* New primop builtins.filterSource, which can be used to filter files

from a source directory.  All files for which a predicate function
  returns true are copied to the store.  Typical example is to leave
  out the .svn directory:

    stdenv.mkDerivation {
      ...
      src = builtins.filterSource
        (path: baseNameOf (toString path) != ".svn")
        ./source-dir;
      # as opposed to
      #   src = ./source-dir;
    }

  This is important because the .svn directory influences the hash in
  a rather unpredictable and variable way.

src/nix-worker/nix-worker.cc

@@ -240,6 +240,7 @@ static void performOp(Source & from, Sink & to, unsigned int op)
         string hashAlgo = readString(from);
         
         Path tmp = createTempDir();
+        AutoDelete delTmp(tmp);
         Path tmp2 = tmp + "/" + baseName;
         restorePath(tmp2, from);
 
@@ -248,8 +249,6 @@ static void performOp(Source & from, Sink & to, unsigned int op)
         stopWork();
         
         writeString(path, to);
-            
-        deletePath(tmp);
         break;
     }