[utils] support "Reverts ${PR}" in commit messages (#112226)

A bisection in ChromeOS ended at a reverted commit, which wasn't flagged
by this revert checking script, since it used `Reverts ${PR}` rather
than `This reverts commit ${SHA}`.

`grep` says that somewhere around 400 reverts in the last year have used
`Reverts ${PR}` syntax. Support it.

Tested in part by running the command that was expected to catch this
revert:

```
$ ./revert_checker.py -C ~/llvm/main/ \
    3b5e7c8 origin/main \
    | grep -q 4b0276d
$ echo $?
0
```

Author: gburgessiv

llvm/utils/revert_checker.py

@@ -45,35 +45,78 @@
 import re
 import subprocess
 import sys
-from typing import Generator, List, NamedTuple, Iterable
+from typing import Dict, Generator, Iterable, List, NamedTuple, Optional, Tuple
 
 assert sys.version_info >= (3, 6), "Only Python 3.6+ is supported."
 
 # People are creative with their reverts, and heuristics are a bit difficult.
-# Like 90% of of reverts have "This reverts commit ${full_sha}".
-# Some lack that entirely, while others have many of them specified in ad-hoc
-# ways, while others use short SHAs and whatever.
+# At a glance, most reverts have "This reverts commit ${full_sha}". Many others
+# have `Reverts llvm/llvm-project#${PR_NUMBER}`.
 #
-# The 90% case is trivial to handle (and 100% free + automatic). The extra 10%
-# starts involving human intervention, which is probably not worth it for now.
+# By their powers combined, we should be able to automatically catch something
+# like 80% of reverts with reasonable confidence. At some point, human
+# intervention will always be required (e.g., I saw
+# ```
+# This reverts commit ${commit_sha_1} and
+# also ${commit_sha_2_shorthand}
+# ```
+# during my sample)
+
+_CommitMessageReverts = NamedTuple(
+    "_CommitMessageReverts",
+    [
+        ("potential_shas", List[str]),
+        ("potential_pr_numbers", List[int]),
+    ],
+)
+
 
+def _try_parse_reverts_from_commit_message(
+    commit_message: str,
+) -> _CommitMessageReverts:
+    """Tries to parse revert SHAs and LLVM PR numbers form the commit message.
 
-def _try_parse_reverts_from_commit_message(commit_message: str) -> List[str]:
+    Returns:
+        A namedtuple containing:
+        - A list of potentially reverted SHAs
+        - A list of potentially reverted LLVM PR numbers
+    """
     if not commit_message:
-        return []
+        return _CommitMessageReverts([], [])
 
-    results = re.findall(r"This reverts commit ([a-f0-9]{40})\b", commit_message)
+    sha_reverts = re.findall(
+        r"This reverts commit ([a-f0-9]{40})\b",
+        commit_message,
+    )
 
     first_line = commit_message.splitlines()[0]
     initial_revert = re.match(r'Revert ([a-f0-9]{6,}) "', first_line)
     if initial_revert:
-        results.append(initial_revert.group(1))
-    return results
+        sha_reverts.append(initial_revert.group(1))
 
+    pr_numbers = [
+        int(x)
+        for x in re.findall(
+            r"Reverts llvm/llvm-project#(\d+)",
+            commit_message,
+        )
+    ]
+
+    return _CommitMessageReverts(
+        potential_shas=sha_reverts,
+        potential_pr_numbers=pr_numbers,
+    )
 
-def _stream_stdout(command: List[str]) -> Generator[str, None, None]:
+
+def _stream_stdout(
+    command: List[str], cwd: Optional[str] = None
+) -> Generator[str, None, None]:
     with subprocess.Popen(
-        command, stdout=subprocess.PIPE, encoding="utf-8", errors="replace"
+        command,
+        cwd=cwd,
+        stdout=subprocess.PIPE,
+        encoding="utf-8",
+        errors="replace",
     ) as p:
         assert p.stdout is not None  # for mypy's happiness.
         yield from p.stdout
@@ -175,10 +218,43 @@ def _find_common_parent_commit(git_dir: str, ref_a: str, ref_b: str) -> str:
     ).strip()
 
 
-def find_reverts(git_dir: str, across_ref: str, root: str) -> List[Revert]:
+def _load_pr_commit_mappings(
+    git_dir: str, root: str, min_ref: str
+) -> Dict[int, List[str]]:
+    git_log = ["git", "log", "--format=%H %s", f"{min_ref}..{root}"]
+    results = collections.defaultdict(list)
+    pr_regex = re.compile(r"\s\(#(\d+)\)$")
+    for line in _stream_stdout(git_log, cwd=git_dir):
+        m = pr_regex.search(line)
+        if not m:
+            continue
+
+        pr_number = int(m.group(1))
+        sha = line.split(None, 1)[0]
+        # N.B., these are kept in log (read: reverse chronological) order,
+        # which is what's expected by `find_reverts`.
+        results[pr_number].append(sha)
+    return results
+
+
+# N.B., max_pr_lookback's default of 20K commits is arbitrary, but should be
+# enough for the 99% case of reverts: rarely should someone land a cleanish
+# revert of a >6 month old change...
+def find_reverts(
+    git_dir: str, across_ref: str, root: str, max_pr_lookback: int = 20000
+) -> List[Revert]:
     """Finds reverts across `across_ref` in `git_dir`, starting from `root`.
 
     These reverts are returned in order of oldest reverts first.
+
+    Args:
+        git_dir: git directory to find reverts in.
+        across_ref: the ref to find reverts across.
+        root: the 'main' ref to look for reverts on.
+        max_pr_lookback: this function uses heuristics to map PR numbers to
+            SHAs. These heuristics require that commit history from `root` to
+            `some_parent_of_root` is loaded in memory. `max_pr_lookback` is how
+            many commits behind `across_ref` should be loaded in memory.
     """
     across_sha = _rev_parse(git_dir, across_ref)
     root_sha = _rev_parse(git_dir, root)
@@ -201,8 +277,41 @@ def find_reverts(git_dir: str, across_ref: str, root: str) -> List[Revert]:
     )
 
     all_reverts = []
+    # Lazily load PR <-> commit mappings, since it can be expensive.
+    pr_commit_mappings = None
     for sha, commit_message in _log_stream(git_dir, root_sha, across_sha):
-        reverts = _try_parse_reverts_from_commit_message(commit_message)
+        reverts, pr_reverts = _try_parse_reverts_from_commit_message(
+            commit_message,
+        )
+        if pr_reverts:
+            if pr_commit_mappings is None:
+                logging.info(
+                    "Loading PR <-> commit mappings. This may take a moment..."
+                )
+                pr_commit_mappings = _load_pr_commit_mappings(
+                    git_dir, root_sha, f"{across_sha}~{max_pr_lookback}"
+                )
+                logging.info(
+                    "Loaded %d PR <-> commit mappings", len(pr_commit_mappings)
+                )
+
+            for reverted_pr_number in pr_reverts:
+                reverted_shas = pr_commit_mappings.get(reverted_pr_number)
+                if not reverted_shas:
+                    logging.warning(
+                        "No SHAs for reverted PR %d (commit %s)",
+                        reverted_pr_number,
+                        sha,
+                    )
+                    continue
+                logging.debug(
+                    "Inferred SHAs %s for reverted PR %d (commit %s)",
+                    reverted_shas,
+                    reverted_pr_number,
+                    sha,
+                )
+                reverts.extend(reverted_shas)
+
         if not reverts:
             continue
 

llvm/utils/revert_checker_test.py

@@ -96,6 +96,7 @@ def test_reverted_noncommit_object_is_a_nop(self) -> None:
             git_dir=get_llvm_project_path(),
             across_ref="c9944df916e41b1014dff5f6f75d52297b48ecdc~",
             root="c9944df916e41b1014dff5f6f75d52297b48ecdc",
+            max_pr_lookback=50,
         )
         self.assertEqual(reverts, [])
 
@@ -113,6 +114,7 @@ def test_known_reverts_across_arbitrary_llvm_rev(self) -> None:
             git_dir=get_llvm_project_path(),
             across_ref="c47f971694be0159ffddfee8a75ae515eba91439",
             root="9f981e9adf9c8d29bb80306daf08d2770263ade6",
+            max_pr_lookback=50,
         )
         self.assertEqual(
             reverts,
@@ -128,6 +130,27 @@ def test_known_reverts_across_arbitrary_llvm_rev(self) -> None:
             ],
         )
 
+    def test_pr_based_revert_works(self) -> None:
+        reverts = revert_checker.find_reverts(
+            git_dir=get_llvm_project_path(),
+            # This SHA is a direct child of the reverted SHA expected below.
+            across_ref="2d5f3b0a61fb171617012a2c3ba05fd31fb3bb1d",
+            # This SHA is a direct child of the revert SHA listed below.
+            root="2c01b278580212914ec037bb5dd9b73702dfe7f1",
+            max_pr_lookback=50,
+        )
+        self.assertEqual(
+            reverts,
+            [
+                revert_checker.Revert(
+                    # This SHA is a `Reverts ${PR}` for #111004.
+                    sha="50866e84d1da8462aeb96607bf6d9e5bbd5869c5",
+                    # ...And this was the commit for #111004.
+                    reverted_sha="67160c5ab5f5b7fd5fa7851abcfde367c8a9f91b",
+                ),
+            ],
+        )
+
 
 if __name__ == "__main__":
     unittest.main()