Add tools/find-commit-for-patch.py

It searches for a commit that a given patch can cleanly be applied to.

Review URL: https://codereview.chromium.org/799273003

Cr-Commit-Position: refs/heads/master@{#25845}

tools/find-commit-for-patch.py

+#!/usr/bin/env python
+# Copyright 2014 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import argparse
+import subprocess
+import sys
+
+
+def GetArgs():
+  parser = argparse.ArgumentParser(
+      description="Finds a commit that a given patch can be applied to. "
+                  "Does not actually apply the patch or modify your checkout "
+                  "in any way.")
+  parser.add_argument("patch_file", help="Patch file to match")
+  parser.add_argument(
+      "--branch", "-b", default="origin/master", type=str,
+      help="Git tree-ish where to start searching for commits, "
+           "default: %(default)s")
+  parser.add_argument(
+      "--limit", "-l", default=500, type=int,
+      help="Maximum number of commits to search, default: %(default)s")
+  parser.add_argument(
+      "--verbose", "-v", default=False, action="store_true",
+      help="Print verbose output for your entertainment")
+  return parser.parse_args()
+
+
+def FindFilesInPatch(patch_file):
+  files = {}
+  next_file = ""
+  with open(patch_file) as patch:
+    for line in patch:
+      if line.startswith("diff --git "):
+        # diff --git a/src/objects.cc b/src/objects.cc
+        words = line.split()
+        assert words[2].startswith("a/") and len(words[2]) > 2
+        next_file = words[2][2:]
+      elif line.startswith("index "):
+        # index add3e61..d1bbf6a 100644
+        hashes = line.split()[1]
+        old_hash = hashes.split("..")[0]
+        if old_hash.startswith("0000000"): continue  # Ignore new files.
+        files[next_file] = old_hash
+  return files
+
+
+def GetGitCommitHash(treeish):
+  cmd = ["git", "log", "-1", "--format=%H", treeish]
+  return subprocess.check_output(cmd).strip()
+
+
+def CountMatchingFiles(commit, files):
+  matched_files = 0
+  # Calling out to git once and parsing the result Python-side is faster
+  # than calling 'git ls-tree' for every file.
+  cmd = ["git", "ls-tree", "-r", commit] + [f for f in files]
+  output = subprocess.check_output(cmd)
+  for line in output.splitlines():
+    # 100644 blob c6d5daaa7d42e49a653f9861224aad0a0244b944      src/objects.cc
+    _, _, actual_hash, filename = line.split()
+    expected_hash = files[filename]
+    if actual_hash.startswith(expected_hash): matched_files += 1
+  return matched_files
+
+
+def FindFirstMatchingCommit(start, files, limit, verbose):
+  commit = GetGitCommitHash(start)
+  num_files = len(files)
+  if verbose: print(">>> Found %d files modified by patch." % num_files)
+  for _ in range(limit):
+    matched_files = CountMatchingFiles(commit, files)
+    if verbose: print("Commit %s matched %d files" % (commit, matched_files))
+    if matched_files == num_files:
+      return commit
+    commit = GetGitCommitHash("%s^" % commit)
+  print("Sorry, no matching commit found. "
+        "Try running 'git fetch', specifying the correct --branch, "
+        "and/or setting a higher --limit.")
+  sys.exit(1)
+
+
+if __name__ == "__main__":
+  args = GetArgs()
+  files = FindFilesInPatch(args.patch_file)
+  commit = FindFirstMatchingCommit(args.branch, files, args.limit, args.verbose)
+  if args.verbose:
+    print(">>> Matching commit: %s" % commit)
+    print(subprocess.check_output(["git", "log", "-1", commit]))
+    print(">>> Kthxbai.")
+  else:
+    print(commit)