test: fix flaky test_background_task_runs_and_exits near the exit tick#216
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The background-shell poll helper (_poll_output) returned the instant it saw status=='exited', but shell_task_output reads are incremental and the final output chunk can still be draining into the task buffer on the same tick the status flips to exited. Under load the first post-exit read returns empty and the helper returns acc='', failing 'quick_done' in acc. This is a harness race, not a shell bug (the incremental-read contract is correct). Keep reading until a read yields nothing new (bounded 2s) after the terminal status so the tail is captured deterministically. Verified 15/15.
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_poll_outputreturns the moment it seesstatus == "exited", butshell_task_outputreads are incremental and the final output chunk can still be draining into the task buffer on the same tick the status flips to exited. Under load the first post-exit read returns empty, so the helper returnsacc == ""and the test fails with'quick_done' not in ''.This is a harness race, not a shell bug — the incremental-read contract is correct; the poll helper just returned before the tail was drained. The fix keeps reading until a read yields nothing new (bounded 2s) after the terminal status, so the final output is captured deterministically. The existing
again["output"] == ""assertion still holds (the drain leaves nothing buffered). Verified 15/15 in a loop.