From 470553bd0572c4010407acb4e410b45a521e5f11 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Wed, 1 May 2013 13:21:39 +0200
Subject: [PATCH] Don't let stderr writes in substituters cause a deadlock

---
 src/libstore/local-store.cc | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 687ad7e47..dce57f751 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1007,6 +1007,10 @@ void LocalStore::startSubstituter(const Path & substituter, RunningSubstituter &
     fromPipe.create();
     errorPipe.create();
 
+    /* Hack: prevent substituters that write too much to stderr from
+       deadlocking our read() from stdout. */
+    fcntl(errorPipe.writeSide, F_SETFL, O_NONBLOCK);
+
     setSubstituterEnv();
 
     run.pid = maybeVfork();