* The policy-free derivate sharing now *almost* works. :-) For any

  hash for which no local expansion is available, Nix can execute a
  `substitute' which should produce a path with such a hash.

  This is policy-free since Nix does not in any way specify how the
  substitute should work, i.e., it's an arbitrary (unnormalised)
  fstate expression.  For example, `nix-pull' registers substitutes
  that fetch Nix archives from the network (through `wget') and unpack
  them, but any other method is possible as well.  This is an
  improvement over the old Nix sharing scheme, which had a policy
  (fetching through `wget') built in.

  The sharing scheme doesn't work completely yet because successors
  from fstate rewriting have to be registered on the receiving side.
  Probably the whole successor stuff can be folded up into the
  substitute mechanism; this would be a nice simplification.

5 files changed, 73 insertions, 9 deletions

diff --git a/src/fstate.cc b/src/fstate.cc
index fdd43d1b1384..97532c162c1f 100644
--- a/src/fstate.cc
+++ b/src/fstate.cc
@@ -147,6 +147,12 @@ Hash hashTerm(ATerm t)
 }
 
 
+FState hash2fstate(Hash hash)
+{
+    return ATmake("Include(<str>)", ((string) hash).c_str());
+}
+
+
 ATerm termFromHash(const Hash & hash, string * p)
 {
     string path = expandHash(hash);
diff --git a/src/fstate.hh b/src/fstate.hh
index 159c7ba46338..8a873a5acd22 100644
--- a/src/fstate.hh
+++ b/src/fstate.hh
@@ -85,6 +85,8 @@ Error badTerm(const format & f, ATerm t);
 /* Hash an aterm. */
 Hash hashTerm(ATerm t);
 
+FState hash2fstate(Hash hash);
+
 /* Read an aterm from disk, given its hash. */
 ATerm termFromHash(const Hash & hash, string * p = 0);
 
diff --git a/src/nix.cc b/src/nix.cc
index 4721563fdff5..53057328dd57 100644
--- a/src/nix.cc
+++ b/src/nix.cc
@@ -26,6 +26,8 @@ static ArgType argType = atpUnknown;
      --add / -A: copy a path to the Nix store
      --query / -q: query information
 
+     --substitute: register a substitute expression
+
      --dump: dump a path as a Nix archive
      --restore: restore a path from a Nix archive
 
@@ -87,12 +89,6 @@ static Hash argToHash(const string & arg)
 }
 
 
-static FState hash2fstate(Hash hash)
-{
-    return ATmake("Include(<str>)", ((string) hash).c_str());
-}
-
-
 /* Realise (or install) paths from the given Nix fstate
    expressions. */
 static void opInstall(Strings opFlags, Strings opArgs)
@@ -187,6 +183,21 @@ static void opQuery(Strings opFlags, Strings opArgs)
 }
 
 
+static void opSubstitute(Strings opFlags, Strings opArgs)
+{
+    if (!opFlags.empty()) throw UsageError("unknown flag");
+    if (opArgs.size() % 2) throw UsageError("expecting even number of arguments");
+    
+    for (Strings::iterator i = opArgs.begin();
+         i != opArgs.end(); )
+    {
+        Hash srcHash = parseHash(*i++);
+        Hash subHash = parseHash(*i++);
+        registerSubstitute(srcHash, subHash);
+    }
+}
+
+
 /* A sink that writes dump output to stdout. */
 struct StdoutSink : DumpSink
 {
@@ -277,6 +288,8 @@ void run(Strings args)
             op = opAdd;
         else if (arg == "--query" || arg == "-q")
             op = opQuery;
+        else if (arg == "--substitute")
+            op = opSubstitute;
         else if (arg == "--dump")
             op = opDump;
         else if (arg == "--restore")
diff --git a/src/store.cc b/src/store.cc
index 5a3a4e0678fe..435ac5cc69ce 100644
--- a/src/store.cc
+++ b/src/store.cc
@@ -7,6 +7,7 @@
 #include "globals.hh"
 #include "db.hh"
 #include "archive.hh"
+#include "fstate.hh"
 
 
 struct CopySink : DumpSink
@@ -83,6 +84,20 @@ void copyPath(string src, string dst)
 }
 
 
+void registerSubstitute(const Hash & srcHash, const Hash & subHash)
+{
+    Strings subs;
+    queryListDB(nixDB, dbSubstitutes, srcHash, subs); /* non-existence = ok */
+
+    for (Strings::iterator it = subs.begin(); it != subs.end(); it++)
+        if (parseHash(*it) == subHash) return;
+    
+    subs.push_back(subHash);
+    
+    setListDB(nixDB, dbSubstitutes, srcHash, subs);
+}
+
+
 Hash registerPath(const string & _path, Hash hash)
 {
     string path(canonPath(_path));
@@ -139,8 +154,7 @@ string expandHash(const Hash & hash, const string & target,
     if (!target.empty() && !isInPrefix(target, prefix))
         abort();
 
-    if (!queryListDB(nixDB, dbHash2Paths, hash, paths))
-        throw Error(format("no paths known with hash `%1%'") % (string) hash);
+    queryListDB(nixDB, dbHash2Paths, hash, paths);
 
     /* !!! we shouldn't check for staleness by default --- too slow */
 
@@ -181,8 +195,32 @@ string expandHash(const Hash & hash, const string & target,
             /* try next one */
         }
     }
+
+    /* Try to realise the substitutes. */
+
+    Strings subs;
+    queryListDB(nixDB, dbSubstitutes, hash, subs); /* non-existence = ok */
+
+    for (Strings::iterator it = subs.begin(); it != subs.end(); it++) {
+        StringSet dummy;
+        FState nf = realiseFState(hash2fstate(parseHash(*it)), dummy);
+        string path = fstatePath(nf);
+
+        if (hashPath(path) != hash)
+            throw Error(format("bad substitute in `%1%'") % (string) path);
+
+        if (target.empty())
+            return path; /* !!! prefix */
+        else {
+            if (path != target) {
+                copyPath(path, target);
+                registerPath(target, hash);
+            }
+            return target;
+        }
+    }
     
-    throw Error(format("all paths with hash `%1%' are stale") % (string) hash);
+    throw Error(format("cannot expand hash `%1%'") % (string) hash);
 }
 
     
@@ -193,6 +231,7 @@ void addToStore(string srcPath, string & dstPath, Hash & hash)
     hash = hashPath(srcPath);
 
     try {
+        /* !!! should not use the substitutes! */
         dstPath = expandHash(hash, "", nixStore);
         return;
     } catch (...) {
diff --git a/src/store.hh b/src/store.hh
index 8b02cba99681..8b41478a24af 100644
--- a/src/store.hh
+++ b/src/store.hh
@@ -8,8 +8,12 @@
 using namespace std;
 
 
+/* Copy a path recursively. */
 void copyPath(string src, string dst);
 
+/* Register a substitute. */
+void registerSubstitute(const Hash & srcHash, const Hash & subHash);
+
 /* Register a path keyed on its hash. */
 Hash registerPath(const string & path, Hash hash = Hash());