compiler_version: suppress stderr unless command fails

gyp considers a shell variable substitution to have failed if it
generates any output to stderr, even if the exit status was zero.
This causes problems when gomacc automatically starts its proxy,
which helpfully reports "GOMA: Starting compiler proxy" to stderr.

Fixed by dropping stderr unless the underlying process returns
a non-zero exit status.

BUG=chromium-os:28670
TEST=Ran hooks normally and inside CrOS chroot, with and without
  $CXX pointing to goma. A $CXX which writes to stderr and then
  exits with failure still prints its output before the exception.


Review URL: http://codereview.chromium.org/10288005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@135663 0039d316-1c4b-4281-b951-d872f2087c98

build/compiler_version.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# Copyright (c) 2011 The Chromium Authors. All rights reserved.
+# Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
@@ -18,11 +18,17 @@ def GetVersion(compiler):
   try:
     # Note that compiler could be something tricky like "distcc g++".
     compiler = compiler + " -dumpversion"
-    pipe = subprocess.Popen(compiler, stdout=subprocess.PIPE, shell=True)
-    gcc_output = pipe.communicate()[0]
+    pipe = subprocess.Popen(compiler, shell=True,
+                            stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    gcc_output, gcc_error = pipe.communicate()
+    if pipe.returncode:
+      raise subprocess.CalledProcessError(pipe.returncode, compiler)
+
     result = re.match(r"(\d+)\.(\d+)", gcc_output)
     return result.group(1) + result.group(2)
   except Exception, e:
+    if gcc_error:
+      sys.stderr.write(gcc_error)
     print >> sys.stderr, "compiler_version.py failed to execute:", compiler
     print >> sys.stderr, e
     return ""