git_llvm_rev: handle merge commits more gracefully

llvm grew a relatively large merge commit recently, which broke this
script noticeably.

There's analysis of why this happened in the bug. tl;dr is that
`rev-parse ${sha}~${N}` and `rev-list --count` don't do the same thing
without extra flags.

BUG=chromium:1041016
TEST=Unit tests. A few random runs in my LLVM tree.

Change-Id: I393d1ada842b5f19846d961b403641bc958b191f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/1995315
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>

2 files changed, 42 insertions, 3 deletions

diff --git a/llvm_tools/git_llvm_rev.py b/llvm_tools/git_llvm_rev.py
index c8f8a89d..793b56f9 100755
--- a/llvm_tools/git_llvm_rev.py
+++ b/llvm_tools/git_llvm_rev.py
@@ -129,7 +129,13 @@ def translate_sha_to_rev(llvm_config: LLVMConfig, sha_or_ref: str) -> Rev:
 
   if merge_base == base_llvm_sha:
     result = check_output(
-        ['git', 'rev-list', '--count', f'{base_llvm_sha}..{sha}'],
+        [
+            'git',
+            'rev-list',
+            '--count',
+            '--first-parent',
+            f'{base_llvm_sha}..{sha}',
+        ],
         cwd=llvm_config.dir,
     )
     count = int(result.strip())
@@ -145,7 +151,13 @@ def translate_sha_to_rev(llvm_config: LLVMConfig, sha_or_ref: str) -> Rev:
     return Rev(branch='master', number=merge_base_number)
 
   distance_from_base = check_output(
-      ['git', 'rev-list', '--count', f'{merge_base}..{sha}'],
+      [
+          'git',
+          'rev-list',
+          '--count',
+          '--first-parent',
+          f'{merge_base}..{sha}',
+      ],
       cwd=llvm_config.dir,
   )
 
@@ -283,7 +295,13 @@ def translate_rev_to_sha(llvm_config: LLVMConfig, rev: Rev) -> str:
 
   commit_number = number - base_revision_number
   revs_between_str = check_output(
-      ['git', 'rev-list', '--count', f'{base_sha}..{branch_head_sha}'],
+      [
+          'git',
+          'rev-list',
+          '--count',
+          '--first-parent',
+          f'{base_sha}..{branch_head_sha}',
+      ],
       cwd=llvm_config.dir,
   )
   revs_between = int(revs_between_str.strip())
diff --git a/llvm_tools/git_llvm_rev_test.py b/llvm_tools/git_llvm_rev_test.py
index ebb654ff..06b83cd1 100755
--- a/llvm_tools/git_llvm_rev_test.py
+++ b/llvm_tools/git_llvm_rev_test.py
@@ -4,6 +4,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+"""Tests for git_llvm_rev."""
+
+from __future__ import print_function
+
 import os
 import sys
 import unittest
@@ -21,6 +25,7 @@ def get_llvm_config() -> git_llvm_rev.LLVMConfig:
 
 
 class Test(unittest.TestCase):
+  """Test cases for git_llvm_rev."""
 
   def rev_to_sha_with_round_trip(self, rev: git_llvm_rev.Rev) -> str:
     config = get_llvm_config()
@@ -66,6 +71,22 @@ class Test(unittest.TestCase):
 
     self.assertIn('Try updating your tree?', str(r.exception))
 
+  def test_merge_commits_count_as_one_commit_crbug1041079(self) -> None:
+    # This CL merged _a lot_ of commits in. Verify a few hand-computed
+    # properties about it.
+    merge_sha_rev_number = 4496 + git_llvm_rev.base_llvm_revision
+    sha = self.rev_to_sha_with_round_trip(
+        git_llvm_rev.Rev(branch='master', number=merge_sha_rev_number))
+    self.assertEqual(sha, '0f0d0ed1c78f1a80139a1f2133fad5284691a121')
+
+    sha = self.rev_to_sha_with_round_trip(
+        git_llvm_rev.Rev(branch='master', number=merge_sha_rev_number - 1))
+    self.assertEqual(sha, '6f635f90929da9545dd696071a829a1a42f84b30')
+
+    sha = self.rev_to_sha_with_round_trip(
+        git_llvm_rev.Rev(branch='master', number=merge_sha_rev_number + 1))
+    self.assertEqual(sha, '199700a5cfeedf227619f966aa3125cef18bc958')
+
   # NOTE: The below tests have _zz_ in their name as an optimization. Iterating
   # on a quick test is painful when these larger tests come before it and take
   # 7secs to run. Python's unittest module guarantees tests are run in