[test] Fix flaky build CLI output capture - #97900
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Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
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sokra
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August 25, 2026 22:00
eps1lon
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What?
Stabilize
NextStartInstance.build()CLI output capture so failed production builds return all trailing stdout/stderr diagnostics.The existing
sync-io-blocks-rootbehavioral assertions remain unchanged, including the route-specificDate.now()diagnostic, timeout/retry guard, and exit-code check.Why?
The
--debug-prerendercases intermittently lost the detailedDate.now()diagnostic fromresult.cliOutputin CI even though builds exited normally. Across the reported CI run and two retries, the missing route set varied from 4 to 2 to 3 routes, consistent with an output-drain race rather than a rendering or resource failure.On latest canary with Node 20,
NEXT_TEST_MODE=start, andIS_WEBPACK_TEST=1, 5 baseline campaigns completed 24 builds (20 debug-prerender) without reproducing the intermittent failure locally. A controlled Node 20 reproduction established the lifecycle race directly: the childexitevent observed no trailing output, whilecloseobserved the trailing diagnostic.How?
Wait for the child process
closeevent before snapshottingcliOutput. Unlikeexit,closeis emitted after the piped stdout and stderr streams close, so diagnostics emitted near process shutdown are fully drained before the build result is returned.This is intentionally limited to the affected start-mode build harness and is independent of the mmap persistence PR stack (#97873, #97888, #97889).
Verification
exitcaptured empty output;closecaptured the trailing diagnosticpnpm build-alltest/lib/next-modes/next-start.tspnpm types