nix-store -r: do substitutions in parallel

I.e. when multiple non-derivation arguments are passed to ‘nix-store
-r’ to be substituted, do them in parallel.

src/libstore/build.cc

@@ -2275,6 +2275,8 @@ public:
     /* Callback used by the worker to write to the log. */
     void handleChildOutput(int fd, const string & data);
     void handleEOF(int fd);
+
+    Path getStorePath() { return storePath; }
 };
 
 
@@ -2938,7 +2940,7 @@ unsigned int Worker::exitStatus()
 //////////////////////////////////////////////////////////////////////
 
 
-void LocalStore::buildDerivations(const PathSet & drvPaths)
+void LocalStore::buildPaths(const PathSet & drvPaths)
 {
     startNest(nest, lvlDebug,
         format("building %1%") % showPaths(drvPaths));
@@ -2947,7 +2949,10 @@ void LocalStore::buildDerivations(const PathSet & drvPaths)
 
     Goals goals;
     foreach (PathSet::const_iterator, i, drvPaths)
-        goals.insert(worker.makeDerivationGoal(*i));
+        if (isDerivation(*i))
+            goals.insert(worker.makeDerivationGoal(*i));
+        else
+            goals.insert(worker.makeSubstitutionGoal(*i));
 
     worker.run(goals);
 
@@ -2955,8 +2960,8 @@ void LocalStore::buildDerivations(const PathSet & drvPaths)
     foreach (Goals::iterator, i, goals)
         if ((*i)->getExitCode() == Goal::ecFailed) {
             DerivationGoal * i2 = dynamic_cast<DerivationGoal *>(i->get());
-            assert(i2);
-            failed.insert(i2->getDrvPath());
+            if (i2) failed.insert(i2->getDrvPath());
+            else failed.insert(dynamic_cast<SubstitutionGoal *>(i->get())->getStorePath());
         }
             
     if (!failed.empty())

src/libstore/local-store.hh

@@ -150,7 +150,7 @@ public:
 
     Paths importPaths(bool requireSignature, Source & source);
     
-    void buildDerivations(const PathSet & drvPaths);
+    void buildPaths(const PathSet & paths);
 
     void ensurePath(const Path & path);
 

src/libstore/remote-store.cc

@@ -397,10 +397,10 @@ Paths RemoteStore::importPaths(bool requireSignature, Source & source)
 }
 
 
-void RemoteStore::buildDerivations(const PathSet & drvPaths)
+void RemoteStore::buildPaths(const PathSet & drvPaths)
 {
     openConnection();
-    writeInt(wopBuildDerivations, to);
+    writeInt(wopBuildPaths, to);
     writeStrings(drvPaths, to);
     processStderr();
     readInt(from);

src/libstore/remote-store.hh

@@ -60,7 +60,7 @@ public:
 
     Paths importPaths(bool requireSignature, Source & source);
     
-    void buildDerivations(const PathSet & drvPaths);
+    void buildPaths(const PathSet & paths);
 
     void ensurePath(const Path & path);
 

src/libstore/store-api.hh

@@ -172,13 +172,15 @@ public:
        the Nix store. */
     virtual Paths importPaths(bool requireSignature, Source & source) = 0;
 
-    /* Ensure that the output paths of the derivation are valid.  If
+    /* For each path, if it's a derivation, build it.  Building a
+       derivation means ensuring that the output paths are valid.  If
        they are already valid, this is a no-op.  Otherwise, validity
        can be reached in two ways.  First, if the output paths is
        substitutable, then build the path that way.  Second, the
        output paths can be created by running the builder, after
-       recursively building any sub-derivations. */
-    virtual void buildDerivations(const PathSet & drvPaths) = 0;
+       recursively building any sub-derivations. For inputs that are
+       not derivations, substitute them. */
+    virtual void buildPaths(const PathSet & paths) = 0;
 
     /* Ensure that a path is valid.  If it is not currently valid, it
        may be made valid by running a substitute (if defined for the

src/libstore/worker-protocol.hh

@@ -22,7 +22,7 @@ typedef enum {
     wopQueryReferrers = 6,
     wopAddToStore = 7,
     wopAddTextToStore = 8,
-    wopBuildDerivations = 9,
+    wopBuildPaths = 9,
     wopEnsurePath = 10,
     wopAddTempRoot = 11,
     wopAddIndirectRoot = 12,