fix(ci): serialize mutation-testing runs globally, not per-branch - #1049
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…-ref The concurrency group was scoped to github.ref, which only stops the SAME branch from running two overlapping mutation jobs -- it does nothing to stop main and several PR branches each spinning up their own full 8-shard run at once. Confirmed directly: four simultaneous mutation runs across main and three PR branches starved the shared runner pool of capacity every other workflow needs, leaving every open PR's required checks (and main's own post-merge CI) queued with zero progress for over an hour. A single fixed group across every ref queues a superseding run behind whichever one is already active instead of running it at all -- exactly what a non-required, delay-tolerant job like this one should do. cancel-in-progress stays false: this is now a queue, not a per-ref supersession, so there is nothing stale to cancel -- every queued run still executes eventually, just serialised.
Mearman
marked this pull request as ready for review
September 6, 2026 20:48
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The mutation-testing workflow's concurrency group was scoped to `github.ref`, which only stops the same branch from running two overlapping mutation jobs. It does nothing to stop main and several PR branches each spinning up their own full 8-shard run at the same time.
Confirmed directly today: four simultaneous mutation runs (main plus three PR branches) starved the shared runner pool of capacity every other workflow needs, leaving every open PR's required checks -- and main's own post-merge CI -- queued with zero progress for over an hour, even though GitHub's status page showed no incident.
Fix: a single fixed concurrency group across every ref, so a new mutation run queues behind whichever one is already active instead of running alongside it. `cancel-in-progress` stays false -- this is a queue, not a per-ref supersession, so every run still executes eventually, just serialised. Mutation testing is deliberately not a required check, so the delay costs nothing merge-blocking currently depends on.