#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# ===----------------------------------------------------------------------===##
#
# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
# See https://llvm.org/LICENSE.txt for license information.
# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#
# ===----------------------------------------------------------------------===##
"""Checks for reverts of commits across a given git commit.

To clarify the meaning of 'across' with an example, if we had the following
commit history (where `a -> b` notes that `b` is a direct child of `a`):

123abc -> 223abc -> 323abc -> 423abc -> 523abc

And where 423abc is a revert of 223abc, this revert is considered to be 'across'
323abc. More generally, a revert A of a parent commit B is considered to be
'across' a commit C if C is a parent of A and B is a parent of C.

Please note that revert detection in general is really difficult, since merge
conflicts/etc always introduce _some_ amount of fuzziness. This script just
uses a bundle of heuristics, and is bound to ignore / incorrectly flag some
reverts. The hope is that it'll easily catch the vast majority (>90%) of them,
though.

This is designed to be used in one of two ways: an import in Python, or run
directly from a shell. If you want to import this, the `find_reverts`
function is the thing to look at. If you'd rather use this from a shell, have a
usage example:

```
./revert_checker.py c47f97169 origin/main origin/release/12.x
```

This checks for all reverts from the tip of origin/main to c47f97169, which are
across the latter. It then does the same for origin/release/12.x to c47f97169.
Duplicate reverts discovered when walking both roots (origin/main and
origin/release/12.x) are deduplicated in output.
"""

import argparse
import collections
import logging
import re
import subprocess
import sys
from typing import Generator, List, NamedTuple, Iterable

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.
#
# 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.


def _try_parse_reverts_from_commit_message(commit_message: str) -> List[str]:
    if not commit_message:
        return []

    results = 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


def _stream_stdout(command: List[str]) -> Generator[str, None, None]:
    with subprocess.Popen(
        command, stdout=subprocess.PIPE, encoding="utf-8", errors="replace"
    ) as p:
        assert p.stdout is not None  # for mypy's happiness.
        yield from p.stdout


def _resolve_sha(git_dir: str, sha: str) -> str:
    if len(sha) == 40:
        return sha

    return subprocess.check_output(
        ["git", "-C", git_dir, "rev-parse", sha],
        encoding="utf-8",
        stderr=subprocess.DEVNULL,
    ).strip()


_LogEntry = NamedTuple(
    "_LogEntry",
    [
        ("sha", str),
        ("commit_message", str),
    ],
)


def _log_stream(git_dir: str, root_sha: str, end_at_sha: str) -> Iterable[_LogEntry]:
    sep = 50 * "<>"
    log_command = [
        "git",
        "-C",
        git_dir,
        "log",
        "^" + end_at_sha,
        root_sha,
        "--format=" + sep + "%n%H%n%B%n",
    ]

    stdout_stream = iter(_stream_stdout(log_command))

    # Find the next separator line. If there's nothing to log, it may not exist.
    # It might not be the first line if git feels complainy.
    found_commit_header = False
    for line in stdout_stream:
        if line.rstrip() == sep:
            found_commit_header = True
            break

    while found_commit_header:
        sha = next(stdout_stream, None)
        assert sha is not None, "git died?"
        sha = sha.rstrip()

        commit_message = []

        found_commit_header = False
        for line in stdout_stream:
            line = line.rstrip()
            if line.rstrip() == sep:
                found_commit_header = True
                break
            commit_message.append(line)

        yield _LogEntry(sha, "\n".join(commit_message).rstrip())


def _shas_between(git_dir: str, base_ref: str, head_ref: str) -> Iterable[str]:
    rev_list = [
        "git",
        "-C",
        git_dir,
        "rev-list",
        "--first-parent",
        f"{base_ref}..{head_ref}",
    ]
    return (x.strip() for x in _stream_stdout(rev_list))


def _rev_parse(git_dir: str, ref: str) -> str:
    return subprocess.check_output(
        ["git", "-C", git_dir, "rev-parse", ref],
        encoding="utf-8",
    ).strip()


Revert = NamedTuple(
    "Revert",
    [
        ("sha", str),
        ("reverted_sha", str),
    ],
)


def _find_common_parent_commit(git_dir: str, ref_a: str, ref_b: str) -> str:
    """Finds the closest common parent commit between `ref_a` and `ref_b`."""
    return subprocess.check_output(
        ["git", "-C", git_dir, "merge-base", ref_a, ref_b],
        encoding="utf-8",
    ).strip()


def find_reverts(git_dir: str, across_ref: str, root: str) -> List[Revert]:
    """Finds reverts across `across_ref` in `git_dir`, starting from `root`.

    These reverts are returned in order of oldest reverts first.
    """
    across_sha = _rev_parse(git_dir, across_ref)
    root_sha = _rev_parse(git_dir, root)

    common_ancestor = _find_common_parent_commit(git_dir, across_sha, root_sha)
    if common_ancestor != across_sha:
        raise ValueError(
            f"{across_sha} isn't an ancestor of {root_sha} "
            "(common ancestor: {common_ancestor})"
        )

    intermediate_commits = set(_shas_between(git_dir, across_sha, root_sha))
    assert across_sha not in intermediate_commits

    logging.debug(
        "%d commits appear between %s and %s",
        len(intermediate_commits),
        across_sha,
        root_sha,
    )

    all_reverts = []
    for sha, commit_message in _log_stream(git_dir, root_sha, across_sha):
        reverts = _try_parse_reverts_from_commit_message(commit_message)
        if not reverts:
            continue

        resolved_reverts = sorted(set(_resolve_sha(git_dir, x) for x in reverts))
        for reverted_sha in resolved_reverts:
            if reverted_sha in intermediate_commits:
                logging.debug(
                    "Commit %s reverts %s, which happened after %s",
                    sha,
                    reverted_sha,
                    across_sha,
                )
                continue

            try:
                object_type = subprocess.check_output(
                    ["git", "-C", git_dir, "cat-file", "-t", reverted_sha],
                    encoding="utf-8",
                    stderr=subprocess.DEVNULL,
                ).strip()
            except subprocess.CalledProcessError:
                logging.warning(
                    "Failed to resolve reverted object %s (claimed to be reverted "
                    "by sha %s)",
                    reverted_sha,
                    sha,
                )
                continue

            if object_type == "commit":
                all_reverts.append(Revert(sha, reverted_sha))
                continue

            logging.error(
                "%s claims to revert %s -- which isn't a commit -- %s",
                sha,
                object_type,
                reverted_sha,
            )

    # Since `all_reverts` contains reverts in log order (e.g., newer comes before
    # older), we need to reverse this to keep with our guarantee of older =
    # earlier in the result.
    all_reverts.reverse()
    return all_reverts


def _main() -> None:
    parser = argparse.ArgumentParser(
        description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter
    )
    parser.add_argument("base_ref", help="Git ref or sha to check for reverts around.")
    parser.add_argument("-C", "--git_dir", default=".", help="Git directory to use.")
    parser.add_argument("root", nargs="+", help="Root(s) to search for commits from.")
    parser.add_argument("--debug", action="store_true")
    parser.add_argument(
        "-u",
        "--review_url",
        action="store_true",
        help="Format SHAs as llvm review URLs",
    )
    opts = parser.parse_args()

    logging.basicConfig(
        format="%(asctime)s: %(levelname)s: %(filename)s:%(lineno)d: %(message)s",
        level=logging.DEBUG if opts.debug else logging.INFO,
    )

    # `root`s can have related history, so we want to filter duplicate commits
    # out. The overwhelmingly common case is also to have one root, and it's way
    # easier to reason about output that comes in an order that's meaningful to
    # git.
    seen_reverts = set()
    all_reverts = []
    for root in opts.root:
        for revert in find_reverts(opts.git_dir, opts.base_ref, root):
            if revert not in seen_reverts:
                seen_reverts.add(revert)
                all_reverts.append(revert)

    for revert in all_reverts:
        sha_fmt = (
            f"https://reviews.llvm.org/rG{revert.sha}"
            if opts.review_url
            else revert.sha
        )
        reverted_sha_fmt = (
            f"https://reviews.llvm.org/rG{revert.reverted_sha}"
            if opts.review_url
            else revert.reverted_sha
        )
        print(f"{sha_fmt} claims to revert {reverted_sha_fmt}")


if __name__ == "__main__":
    _main()