Add presubmit check for #pragma once.

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index aaf6544260c0ae6a01f42962b3f13d33ad133ffc..cf207823cc0f7f415f008f4c5563d35ff9248504 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -354,6 +354,25 @@ def _CheckNoBannedFunctions(input_api, output_api):
   return result
 
 
+def _CheckNoPragmaOnce(input_api, output_api):
+  """Make sure that banned functions are not used."""
+  files = []
+  pattern = input_api.re.compile(r'^#pragma\s+once',
+                                 input_api.re.MULTILINE)
+  for f in input_api.AffectedSourceFiles(input_api.FilterSourceFile):
+    if not f.LocalPath().endswith('.h'):
+      continue
+    contents = input_api.ReadFile(f)
+    if pattern.search(contents):
+      files.append(f)
+
+  if files:
+    return [output_api.PresubmitError(
+        'Do not use #pragma once in header files.\n' +

[awong, 2012/07/11 05:41:06]

I don't think you need this plus to concat these strings.  In fact, I think if
you do, you end up making an o(n) copy...

[dcheng, 2012/07/11 05:44:19]

Oops, you're right. I copied this from the iostream check so I'll fix that site
as well.

+        'See http://www.chromium.org/developers/coding-style#TOC-File-headers',
+        files)]
+  return []
+
 
 def _CommonChecks(input_api, output_api):
   """Checks common to both upload and commit."""
@@ -368,6 +387,7 @@ def _CommonChecks(input_api, output_api):
   results.extend(_CheckNoNewWStrings(input_api, output_api))
   results.extend(_CheckNoDEPSGIT(input_api, output_api))
   results.extend(_CheckNoBannedFunctions(input_api, output_api))
+  results.extend(_CheckNoPragmaOnce(input_api, output_api))
   return results