ci: pull the base images at once

.github/scripts/mirror_base_images.py

@@ -9,7 +9,7 @@
 are also named in containerd, whose images Docker reads only on the containerd image store. Switching dockerd to it
 restarts dockerd, which stops the containers it runs, so this runs before any start.
 
-``mirror_base_images.py PATH...`` does this for the Dockerfiles at or under each PATH.
+``mirror_base_images.py PATH...`` does this for the Dockerfiles at or under each PATH, pulling their bases at once.
 """
 
 from __future__ import annotations
@@ -19,6 +19,7 @@
 import subprocess
 import sys
 import time
+from concurrent.futures import ThreadPoolExecutor
 from pathlib import Path
 
 ECR = "public.ecr.aws/docker/library/"
@@ -33,8 +34,8 @@ def main(argv: list[str]) -> int:
     bases = _bases([Path(arg) for arg in argv])
     if any("@" in base for base in bases):
         _use_containerd_store()
-    for base in bases:
-        _pull(base)
+    with ThreadPoolExecutor(max_workers=max(len(bases), 1)) as pool:
+        list(pool.map(_pull, bases))
     return 0
 
 

tst/unit/test_mirror_base_images_script.py

@@ -7,6 +7,7 @@
 import json
 import subprocess
 import sys
+import threading
 
 import pytest
 
@@ -139,3 +140,22 @@ def test_a_pull_that_fails_once_is_tried_again(script, monkeypatch, tmp_path):
 
     assert ran[-1] == ["docker", "tag", "mirror.gcr.io/library/postgres:16-alpine", _TAGGED]
     assert ["sleep", 10] in ran and ["sleep", 20] not in ran
+
+
+def test_the_bases_are_pulled_at_once(script, monkeypatch, tmp_path):
+    ran = _docker(monkeypatch, script)
+    fake = script.subprocess.run
+    both_pulling = threading.Barrier(2, timeout=10)
+
+    def run(argv, **kwargs):
+        if argv[:2] == ["docker", "pull"]:
+            both_pulling.wait()
+        return fake(argv, **kwargs)
+
+    monkeypatch.setattr(script.subprocess, "run", run)
+    dockerfile = tmp_path / "Dockerfile"
+    dockerfile.write_text(f"FROM {_TAGGED}\nFROM public.ecr.aws/docker/library/busybox:1.37\n")
+
+    assert script.main([str(dockerfile)]) == 0
+
+    assert sorted(argv[1] for argv in ran) == ["pull", "pull", "tag", "tag"]