daemon: Buffer data sent to clients by the 'export-path' RPC.

Before that we'd have STDERR_WRITE round trips for very small amounts of
data, ranging from a few bytes for the metadata of nars to the size of
one file being exported.

With this change, something like:

  guix archive --export /gnu/store/5rrsbaghh5ix1vjcicsl60gsxilhjnf2-coreutils-8.25 | dd of=/dev/null

reports a throughput of 35 MB/s instead of 25 MB/s before.

* nix/nix-daemon/nix-daemon.cc (TunnelSink): Inherit from 'BufferedSink'
rather than 'Sink'.  Rename 'operator ()' to 'write'.
(performOp) <wopExportPath>: Add 'sink.flush' call.

1 files changed, 4 insertions, 3 deletions

diff --git a/nix/nix-daemon/nix-daemon.cc b/nix/nix-daemon/nix-daemon.cc
index 9b29b3e91d..682f9a2b5b 100644
--- a/nix/nix-daemon/nix-daemon.cc
+++ b/nix/nix-daemon/nix-daemon.cc
@@ -203,11 +203,11 @@ static void stopWork(bool success = true, const string & msg = "", unsigned int
 }
 
 
-struct TunnelSink : Sink
+struct TunnelSink : BufferedSink
 {
     Sink & to;
-    TunnelSink(Sink & to) : to(to) { }
-    virtual void operator () (const unsigned char * data, size_t len)
+    TunnelSink(Sink & to) : BufferedSink(64 * 1024), to(to) { }
+    virtual void write(const unsigned char * data, size_t len)
     {
         writeInt(STDERR_WRITE, to);
         writeString(data, len, to);
@@ -433,6 +433,7 @@ static void performOp(bool trusted, unsigned int clientVersion,
         startWork();
         TunnelSink sink(to);
         store->exportPath(path, sign, sink);
+        sink.flush();
         stopWork();
         writeInt(1, to);
         break;