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Marc-André BombeckandClaude Opus 4.7
249c42bdd2
The v1.4.14 release exposed a hard hang: the `main` push and the
`v1.4.14` tag push both fired at SHA `e5fae9bce6cb` simultaneously.
The tag run finished cleanly in 17 minutes; the main run sat on
`#26 [linux/amd64 runner 2/14] RUN apk add tzdata` from 16:14:52Z
until Marc cancelled it at 17:00:15Z — 47 minutes of zero buildkit
output. Same SHA, same Dockerfile, same runner image, vastly
different outcome.
Root cause: both `cache-from: type=gha` and `cache-to: type=gha,mode=max`
used buildx's default scope name (`buildkit`). With `mode=max` every
stage of every platform is exported to the cache; the two concurrent
writers raced on the gha cache manifest and one deadlocked on the
exporter. This is the classic "shared buildkit cache between
parallel refs" failure mode — pinned down by the symmetry of
"same SHA, parallel runs, only the slower-to-arrive one hangs".
Fix is two layers:
1. Per-ref cache scope (`scope=build-${{ github.ref_name }}`). Each
workflow invocation now writes to its own slot:
- `build-main` for branch pushes
- `build-v1.4.x` for tag pushes
- `build-pr-N` for pull-request pushes
`cache-from` reads BOTH the ref-specific scope AND `build-main`,
so a tag built right after a main push still reuses main's
layers — just without contending on the writer side.
2. Job-level `timeout-minutes: 30`. The fastest healthy build of the
v1.4.7..v1.4.13 series finished in 12 min, the slowest in 18 min.
30 min covers a slow exporter while still rescuing the runner
automatically if buildkit deadlocks again, instead of waiting for
a human to notice and click Cancel an hour later.
Pass-rate impact in the v1.4.15 marathon: 30/60 successes over
30 days (most "cancelled" entries are by-design — concurrency group
cancels superseded runs — but the v1.4.14 hang counts as a
preventable outlier). After this commit a deadlocked run is bounded
to 30 min and a release commit's two refs no longer fight each other.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>