Display prompt for caution only if it is an interactive shell

PRESUBMIT.py

 # Copyright (c) 2013 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.
 
 
 """Top-level presubmit script for Skia.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
+import os
+import sys
+
 
 def _CheckChangeHasEol(input_api, output_api, source_file_filter=None):
   """Checks that files end with atleast one \n (LF)."""
   eof_files = []
   for f in input_api.AffectedSourceFiles(source_file_filter):
     contents = input_api.ReadFile(f, 'rb')
     # Check that the file ends in atleast one newline character.
     if len(contents) > 1 and contents[-1:] != '\n':
       eof_files.append(f.LocalPath())
 
@@ skipping 38 matching lines @@
     output_api: output related apis.
     json_url: url to download json style status.
   """
   tree_status_results = input_api.canned_checks.CheckTreeIsOpen(
       input_api, output_api, json_url=json_url)
   if not tree_status_results:
     # Check for caution state only if tree is not closed.
     connection = input_api.urllib2.urlopen(json_url)
     status = input_api.json.loads(connection.read())
     connection.close()
-    if 'caution' in status['message'].lower():
+    if ('caution' in status['message'].lower() and
+        os.isatty(sys.stdout.fileno())):
+      # Display a prompt only if we are in an interactive shell. Without this
+      # check the commit queue behaves incorrectly because it considers
+      # prompts to be failures.
       short_text = 'Tree state is: ' + status['general_state']
       long_text = status['message'] + '\n' + json_url
       tree_status_results.append(
           output_api.PresubmitPromptWarning(
               message=short_text, long_text=long_text))
   return tree_status_results
 
 
 def CheckChangeOnCommit(input_api, output_api):
   """Presubmit checks for the change on commit.
 
   The following are the presubmit checks:
   * Check change has one and only one EOL.
   * Ensures that the Skia tree is open in
     http://skia-tree-status.appspot.com/. Shows a warning if it is in 'Caution'
     state and an error if it is in 'Closed' state.
   """
   results = []
   results.extend(_CommonChecks(input_api, output_api))
   results.extend(
       _CheckTreeStatus(input_api, output_api, json_url=(
           'http://skia-tree-status.appspot.com/banner-status?format=json')))
   return results