#!/usr/bin/env python3
"""
Fetch the latest patchset of a Gerrit change via `git fetch`. This allows reading the patch locally
via e.g. `git show FETCH_HEAD`, even if it's not cherry-picked anywhere.

Usage: git-review-fetch <gerrit-change-url>

Example:
    git-review-fetch https://gerrit.collaboraoffice.com/c/online/+/1389
"""

import json
import re
import subprocess
import sys
import urllib.request


def parse_url(url):
    """Return (host_base, project, change_number) parsed from a Gerrit URL."""
    m = re.match(r"^(https?://[^/]+)/c/(.+?)/\+/(\d+)", url)
    if not m:
        sys.exit("Cannot parse Gerrit URL: " + url)
    return m.group(1), m.group(2), int(m.group(3))


def get_current_patchset(host_base, change_number):
    """Query Gerrit REST API for the current patchset number."""
    api_url = "%s/changes/%d?o=CURRENT_REVISION" % (host_base, change_number)
    req = urllib.request.Request(api_url, headers={"User-Agent": "git-review-fetch"})
    with urllib.request.urlopen(req) as resp:
        raw = resp.read().decode("utf-8")
    # Gerrit prefixes JSON responses with )]}'.
    if raw.startswith(")]}'"):
        raw = raw.split("\n", 1)[1]
    data = json.loads(raw)
    current_rev = data["current_revision"]
    return data["revisions"][current_rev]["_number"]


def build_ref(change_number, patchset):
    """refs/changes/XX/YYYY/N where XX is the last two digits of YYYY."""
    last_two = "%02d" % (change_number % 100)
    return "refs/changes/%s/%d/%d" % (last_two, change_number, patchset)


def main(argv):
    if len(argv) != 2:
        print(__doc__.strip(), file=sys.stderr)
        return 2
    host_base, project, change_number = parse_url(argv[1])
    patchset = get_current_patchset(host_base, change_number)
    remote = "%s/%s" % (host_base, project)
    ref = build_ref(change_number, patchset)
    print("Fetching patchset %d of change %d: %s %s" % (patchset, change_number, remote, ref))
    return subprocess.call(["git", "fetch", remote, ref])


if __name__ == "__main__":
    sys.exit(main(sys.argv))

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