scripts: prepare for nightly and beta cut-off

To prevent unbounded size growth, we want to delete older nightly and beta
manifests from master branch regularly. Users of old nightly and beta
toolchains can still access from the `before/` tags, but these tags will
not receive patch updates anymore. Currently cut-off can only be
deployed manually.

Stable versions are not affected and are all kept currently.

scripts/cut_off_nightly_beta.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i python3 -p "python3.withPackages (ps: with ps; [ toml requests ])"
+from typing import Callable
+from fetch import update_nightly_index, update_beta_index
+from pathlib import Path
+import re
+import sys
+
+CHANNELS: list[tuple[Path, Callable[[], None]]] = [
+    (Path('manifests/nightly'), update_nightly_index),
+    (Path('manifests/beta'), update_beta_index),
+]
+RE_LATEST_DATE = re.compile('latest = "(.*?)"')
+
+def main():
+    args = sys.argv[1:]
+    if len(args) != 1:
+        print('''
+Usage:
+    {0} <year>
+        Delete all nightly and beta manifests before (excluding) <year>.
+''')
+        exit(1)
+
+    cut_year = int(args[0])
+    for (channel_root, update_index) in CHANNELS:
+        for year_dir in channel_root.iterdir():
+            if year_dir.is_dir() and int(year_dir.name) < cut_year:
+                print(f'deleting {year_dir}')
+                for ver in year_dir.iterdir():
+                    ver.unlink()
+                year_dir.rmdir()
+        def latest_ver():
+            src = (channel_root / 'default.nix').read_text()
+            m = RE_LATEST_DATE.search(src)
+            assert m is not None, f'No latest version:\n{src}'
+            return m[1]
+        before_latest =  latest_ver()
+        update_index()
+        after_latest =  latest_ver()
+        assert before_latest == after_latest, \
+            f'Latest version must not be affected: {before_latest} -> {after_latest}'
+
+if __name__ == '__main__':
+    main()