Add --test to gen_tasks.go, add presubmit check

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 8115b497b654ec87aa198fe311105f2e0f89ff23..4798a02d594df20f6dbf4ea6f0b703bce93fc32e 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -183,6 +183,24 @@ def _RecipeSimulationTest(input_api, output_api):
         '`%s` failed:\n%s' % (' '.join(cmd), e.output)))
   return results
 
+
+def _GenTasksTest(input_api, output_api):
+  """Run gen_tasks.go test."""
+  results = []
+  if not any(f.LocalPath().startswith('infra')
+             for f in input_api.AffectedFiles()):
+    return results
+
+  gen_tasks = os.path.join('infra', 'bots', 'gen_tasks.go')
+  cmd = ['go', 'run', gen_tasks, '--test']
+  try:
+    subprocess.check_output(cmd)
+  except subprocess.CalledProcessError as e:
+    results.append(output_api.PresubmitError(
+        '`%s` failed:\n%s' % (' '.join(cmd), e.output)))
+  return results
+
+
 def _CheckGNFormatted(input_api, output_api):
   """Make sure any .gn files we're changing have been formatted."""
   results = []
@@ -234,8 +252,10 @@ def CheckChangeOnUpload(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
   # Run on upload, not commit, since the presubmit bot apparently doesn't have
-  # coverage installed.
+  # coverage or Go installed.
   results.extend(_RecipeSimulationTest(input_api, output_api))
+  results.extend(_GenTasksTest(input_api, output_api))

[borenet, 2016/10/04 18:59:47]

This is really unfortunate. We need to be able to test these on the CQ.

[dogben, 2016/10/04 19:16:23]

Is there a reason skia_presubmit-Trybot doesn't run on swarming (or
swarmbucket)?

[borenet, 2016/10/04 19:26:55]

That bot was brought up by chrome-infra after they turned off presubmit checks
on the CQ itself. It's running a non-skia recipe
(https://chromium.googlesource.com/chromium/tools/build/+/master/scripts/slave...),
and I had previously dismissed it as not under our control. I guess all that
matters is that we run the presubmit on a trybot somewhere, so we could possibly
create our own presubmit recipe, add a bot which runs that in Swarming, and then
turn the current presubmit bot off.

[borenet, 2016/10/04 19:32:22]

Filed https://bugs.chromium.org/p/skia/issues/detail?id=5823

+
   results.extend(_CheckGNFormatted(input_api, output_api))
   return results