Run the daemon worker on the same CPU as the client

On a system with multiple CPUs, running Nix operations through the
daemon is significantly slower than "direct" mode:

$ NIX_REMOTE= nix-instantiate '<nixos>' -A system
real    0m0.974s
user    0m0.875s
sys     0m0.088s

$ NIX_REMOTE=daemon nix-instantiate '<nixos>' -A system
real    0m2.118s
user    0m1.463s
sys     0m0.218s

The main reason seems to be that the client and the worker get moved
to a different CPU after every call to the worker.  This patch adds a
hack to lock them to the same CPU.  With this, the overhead of going
through the daemon is very small:

$ NIX_REMOTE=daemon nix-instantiate '<nixos>' -A system
real    0m1.074s
user    0m0.809s
sys     0m0.098s

1 files changed, 4 insertions, 0 deletions

diff --git a/src/nix-daemon/nix-daemon.cc b/src/nix-daemon/nix-daemon.cc
index a6f29b7c97..86f5c0a244 100644
--- a/src/nix-daemon/nix-daemon.cc
+++ b/src/nix-daemon/nix-daemon.cc
@@ -4,6 +4,7 @@
 #include "serialise.hh"
 #include "worker-protocol.hh"
 #include "archive.hh"
+#include "affinity.hh"
 #include "globals.hh"
 
 #include <cstring>
@@ -671,6 +672,9 @@ static void processConnection(bool trusted)
     to.flush();
     unsigned int clientVersion = readInt(from);
 
+    if (GET_PROTOCOL_MINOR(clientVersion) >= 14 && readInt(from))
+        setAffinityTo(readInt(from));
+
     bool reserveSpace = true;
     if (GET_PROTOCOL_MINOR(clientVersion) >= 11)
         reserveSpace = readInt(from) != 0;