overridden. */
     virtual size_t readUnbuffered(unsigned char * data, size_t len) = 0;
@@ -80,7 +83,12 @@ struct FdSink : BufferedSink
     FdSink(int fd) : fd(fd), warn(false), written(0) { }
     ~FdSink();
 
-    void write(const unsigned char * data, size_t len);
+    void write(const unsigned char * data, size_t len) override;
+
+    bool good() override;
+
+private:
+    bool _good = true;
 };
 
 
@@ -90,7 +98,10 @@ struct FdSource : BufferedSource
     int fd;
     FdSource() : fd(-1) { }
     FdSource(int fd) : fd(fd) { }
-    size_t readUnbuffered(unsigned char * data, size_t len);
+    size_t readUnbuffered(unsigned char * data, size_t len) override;
+    bool good() override;
+private:
+    bool _good = true;
 };
 
 
@@ -98,7 +109,7 @@ struct FdSource : BufferedSource
 struct StringSink : Sink
 {
     string s;
-    void operator () (const unsigned char * data, size_t len);
+    void operator () (const unsigned char * data, size_t len) override;
 };
 
 
@@ -108,7 +119,7 @@ struct StringSource : Source
     const string & s;
     size_t pos;
     StringSource(const string & _s) : s(_s), pos(0) { }
-    size_t read(unsigned char * data, size_t len);
+    size_t read(unsigned char * data, size_t len) override;
 };
 
 
diff --git a/src/libutil/sync.hh b/src/libutil/sync.hh
new file mode 100644
index 000000000000..c99c098ac9c6
--- /dev/null
+++ b/src/libutil/sync.hh
@@ -0,0 +1,78 @@
+#pragma once
+
+#include <mutex>
+#include <condition_variable>
+#include <cassert>
+
+namespace nix {
+
+/* This template class ensures synchronized access to a value of type
+   T. It is used as follows:
+
+     struct Data { int x; ... };
+
+     Sync<Data> data;
+
+     {
+       auto data_(data.lock());
+       data_->x = 123;
+     }
+
+   Here, "data" is automatically unlocked when "data_" goes out of
+   scope.
+*/
+
+template<class T>
+class Sync
+{
+private:
+    std::mutex mutex;
+    T data;
+
+public:
+
+    Sync() { }
+    Sync(const T & data) : data(data) { }
+
+    class Lock
+    {
+    private:
+        Sync * s;
+        std::unique_lock<std::mutex> lk;
+        friend Sync;
+        Lock(Sync * s) : s(s), lk(s->mutex) { }
+    public:
+        Lock(Lock && l) : s(l.s) { abort(); }
+        Lock(const Lock & l) = delete;
+        ~Lock() { }
+        T * operator -> () { return &s->data; }
+        T & operator * () { return s->data; }
+
+        void wait(std::condition_variable & cv)
+        {
+            assert(s);
+            cv.wait(lk);
+        }
+
+        template<class Rep, class Period, class Predicate>
+        bool wait_for(std::condition_variable & cv,
+            const std::chrono::duration<Rep, Period> & duration,
+            Predicate pred)
+        {
+            assert(s);
+            return cv.wait_for(lk, duration, pred);
+        }
+
+        template<class Clock, class Duration>
+        std::cv_status wait_until(std::condition_variable & cv,
+            const std::chrono::time_point<Clock, Duration> & duration)
+        {
+            assert(s);
+            return cv.wait_until(lk, duration);
+        }
+    };
+
+    Lock lock() { return Lock(this); }
+};
+
+}
diff --git a/src/libutil/types.hh b/src/libutil/types.hh
index 23eb5251209e..33aaf5fc9c4d 100644
--- a/src/libutil/types.hh
+++ b/src/libutil/types.hh
@@ -2,6 +2,8 @@
 
 #include "config.h"
 
+#include "ref.hh"
+
 #include <string>
 #include <list>
 #include <set>
@@ -97,70 +99,4 @@ typedef enum {
 } Verbosity;
 
 
-/* A simple non-nullable reference-counted pointer. Actually a wrapper
-   around std::shared_ptr that prevents non-null constructions. */
-template<typename T>
-class ref
-{
-private:
-
-    std::shared_ptr<T> p;
-
-public:
-
-    ref<T>(const ref<T> & r)
-        : p(r.p)
-    { }
-
-    explicit ref<T>(const std::shared_ptr<T> & p)
-        : p(p)
-    {
-        if (!p)
-            throw std::invalid_argument("null pointer cast to ref");
-    }
-
-    T* operator ->() const
-    {
-        return &*p;
-    }
-
-    T& operator *() const
-    {
-        return *p;
-    }
-
-    operator std::shared_ptr<T> ()
-    {
-        return p;
-    }
-
-private:
-
-    template<typename T2, typename... Args>
-    friend ref<T2>
-    make_ref(Args&&... args);
-
-    template<typename T2, typename T3, typename... Args>
-    friend ref<T2>
-    make_ref(Args&&... args);
-
-};
-
-template<typename T, typename... Args>
-inline ref<T>
-make_ref(Args&&... args)
-{
-    auto p = std::make_shared<T>(std::forward<Args>(args)...);
-    return ref<T>(p);
-}
-
-template<typename T, typename T2, typename... Args>
-inline ref<T>
-make_ref(Args&&... args)
-{
-    auto p = std::make_shared<T2>(std::forward<Args>(args)...);
-    return ref<T>(p);
-}
-
-
 }
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index def0525abc18..3becbbabc2bf 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -320,9 +320,11 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed)
 {
     checkInterrupt();
 
-    printMsg(lvlVomit, format("%1%") % path);
-
-    struct stat st = lstat(path);
+    struct stat st;
+    if (lstat(path.c_str(), &st) == -1) {
+        if (errno == ENOENT) return;
+        throw SysError(format("getting status of ‘%1%’") % path);
+    }
 
     if (!S_ISDIR(st.st_mode) && st.st_nlink == 1)
         bytesFreed += st.st_blocks * 512;
@@ -338,8 +340,10 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed)
             _deletePath(path + "/" + i.name, bytesFreed);
     }
 
-    if (remove(path.c_str()) == -1)
+    if (remove(path.c_str()) == -1) {
+        if (errno == ENOENT) return;
         throw SysError(format("cannot unlink ‘%1%’") % path);
+    }
 }
 
 
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index b714cdc64a5a..3606f6ec9eb2 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -92,8 +92,8 @@ string readLine(int fd);
 void writeLine(int fd, string s);
 
 /* Delete a path; i.e., in the case of a directory, it is deleted
-   recursively.  Don't use this at home, kids.  The second variant
-   returns the number of bytes and blocks freed. */
+   recursively. It's not an error if the path does not exist. The
+   second variant returns the number of bytes and blocks freed. */
 void deletePath(const Path & path);
 
 void deletePath(const Path & path, unsigned long long & bytesFreed);
@@ -366,6 +366,14 @@ template<class N> bool string2Int(const string & s, N & n)
     return str && str.get() == EOF;
 }
 
+/* Parse a string into a float. */
+template<class N> bool string2Float(const string & s, N & n)
+{
+    std::istringstream str(s);
+    str >> n;
+    return str && str.get() == EOF;
+}
+
 
 /* Return true iff `s' ends in `suffix'. */
 bool hasSuffix(const string & s, const string & suffix);
diff --git a/src/nix-collect-garbage/nix-collect-garbage.cc b/src/nix-collect-garbage/nix-collect-garbage.cc
index b9ccafb751c1..3aa348581b19 100644
--- a/src/nix-collect-garbage/nix-collect-garbage.cc
+++ b/src/nix-collect-garbage/nix-collect-garbage.cc
@@ -82,7 +82,7 @@ int main(int argc, char * * argv)
 
         // Run the actual garbage collector.
         if (!dryRun) {
-            auto store = openStore(false);
+            auto store = openStore();
             options.action = GCOptions::gcDeleteDead;
             GCResults results;
             PrintFreed freed(true, results);
diff --git a/src/nix-daemon/nix-daemon.cc b/src/nix-daemon/nix-daemon.cc
index aa20aa67daa8..e3c12ea0e0ea 100644
--- a/src/nix-daemon/nix-daemon.cc
+++ b/src/nix-daemon/nix-daemon.cc
@@ -515,7 +515,7 @@ static void performOp(ref<LocalStore> store, bool trusted, unsigned int clientVe
         startWork();
         ValidPathInfo info = store->queryPathInfo(path);
         stopWork();
-        to << info.deriver << printHash(info.hash) << info.references
+        to << info.deriver << printHash(info.narHash) << info.references
            << info.registrationTime << info.narSize;
         break;
     }
@@ -562,9 +562,8 @@ static void processConnection(bool trusted)
     if (GET_PROTOCOL_MINOR(clientVersion) >= 14 && readInt(from))
         setAffinityTo(readInt(from));
 
-    bool reserveSpace = true;
     if (GET_PROTOCOL_MINOR(clientVersion) >= 11)
-        reserveSpace = readInt(from) != 0;
+        readInt(from); // obsolete reserveSpace
 
     /* Send startup error messages to the client. */
     startWork();
@@ -582,7 +581,7 @@ static void processConnection(bool trusted)
 #endif
 
         /* Open the store. */
-        auto store = make_ref<LocalStore>(reserveSpace);
+        auto store = make_ref<LocalStore>();
 
         stopWork();
         to.flush();
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 12e19d4af797..a9d1ed024dd3 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -1127,6 +1127,10 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
                                     attrs2["type"] = "int";
                                     attrs2["value"] = (format("%1%") % v->integer).str();
                                     xml.writeEmptyElement("meta", attrs2);
+                                } else if (v->type == tFloat) {
+                                    attrs2["type"] = "float";
+                                    attrs2["value"] = (format("%1%") % v->fpoint).str();
+                                    xml.writeEmptyElement("meta", attrs2);
                                 } else if (v->type == tBool) {
                                     attrs2["type"] = "bool";
                                     attrs2["value"] = v->boolean ? "true" : "false";
diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 1604c0817587..81c1c8d5637c 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -32,7 +32,7 @@ static bool indirectRoot = false;
 enum OutputKind { okPlain, okXML, okJSON };
 
 
-void processExpr(ref<Store> store, EvalState & state, const Strings & attrPaths,
+void processExpr(EvalState & state, const Strings & attrPaths,
     bool parseOnly, bool strict, Bindings & autoArgs,
     bool evalOnly, OutputKind output, bool location, Expr * e)
 {
@@ -79,7 +79,7 @@ void processExpr(ref<Store> store, EvalState & state, const Strings & attrPaths,
                 else {
                     Path rootName = gcRoot;
                     if (++rootNr > 1) rootName += "-" + std::to_string(rootNr);
-                    drvPath = addPermRoot(store, drvPath, rootName, indirectRoot);
+                    drvPath = state.store->addPermRoot(drvPath, rootName, indirectRoot);
                 }
                 std::cout << format("%1%%2%\n") % drvPath % (outputName != "out" ? "!" + outputName : "");
             }
@@ -177,7 +177,7 @@ int main(int argc, char * * argv)
 
         if (readStdin) {
             Expr * e = parseStdin(state);
-            processExpr(store, state, attrPaths, parseOnly, strict, autoArgs,
+            processExpr(state, attrPaths, parseOnly, strict, autoArgs,
                 evalOnly, outputKind, xmlOutputSourceLocation, e);
         } else if (files.empty() && !fromArgs)
             files.push_back("./default.nix");
@@ -186,7 +186,7 @@ int main(int argc, char * * argv)
             Expr * e = fromArgs
                 ? state.parseExprFromString(i, absPath("."))
                 : state.parseExprFromFile(resolveExprPath(lookupFileArg(state, i)));
-            processExpr(store, state, attrPaths, parseOnly, strict, autoArgs,
+            processExpr(state, attrPaths, parseOnly, strict, autoArgs,
                 evalOnly, outputKind, xmlOutputSourceLocation, e);
         }
 
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 4e706f93c12f..48c2865e5ecc 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -84,7 +84,7 @@ static PathSet realisePath(Path path, bool build = true)
                 Path rootName = gcRoot;
                 if (rootNr > 1) rootName += "-" + std::to_string(rootNr);
                 if (i->first != "out") rootName += "-" + i->first;
-                outPath = addPermRoot(ref<Store>(store), outPath, rootName, indirectRoot);
+                outPath = store->addPermRoot(outPath, rootName, indirectRoot);
             }
             outputs.insert(outPath);
         }
@@ -100,7 +100,7 @@ static PathSet realisePath(Path path, bool build = true)
             Path rootName = gcRoot;
             rootNr++;
             if (rootNr > 1) rootName += "-" + std::to_string(rootNr);
-            path = addPermRoot(ref<Store>(store), path, rootName, indirectRoot);
+            path = store->addPermRoot(path, rootName, indirectRoot);
         }
         return singleton<PathSet>(path);
     }
@@ -374,8 +374,8 @@ static void opQuery(Strings opFlags, Strings opArgs)
                 for (auto & j : paths) {
                     ValidPathInfo info = store->queryPathInfo(j);
                     if (query == qHash) {
-                        assert(info.hash.type == htSHA256);
-                        cout << format("sha256:%1%\n") % printHash32(info.hash);
+                        assert(info.narHash.type == htSHA256);
+                        cout << format("sha256:%1%\n") % printHash32(info.narHash);
                     } else if (query == qSize)
                         cout << format("%1%\n") % info.narSize;
                 }
@@ -567,7 +567,7 @@ static void registerValidity(bool reregister, bool hashGiven, bool canonicalise)
                 canonicalisePathMetaData(info.path, -1);
             if (!hashGiven) {
                 HashResult hash = hashPath(htSHA256, info.path);
-                info.hash = hash.first;
+                info.narHash = hash.first;
                 info.narSize = hash.second;
             }
             infos.push_back(info);
@@ -783,11 +783,11 @@ static void opVerifyPath(Strings opFlags, Strings opArgs)
         Path path = followLinksToStorePath(i);
         printMsg(lvlTalkative, format("checking path ‘%1%’...") % path);
         ValidPathInfo info = store->queryPathInfo(path);
-        HashResult current = hashPath(info.hash.type, path);
-        if (current.first != info.hash) {
+        HashResult current = hashPath(info.narHash.type, path);
+        if (current.first != info.narHash) {
             printMsg(lvlError,
                 format("path ‘%1%’ was modified! expected hash ‘%2%’, got ‘%3%’")
-                % path % printHash(info.hash) % printHash(current.first));
+                % path % printHash(info.narHash) % printHash(current.first));
             status = 1;
         }
     }
@@ -1131,7 +1131,7 @@ int main(int argc, char * * argv)
         if (!op) throw UsageError("no operation specified");
 
         if (op != opDump && op != opRestore) /* !!! hack */
-            store = openStore(op != opGC);
+            store = openStore();
 
         op(opFlags, opArgs);
     });