Make 'nix copy --from ssh://...' run in constant memory

For instance, this reduced the memory consumption of

  $ nix copy --from ssh://localhost --to ~/my-nix /nix/store/1n7x0yv8vq6zi90hfmian84vdhd04bgp-blender-2.79a

from 632 MiB to 16 MiB.

src/libstore/legacy-ssh-store.cc

@@ -151,12 +151,7 @@ struct LegacySSHStore : public Store
 
         conn->to << cmdDumpStorePath << path;
         conn->to.flush();
-
+        copyNAR(conn->from, sink);
-        /* FIXME: inefficient. */
-        ParseSink parseSink; /* null sink; just parse the NAR */
-        TeeSource savedNAR(conn->from);
-        parseDump(parseSink, savedNAR);
-        sink(*savedNAR.data);
     }
 
     PathSet queryAllValidPaths() override { unsupported(); }

src/libstore/ssh-store.cc

@@ -63,29 +63,12 @@ private:
     };
 };
 
-
-class ForwardSource : public Source
-{
-    Source & readSource;
-    Sink & writeSink;
-public:
-    ForwardSource(Source & readSource, Sink & writeSink) : readSource(readSource), writeSink(writeSink) {}
-    size_t read(unsigned char * data, size_t len) override
-    {
-        auto n = readSource.read(data, len);
-        writeSink(data, n);
-        return n;
-    }
-};
-
 void SSHStore::narFromPath(const Path & path, Sink & sink)
 {
     auto conn(connections->get());
     conn->to << wopNarFromPath << path;
     conn->processStderr();
-    ParseSink ps;
+    copyNAR(conn->from, sink);
-    auto fwd = ForwardSource(conn->from, sink);
-    parseDump(ps, fwd);
 }
 
 ref<FSAccessor> SSHStore::getFSAccessor()

src/libutil/archive.cc

@@ -350,4 +350,21 @@ void restorePath(const Path & path, Source & source)
 }
 
 
+void copyNAR(Source & source, Sink & sink)
+{
+    // FIXME: if 'source' is the output of dumpPath() followed by EOF,
+    // we should just forward all data directly without parsing.
+
+    ParseSink parseSink; /* null sink; just parse the NAR */
+
+    LambdaSource wrapper([&](unsigned char * data, size_t len) {
+        auto n = source.read(data, len);
+        sink(data, n);
+        return n;
+    });
+
+    parseDump(parseSink, wrapper);
+}
+
+
 }

src/libutil/archive.hh

@@ -74,6 +74,9 @@ void parseDump(ParseSink & sink, Source & source);
 
 void restorePath(const Path & path, Source & source);
 
+/* Read a NAR from 'source' and write it to 'sink'. */
+void copyNAR(Source & source, Sink & sink);
+
 
 // FIXME: global variables are bad m'kay.
 extern bool useCaseHack;