fix(taskdump): propagate Ready from the capture re-poll#683
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To sample a task-dump backtrace, TaskDumped polls the inner future a second time and discarded the result. On a multi-threaded runtime that re-poll can legitimately return Ready (a resource the future is parked on is readied by another thread between the primary poll and the capture re-poll). The completion was dropped and Pending returned to the caller; the pending wake then reschedules the task and the completed future is polled again — a poll-after-Ready contract violation. It panics (e.g. "async fn resumed after completion", or "Map must not be polled after it returned Poll::Ready") and, under panic=abort, aborts the process. just_captured (#375) guards the capture, not the primary poll, so it does not prevent this. Make FrameBuf::capture return the re-poll's Poll, and have TaskDumped::poll adopt a Ready (running the same cleanup as the Ready arm and returning it) so the future is never polled after completion. Adds a regression test that spawns a future which completes on its second poll (the capture re-poll); it panicked before this fix and passes now. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> (cherry picked from commit d913c63)
rcoh
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July 18, 2026 18:23
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To sample a task-dump backtrace, TaskDumped polls the inner future a second time and discarded the result. On a multi-threaded runtime that re-poll can legitimately return Ready (a resource the future is parked on is readied by another thread between the primary poll and the capture re-poll). The completion was dropped and Pending returned to the caller; the pending wake then reschedules the task and the completed future is polled again — a poll-after-Ready contract violation. It panics (e.g. "async fn resumed after completion", or "Map must not be polled after it returned Poll::Ready") and, under panic=abort, aborts the process.
just_captured (#375) guards the capture, not the primary poll, so it does not prevent this.
Make FrameBuf::capture return the re-poll's Poll, and have TaskDumped::poll adopt a Ready (running the same cleanup as the Ready arm and returning it) so the future is never polled after completion.
Adds a regression test that spawns a future which completes on its second poll (the capture re-poll); it panicked before this fix and passes now.
(cherry picked from commit d913c63)