Display prompt for caution only if it is an interactive shell

PRESUBMIT.py

Index: PRESUBMIT.py
===================================================================
--- PRESUBMIT.py	(revision 8445)
+++ PRESUBMIT.py	(working copy)
@@ -9,7 +9,10 @@
 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 = []
@@ -67,7 +70,11 @@
     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(