From 98a423b75aa9061f0164c316f9d2481ce6d5e2f1 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Mon, 9 Jul 2012 09:59:34 -0400
Subject: [PATCH] prim_import(): prefetch substitute info in parallel using
 queryMissing()

---
 src/libexpr/primops.cc | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 7587dccea..354fc39be 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -51,6 +51,12 @@ static void prim_import(EvalState & state, Value * * args, Value & v)
                 % path % ctx);
         if (isDerivation(ctx))
             try {
+                /* For performance, prefetch all substitute info. */
+                PathSet willBuild, willSubstitute, unknown;
+                unsigned long long downloadSize, narSize;
+                queryMissing(*store, singleton<PathSet>(ctx),
+                    willBuild, willSubstitute, unknown, downloadSize, narSize);
+                  
                 /* !!! If using a substitute, we only need to fetch
                    the selected output of this derivation. */
                 store->buildPaths(singleton<PathSet>(ctx));