Since 22 October the E2E jobs often time out installing browsers before
any test runs: 29 installs failed that day and 30 on 23 October, against
none in the week before. Each job downloads about 119 MB of Ubuntu
packages for Chromium and WebKit from the runner's Azure mirror, and
some GitHub-hosted runners now get tens of kB/s from it
(actions/runner-images#14823 and #15040). The same package set on the
same image versions installed in under a minute before then.
The three browser jobs now restore the newest saved .deb set with
actions/cache/restore and admit it through scripts/browser_os_packages.py.
apt reuses a file already in its archive directory after checking only
its size, so the helper refreshes the index and installs a private copy
of a cached file only when that copy's SHA256 matches a signed index
stanza for the package, version and architecture its filename names.
After a successful install, and before any journey can fail the job,
the helper rebuilds the cache from apt's archives and the job saves it
under a run-unique key only when the set changed, so Ubuntu updates
refresh the cache and an empty or unchanged set is never saved.
The transport bounds and install budgets are unchanged, and the jobs
hold no secrets or write tokens, as the workflow trust contract requires
for caches.
rcourtman