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The scoped Windows lane reached exactly 15m05s and was cancelled on four consecutive shas of PR #3094. A job that exceeds timeout-minutes reports as CANCELLED rather than FAILURE, which is why it first read as infrastructure noise; the giveaway is that the duration equals the cap. The Required tests rollup fans that job in, so it red-blocked merge while every other lane — including all three sharded full-Windows shards — was green. The trigger was a change to the shared test helper, which scopes the install-heavy suites into the selected list. The lane normally runs about eight minutes; with that list it does not fit. It was the only unsharded lane left in this job, so it was the only one without headroom to absorb a large scoped list. Issue #869 hit this exact cliff on the sibling lane and named the durable answer in its own follow-up: a timeout bump moves the cliff, sharding removes it. #2952 then sharded the full lane. This finishes that work. The runner already supports it — the shard partition is applied after scope selection, so it composes with a selected file list rather than only with a suite, and the partition is cost-weighted from the measured timings table. The job name template already renders a shard suffix when one is present, so the three entries name themselves. No individual matrix job is a required status check; the rollup is, and it is name-independent, so renaming these jobs does not touch branch protection. timeout-minutes stays at 15. Each shard now does roughly a third of the work, so the cap goes from binding to backstop without being raised. The lane-shape tests were generalized rather than relaxed: the complete-shard-set invariant now runs per sharded scope instead of only over the full lane, and "only the full lane is sharded" became "targeted is the only unsharded lane". A new assertion pins the shard through to the runner — without it the three shards would each run the entire selected list, triple the cost and no speedup, and every check would stay green. No LANE_COSTS entry is added for the new shards. The only recorded cost for that lane is the pre-sharding run that hit the cap, and inventing a post-sharding number would be exactly the kind of unmeasured claim the rest of that table avoids. The estimate and the reason are written down instead, to be replaced by a real measurement. Refs #3057 Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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