* Start of `nix-store --export' operation for serialising a store

path.  This is like `nix-store --dump', only it also dumps the
  meta-information of the store path (references, deriver).  Will add
  a `--sign' flag later to add a cryptographic signature, which we
  will use for exchanging store paths between build farm machines in a
  secure manner.

src/libstore/local-store.cc

@@ -696,6 +696,24 @@ Path LocalStore::addTextToStore(const string & suffix, const string & s,
 }
 
 
+void LocalStore::exportPath(const Path & path, bool sign,
+    Sink & sink)
+{
+    assertStorePath(path);
+    
+    dumpPath(path, sink);
+
+    writeString(path, sink);
+    
+    PathSet references;
+    queryReferences(path, references);
+    writeStringSet(references, sink);
+
+    Path deriver = queryDeriver(noTxn, path);
+    writeString(deriver, sink);
+}
+
+
 void deleteFromStore(const Path & _path, unsigned long long & bytesFreed)
 {
     bytesFreed = 0;

src/libstore/local-store.hh

@@ -56,6 +56,9 @@ public:
     Path addTextToStore(const string & suffix, const string & s,
         const PathSet & references);
 
+    void exportPath(const Path & path, bool sign,
+        Sink & sink);
+
     void buildDerivations(const PathSet & drvPaths);
 
     void ensurePath(const Path & path);

src/libstore/remote-store.cc

@@ -243,6 +243,13 @@ Path RemoteStore::addTextToStore(const string & suffix, const string & s,
 }
 
 
+void RemoteStore::exportPath(const Path & path, bool sign,
+    Sink & sink)
+{
+    throw Error("not implemented");
+}
+
+
 void RemoteStore::buildDerivations(const PathSet & drvPaths)
 {
     writeInt(wopBuildDerivations, to);

src/libstore/remote-store.hh

@@ -44,6 +44,9 @@ public:
     Path addTextToStore(const string & suffix, const string & s,
         const PathSet & references);
 
+    void exportPath(const Path & path, bool sign,
+        Sink & sink);
+
     void buildDerivations(const PathSet & drvPaths);
 
     void ensurePath(const Path & path);

src/libstore/store-api.hh

@@ -7,6 +7,7 @@
 #include <boost/shared_ptr.hpp>
 
 #include "hash.hh"
+#include "serialise.hh"
 
 
 namespace nix {
@@ -91,6 +92,13 @@ public:
     virtual Path addTextToStore(const string & suffix, const string & s,
         const PathSet & references) = 0;
 
+    /* Export a store path, that is, create a NAR dump of the store
+       path and append its references and its deriver.  Optionally, a
+       cryptographic signature (created by OpenSSL) of the preceding
+       data is attached. */
+    virtual void exportPath(const Path & path, bool sign,
+        Sink & sink) = 0;
+
     /* Ensure that the output paths of the derivation 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 have

src/nix-store/nix-store.cc

@@ -638,6 +638,15 @@ static void opRestore(Strings opFlags, Strings opArgs)
 }
 
 
+static void opExport(Strings opFlags, Strings opArgs)
+{
+    if (!opFlags.empty()) throw UsageError("unknown flag");
+
+    FdSink sink(STDOUT_FILENO);
+    store->exportPath(*opArgs.begin(), false, sink);
+}
+
+
 /* Initialise the Nix databases. */
 static void opInit(Strings opFlags, Strings opArgs)
 {
@@ -707,6 +716,8 @@ void run(Strings args)
             op = opDump;
         else if (arg == "--restore")
             op = opRestore;
+        else if (arg == "--export")
+            op = opExport;
         else if (arg == "--init")
             op = opInit;
         else if (arg == "--verify")