Add an environment variable that will cause md5_check.py to write .stamp file that is debuggable

build/android/gyp/util/md5_check.py

Index: build/android/gyp/util/md5_check.py
diff --git a/build/android/gyp/util/md5_check.py b/build/android/gyp/util/md5_check.py
index 9f365aa08162c83b22192b675a1a53a5e9acb87f..09e86c26b4d12c3ae8e06b4763ef21bde7620b44 100644
--- a/build/android/gyp/util/md5_check.py
+++ b/build/android/gyp/util/md5_check.py
@@ -2,8 +2,24 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import difflib
 import hashlib
 import os
+import re
+import sys
+
+from util import build_utils
+
+if build_utils.COLORAMA_ROOT not in sys.path:
+  sys.path.append(build_utils.COLORAMA_ROOT)
+import colorama
+
+
+# When set and a difference is detected, a diff of what changed is printed.
+_PRINT_MD5_DIFFS = int(os.environ.get('PRINT_MD5_DIFFS', 0))
+
+# Used to strip off temp dir prefix.
+_TEMP_DIR_PATTERN = re.compile(r'^/tmp/.*?/')
 
 
 def CallAndRecordIfStale(
@@ -26,7 +42,13 @@ def CallAndRecordIfStale(
       record_path=record_path,
       input_paths=input_paths,
       input_strings=input_strings)
-  if force or md5_checker.IsStale():
+
+  is_stale = md5_checker.old_digest != md5_checker.new_digest
+  if force or is_stale:
+    if is_stale and _PRINT_MD5_DIFFS:
+      print '%sDifference found in %s:%s' % (
+          colorama.Fore.YELLOW, record_path, colorama.Fore.RESET)
+      print md5_checker.DescribeDifference()
     function()
     md5_checker.Write()
 
@@ -53,6 +75,14 @@ def _UpdateMd5ForPath(md5, path):
     _UpdateMd5ForFile(md5, path)
 
 
+def _TrimPathPrefix(path):
+  """Attempts to remove temp dir prefix from the path.
+
+  Use this only for extended_info (not for the actual md5).
+  """
+  return _TEMP_DIR_PATTERN.sub('{TMP}', path)
+
+
 class _Md5Checker(object):
   def __init__(self, record_path=None, input_paths=None, input_strings=None):
     if not input_paths:
@@ -66,21 +96,41 @@ class _Md5Checker(object):
 
     self.record_path = record_path
 
-    md5 = hashlib.md5()
+    extended_info = []
+    outer_md5 = hashlib.md5()
     for i in sorted(input_paths):
-      _UpdateMd5ForPath(md5, i)
+      inner_md5 = hashlib.md5()
+      _UpdateMd5ForPath(inner_md5, i)
+      i = _TrimPathPrefix(i)
+      extended_info.append(i + '=' + inner_md5.hexdigest())
+      # Include the digest in the overall diff, but not the path
+      outer_md5.update(inner_md5.hexdigest())
+
     for s in input_strings:
-      md5.update(s)
-    self.new_digest = md5.hexdigest()
+      outer_md5.update(s)
+      extended_info.append(s)
+
+    self.new_digest = outer_md5.hexdigest()
+    self.new_extended_info = extended_info
 
     self.old_digest = ''
+    self.old_extended_info = []
     if os.path.exists(self.record_path):
       with open(self.record_path, 'r') as old_record:
-        self.old_digest = old_record.read()
-
-  def IsStale(self):
-    return self.old_digest != self.new_digest
+        self.old_extended_info = [line.strip() for line in old_record]
+        self.old_digest = self.old_extended_info.pop(0)
 
   def Write(self):
     with open(self.record_path, 'w') as new_record:
       new_record.write(self.new_digest)
+      new_record.write('\n' + '\n'.join(self.new_extended_info) + '\n')
+
+  def DescribeDifference(self):
+    if self.old_digest == self.new_digest:
+      return "There's no difference."
+    if not self.old_digest:
+      return 'Previous stamp file not found.'
+    if not self.old_extended_info:
+      return 'Previous stamp file lacks extended info.'
+    diff = difflib.unified_diff(self.old_extended_info, self.new_extended_info)
+    return '\n'.join(diff)