3 files changed, 51 insertions, 6 deletions
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 5a570cefb2fa..615cc8138433 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -59,6 +59,8 @@ void EvalState::realiseContext(const PathSet & context)
             drvs.insert(decoded.first + "!" + decoded.second);
     }
     if (!drvs.empty()) {
+        if (!settings.enableImportFromDerivation)
+            throw EvalError(format("attempted to realize ‘%1%’ during evaluation but 'allow-import-from-derivation' is false") % *(drvs.begin()));
         /* For performance, prefetch all substitute info. */
         PathSet willBuild, willSubstitute, unknown;
         unsigned long long downloadSize, narSize;
@@ -176,6 +178,45 @@ static void prim_importNative(EvalState & state, const Pos & pos, Value * * args
 }
 
 
+/* Execute a program and parse its output */
+static void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceList(*args[0], pos);
+    auto elems = args[0]->listElems();
+    auto count = args[0]->listSize();
+    if (count == 0) {
+        throw EvalError(format("at least one argument to 'exec' required, at %1%") % pos);
+    }
+    PathSet context;
+    auto program = state.coerceToString(pos, *elems[0], context, false, false);
+    Strings commandArgs;
+    for (unsigned int i = 1; i < args[0]->listSize(); ++i) {
+        commandArgs.emplace_back(state.coerceToString(pos, *elems[i], context, false, false));
+    }
+    try {
+        state.realiseContext(context);
+    } catch (InvalidPathError & e) {
+        throw EvalError(format("cannot execute ‘%1%’, since path ‘%2%’ is not valid, at %3%")
+            % program % e.path % pos);
+    }
+
+    auto output = runProgram(program, true, commandArgs);
+    Expr * parsed;
+    try {
+        parsed = state.parseExprFromString(output, pos.file);
+    } catch (Error & e) {
+        e.addPrefix(format("While parsing the output from ‘%1%’, at %2%\n") % program % pos);
+        throw;
+    }
+    try {
+        state.eval(parsed, v);
+    } catch (Error & e) {
+        e.addPrefix(format("While evaluating the output from ‘%1%’, at %2%\n") % program % pos);
+        throw;
+    }
+}
+
+
 /* Return a string representing the type of the expression. */
 static void prim_typeOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
@@ -1901,8 +1942,10 @@ void EvalState::createBaseEnv()
     mkApp(v, *baseEnv.values[baseEnvDispl - 1], *v2);
     forceValue(v);
     addConstant("import", v);
-    if (settings.enableImportNative)
+    if (settings.enableNativeCode) {
         addPrimOp("__importNative", 2, prim_importNative);
+        addPrimOp("__exec", 1, prim_exec);
+    }
     addPrimOp("__typeOf", 1, prim_typeOf);
     addPrimOp("isNull", 1, prim_isNull);
     addPrimOp("__isFunction", 1, prim_isFunction);
diff --git a/src/libexpr/primops/fetchgit.cc b/src/libexpr/primops/fetchgit.cc
index bd440c8c62ad..09e2c077baba 100644
--- a/src/libexpr/primops/fetchgit.cc
+++ b/src/libexpr/primops/fetchgit.cc
@@ -58,12 +58,14 @@ static void prim_fetchgit(EvalState & state, const Pos & pos, Value * * args, Va
 
         for (auto & attr : *args[0]->attrs) {
             string name(attr.name);
-            if (name == "url")
-                url = state.forceStringNoCtx(*attr.value, *attr.pos);
-            else if (name == "rev")
+            if (name == "url") {
+                PathSet context;
+                url = state.coerceToString(*attr.pos, *attr.value, context, false, false);
+                if (hasPrefix(url, "/")) url = "file://" + url;
+            } else if (name == "rev")
                 rev = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
-                throw EvalError(format("unsupported argument ‘%1%’ to ‘fetchgit’, at %3%") % attr.name % attr.pos);
+                throw EvalError("unsupported argument ‘%s’ to ‘fetchgit’, at %s", attr.name, *attr.pos);
         }
 
         if (url.empty())
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 81f918d48de7..802e8ed2ee75 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -256,7 +256,7 @@ size_t valueSize(Value & v);
 
 #if HAVE_BOEHMGC
 typedef std::vector<Value *, gc_allocator<Value *> > ValueVector;
-typedef std::map<Symbol, Value *, std::less<Symbol>, gc_allocator<Value *> > ValueMap;
+typedef std::map<Symbol, Value *, std::less<Symbol>, gc_allocator<std::pair<const Symbol, Value *> > > ValueMap;
 #else
 typedef std::vector<Value *> ValueVector;
 typedef std::map<Symbol, Value *> ValueMap;