mkosi: add helper script to update mkosi hash

This is very similar to tools/fetch-distro.py. The idea is that we extend the
commit to update the mkosi hash with a git log --pretty=oneline output, so that
the reader can know what changes were actually included.

The motivation is that I'm always wondering what changed in mkosi when I see a
commit updating the hash, and it's nicer to have this information shown
directly in the commit.

The script does _not_ pull changes from upstream, on the assumption that the
person doing the commit always has a fresh checkout and that they tested with
that checkout.

tools/fetch-mkosi.py

@@ -0,0 +1,103 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+"""
+Check out mkosi into specified location.
+With -u, if changed, commit the latest hash.
+"""
+
+import argparse
+import shlex
+import subprocess
+import re
+from pathlib import Path
+
+URL = 'https://github.com/systemd/mkosi'
+BRANCH = 'main'  # We only want to ever use commits on upstream 'main' branch
+FILENAME = Path('.github/workflows/mkosi.yml')
+
+def parse_args():
+    p = argparse.ArgumentParser(
+        description=__doc__,
+    )
+    p.add_argument(
+        'dir',
+        type=Path,
+    )
+    p.add_argument(
+        '--update', '-u',
+        action='store_true',
+        default=False,
+    )
+    return p.parse_args()
+
+def read_config():
+    print(f'Reading {FILENAME}…')
+    matches = [m.group(1)
+               for line in open(FILENAME)
+               if (m := re.match('^- uses: systemd/mkosi@([a-z0-9]{40})$',
+                                 line.strip()))]
+    assert len(matches) == 1
+    return matches[0]
+
+def commit_file(args, file: Path, commit: str, changes: str):
+    cmd = [
+        'git', '-C', args.dir.as_posix(),
+        'describe',
+        '--always',
+        commit]
+    print(f"+ {shlex.join(cmd)}")
+    desc = subprocess.check_output(cmd, text=True).strip()
+
+    message = '\n'.join((
+        f'mkosi: update mkosi commit reference to {desc}',
+        '',
+        changes))
+
+    cmd = ['git', 'commit', '-m', message, file.as_posix()]
+    print(f"+ {shlex.join(cmd)}")
+    subprocess.check_call(cmd)
+
+def checkout_mkosi(args):
+    if args.dir.exists():
+        print(f'{args.dir} already exists.')
+        return
+
+    cmd = [
+        'git', 'clone', URL,
+        f'--branch={BRANCH}',
+        args.dir.as_posix(),
+    ]
+    print(f"+ {shlex.join(cmd)}")
+    subprocess.check_call(cmd)
+
+def update_mkosi(args):
+    old_commit = read_config()
+
+    cmd = ['git', '-C', args.dir.as_posix(), 'rev-parse', f'refs/remotes/origin/{BRANCH}']
+    print(f"+ {shlex.join(cmd)}")
+    new_commit = subprocess.check_output(cmd, text=True).strip()
+
+    if old_commit == new_commit:
+        print(f'mkosi: commit {new_commit!s} is still fresh')
+        return
+
+    cmd = ['git', '-C', args.dir.as_posix(), 'log', '--graph',
+           '--pretty=oneline', '--no-decorate', '--abbrev-commit', '--abbrev=10',
+           f'{old_commit}..{new_commit}']
+    print(f"+ {shlex.join(cmd)}")
+    changes = subprocess.check_output(cmd, text=True).strip()
+
+    s = FILENAME.read_text()
+    assert old_commit in s
+    print(f'mkosi: {FILENAME}: found old hash, updating…')
+    new = s.replace(old_commit, new_commit)
+    assert new != s
+    FILENAME.write_text(new)
+    commit_file(args, FILENAME, new_commit, changes)
+
+if __name__ == '__main__':
+    args = parse_args()
+    checkout_mkosi(args)
+    if args.update:
+        update_mkosi(args)