Check for the existence of goma before trying to use it

buildbot/buildbot_pnacl_toolchain.py

Index: buildbot/buildbot_pnacl_toolchain.py
diff --git a/buildbot/buildbot_pnacl_toolchain.py b/buildbot/buildbot_pnacl_toolchain.py
index c8283a94e50282a81c52d2d0622a36b380d2b713..ae1f7aca3fcd2ed97c351ca426fa0f0079fc346b 100755
--- a/buildbot/buildbot_pnacl_toolchain.py
+++ b/buildbot/buildbot_pnacl_toolchain.py
@@ -28,6 +28,9 @@ BUILD_DIR = os.path.join(NACL_DIR, 'build')
 PACKAGE_VERSION_DIR = os.path.join(BUILD_DIR, 'package_version')
 PACKAGE_VERSION_SCRIPT = os.path.join(PACKAGE_VERSION_DIR, 'package_version.py')
 
+GOMA_PATH = '/b/build/goma'
+GOMA_CTL = os.path.join(GOMA_PATH, 'goma_ctl.py')
+
 # As this is a buildbot script, we want verbose logging. Note however, that
 # toolchain_build has its own log settings, controlled by its CLI flags.
 logging.getLogger().setLevel(logging.DEBUG)
@@ -72,7 +75,8 @@ toolchain_install_dir = os.path.join(
     '%s_%s' % (host_os, pynacl.platform.GetArch()),
     'pnacl_newlib')
 
-use_goma = buildbot_lib.RunningOnBuildbot() and not args.no_goma
+use_goma = (buildbot_lib.RunningOnBuildbot() and not args.no_goma
+            and os.path.isfile(GOMA_CTL))
 
 
 def ToolchainBuildCmd(sync=False, extra_flags=[]):
@@ -103,7 +107,7 @@ def ToolchainBuildCmd(sync=False, extra_flags=[]):
     executable_args.append('--disable-llvm-assertions')
 
   if use_goma:
-    executable_args.append('--goma=/b/build/goma')
+    executable_args.append('--goma=' + GOMA_PATH)
 
   return [sys.executable] + executable_args + sync_flag + extra_flags
 
@@ -138,8 +142,7 @@ if host_os != 'win':
              NACL_DIR, '..', 'tools', 'clang', 'scripts', 'update.py')])
 
 if use_goma:
-  buildbot_lib.Command(context, cmd=[
-      sys.executable, '/b/build/goma/goma_ctl.py', 'restart'])
+  buildbot_lib.Command(context, cmd=[sys.executable, GOMA_CTL, 'restart'])
 
 # toolchain_build outputs its own buildbot annotations, so don't use
 # buildbot_lib.Step to run it here.
@@ -153,8 +156,7 @@ if use_goma:
 RunWithLog(ToolchainBuildCmd(sync=True, extra_flags=['--canonical-only']))
 
 if use_goma:
-  buildbot_lib.Command(context, cmd=[
-      sys.executable, '/b/build/goma/goma_ctl.py', 'stop'])
+  buildbot_lib.Command(context, cmd=[sys.executable, GOMA_CTL, 'stop'])
 
 if args.skip_tests:
   sys.exit(0)