fix: check a commit with an empty message as empty, not as HEAD

git log listing dropped such a commit, and piped empty text reads to
the CLI as no input, so it checked HEAD (the merge commit) instead.
The commit is now kept and its empty message goes to the CLI as an
empty file, where allow_empty_commits judges it.

Author: shenxianpeng

main.py

@@ -17,6 +17,7 @@
 import re
 import subprocess
 import sys
+import tempfile
 import uuid
 from dataclasses import dataclass, field
 from typing import Any
@@ -567,14 +568,13 @@ def parse_commit_messages(output: str) -> list[Commit]:
 
     The fields alternate hash, message, hash, message; git puts a newline
     between commits, which lands on the front of the next hash and is
-    stripped along with the message's trailing newline. Commits are paired
-    before empty messages are dropped, so a blank message never shifts the
-    hashes of the commits after it.
+    stripped along with the message's trailing newline. A commit with an
+    empty message is kept, as ``(sha, "")``: it is a commit of the pull
+    request like any other, and ``allow_empty_commits`` is the rule that
+    judges it.
     """
     fields = [f.strip("\n") for f in output.split(COMMIT_MESSAGE_DELIMITER)]
-    return [
-        (sha, message) for sha, message in zip(fields[0::2], fields[1::2]) if message
-    ]
+    return [(sha, message) for sha, message in zip(fields[0::2], fields[1::2]) if sha]
 
 
 def _messages_in_range(revision_range: str) -> list[Commit]:
@@ -831,12 +831,31 @@ def run_pr_message_checks(pr_commits: list[Commit]) -> list[ScopeResult]:
         if author:
             names = ("GIT_AUTHOR_NAME", "GIT_AUTHOR_EMAIL")
             env = {key: value for key, value in zip(names, author) if value} or None
-        results.append(
-            check_scope(label, ["--message"], input_text=msg, sha=sha, env=env)
-        )
+        if msg.strip():
+            scope = check_scope(label, ["--message"], input_text=msg, sha=sha, env=env)
+        else:
+            scope = check_empty_message(label, sha, env)
+        results.append(scope)
     return results
 
 
+def check_empty_message(
+    label: str, sha: str, env: dict[str, str] | None
+) -> ScopeResult:
+    """Check a commit whose message is empty, from an empty file.
+
+    Piped, an empty message reads to the CLI as no input at all, and it
+    checks HEAD in its place: the merge commit, which passes. A file names
+    the message, so the empty one is judged (``allow_empty_commits``).
+    """
+    fd, path = tempfile.mkstemp(prefix="commit-msg-", dir=os.getenv("RUNNER_TEMP"))
+    os.close(fd)
+    try:
+        return check_scope(label, ["--message", path], sha=sha, env=env)
+    finally:
+        os.unlink(path)
+
+
 def run_other_checks(args: list[str], rev: str | None = None) -> list[ScopeResult]:
     """Run each non-message check (branch, author) once, as its own scope.
 

main_test.py

@@ -245,9 +245,12 @@ def test_the_format_asks_git_for_the_hash_and_the_body(self):
         self.assertEqual(main.COMMIT_LOG_FORMAT, "--pretty=format:%H%x00%B%x00")
         self.assertNotIn("\x00", main.COMMIT_LOG_FORMAT)
 
-    def test_an_empty_message_does_not_shift_the_hashes_after_it(self):
+    def test_an_empty_message_is_kept_and_does_not_shift_the_hashes(self):
         output = f"{SHA_A}\x00\n\x00\n{SHA_B}\x00feat: second\n\x00"
-        self.assertEqual(main.parse_commit_messages(output), [(SHA_B, "feat: second")])
+        self.assertEqual(
+            main.parse_commit_messages(output),
+            [(SHA_A, ""), (SHA_B, "feat: second")],
+        )
 
     def test_empty_output_is_no_commits(self):
         self.assertEqual(main.parse_commit_messages(""), [])
@@ -4186,3 +4189,35 @@ def test_comment_client_uses_github_api_url(self):
             main.add_pr_comments([fail_scope()])
         _, kwargs = github_module.Github.call_args
         self.assertEqual(kwargs["base_url"], "https://ghe.example.com/api/v3")
+
+
+class TestEmptyCommitMessage(unittest.TestCase):
+    """An empty message is checked as empty, never as HEAD's message."""
+
+    def test_an_empty_message_goes_to_the_cli_as_an_empty_file(self):
+        seen: list[tuple[list[str], str | None, str]] = []
+
+        def run(command, **kwargs):
+            path = command[-1]
+            with open(path, encoding="utf-8") as f:
+                seen.append((command, kwargs.get("input"), f.read()))
+            return MagicMock(
+                returncode=1,
+                stdout=json_output(make_check("allow_empty_commits", status="fail")),
+                stderr="",
+            )
+
+        with (
+            patch("main.subprocess.run", side_effect=run),
+            patch("main._local_author", return_value=None),
+        ):
+            results = main.run_pr_message_checks([("a1", "")])
+        command, piped, content = seen[0]
+        self.assertEqual(command[:4], ["commit-check", "--format", "json", "--message"])
+        self.assertIsNone(piped)
+        self.assertEqual(content, "")
+        self.assertFalse(os.path.exists(command[-1]))
+        self.assertEqual(
+            [(s.label, s.sha, s.status) for s in results],
+            [("Commit 1/1", "a1", "fail")],
+        )