Document the ~0.3% rotating `trio.TooSlowError` flake under `--spawn-backend=main_thread_forkserver` full-suite runs. Root cause: `hard_kill`'s per-sub 1.6s graceful timeout compounding across N subactors in a cancel cascade, plus cumulative autouse-reaper teardown overhead. Covers symptom, observed flaking tests, root-cause family, ranked mitigations (cap bump -> CPU-count- aware cap -> `pytest-rerunfailures` -> `hard_kill` tuning -> targeted profiling), and a verification protocol. (this commit msg was generated in some part by [`claude-code`][claude-code-gh]) [claude-code-gh]: https://github.com/anthropics/claude-code |
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