Add a function to detect patches that can't be safely applied.

patch.py

Index: patch.py
diff --git a/patch.py b/patch.py
index af18ee43e212583815d27d138d1fc426883eb663..cc86d343e011b1985dfb7c724b15ad06f1af91e2 100644
--- a/patch.py
+++ b/patch.py
@@ -1,8 +1,10 @@
+# coding=utf8
 # Copyright (c) 2010 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.
 """Utility functions to handle patches."""
 
+import logging
 import re
 import subprocess2
 
@@ -29,3 +31,68 @@ def apply_patch(location, patch):
   cmd = ['patch', '-p0', '--forward', '--force']
   subprocess2.check_call(cmd, stdin=patch, cwd=location)
 
+
+def verify_patch(patch_data):
+  """Verifies if a patch can be applied.
+
+  Returns a string if the patch can't be applied explaining the issue.
+  """
+  lines = patch_data.splitlines(False)
+  svn_separator = (
+      '___________________________________________________________________')
+
+  index = 0
+
+  def next_line(i):
+    if index+i >= len(lines):
+      return ''
+    return lines[index+i]
+
+  for index, line in enumerate(lines):
+    if line == svn_separator:
+      # Look for the next line.
+      # TODO: If we ever hit non English patches, we need to
+      # use something like re.match(ur'^\w+$', u'Modifié', re.UNICODE)
+      m = re.match(r'^(\w+)\s*\:\s*svn\:([\w\-]+)$', next_line(1))
+      if not m:
+        # The patch confused the script;
+        continue
+      prop = m.group(2)
+      if prop in ('mergeinfo', 'executable', 'ignore'):
+        # In practice, executable should be treated like eol-style, assuming we
+        # lookup auto-props.
+        return 'Can\'t apply svn property svn:%s' % prop
+      elif prop in ('eol-style', 'mime-type'):
+        # Silently ignore it, assume that the auto-props are ok.
+        # TODO: Verify the auto-props would trigger.
+        continue
+      else:
+        # Unknown property
+        logging.error('Unknown property %s' % prop)
+        continue
+    m = re.match(r'^Index\:\s*(.+)$', line)
+    if m:
+      # We need to find --- on the next 3 lines otherwise discard the patch.
+      if next_line(1).startswith('--- '):
+        continue
+      if not next_line(1):
+        return 'Can\'t patch empty file (svn)'
+      if next_line(2).startswith('--- '):
+        continue
+      if not next_line(2):
+        return 'Can\'t patch empty file (svn)'
+      if next_line(3).startswith('--- '):
+        continue
+      if not next_line(3):
+        return 'Can\'t patch empty file (svn)'
+      if next_line(4).startswith('--- '):
+        # See testSupported3() for a real-world example.
+        continue
+      return 'Can\'t patch empty file (svn)'
+    m = re.match(r'diff \-\-git (.*)$', line)
+    if m:
+      if not next_line(1):
+        return 'Can\'t patch empty file (git)'
+      if next_line(1).startswith('old mode '):
+        return 'Unsupported git file mode change'
+  return None