Make build.py ensure that goma is started if it is enabled

tools/build.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 #
 
 import optparse
 import os
 import re
@@ skipping 409 matching lines @@
 def ShouldRunGN(out_dir):
   return (not os.path.exists(out_dir) or
           not os.path.isfile(os.path.join(out_dir, 'args.gn')))
 
 
 def UseGoma(out_dir):
   args_gn = os.path.join(out_dir, 'args.gn')
   return 'use_goma = true' in open(args_gn, 'r').read()
 
 
+# Try to start goma, but don't bail out if we can't. Instead print an error
+# message, and let the build fail with its own error messages as well.
+def EnsureGomaStarted(out_dir):
+  args_gn_path = os.path.join(out_dir, 'args.gn')
+  goma_dir = None
+  with open(args_gn_path, 'r') as fp:
+    for line in fp:
+      if 'goma_dir' in line:
+        words = line.split()
+        goma_dir = words[2][1:-1]  # goma_dir = "/path/to/goma"
+  if not goma_dir:
+    print 'Could not find goma for ' + out_dir
+    return False
+  if not os.path.exists(goma_dir) or not os.path.isdir(goma_dir):
+    print 'Could not find goma at ' + goma_dir
+    return False
+  goma_ctl = os.path.join(goma_dir, 'goma_ctl.py')
+  goma_ctl_command = [
+    'python',
+    goma_ctl,
+    'ensure_start',
+  ]
+  process = subprocess.Popen(goma_ctl_command)
+  process.wait()
+  if process.returncode != 0:
+    print ("Tried to run goma_ctl.py, but it failed. Try running it manually: "
+           + "\n\t" + ' '.join(goma_ctl_command))
+    return False
+  return True
+
+
+
 def BuildNinjaCommand(options, target, target_os, mode, arch):
   out_dir = utils.GetBuildRoot(HOST_OS, mode, arch, target_os)
   if ShouldRunGN(out_dir):
     RunGN(target_os, mode, arch)
   command = ['ninja', '-C', out_dir]
   if options.verbose:
     command += ['-v']
   if UseGoma(out_dir):
-    command += ['-j1000']
+    if EnsureGomaStarted(out_dir):
+      command += ['-j1000']
+    else:
+      # If we couldn't ensure that goma is started, let the build start, but
+      # slowly so we can see any helpful error messages that pop out.
+      command += ['-j1']
   command += [target]
   return command
 
 
 filter_xcodebuild_output = False
 def BuildOneConfig(options, target, target_os, mode, arch, override_tools):
   global filter_xcodebuild_output
   start_time = time.time()
   args = []
   build_config = utils.GetBuildConf(mode, arch, target_os)
@@ skipping 159 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())