Skip build when nothing changed. - #5157
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`BuildSeries.run` always ran a build, even when the scan of changed files found nothing that could affect any output. A file can be written without changing its content, and one logical write can produce more than one filesystem watch event, so `watch` and `serve` hit this case routinely and respond with a build that reports `wrote 0 outputs`. Move `buildLog.nextBuild` after `updateForFileChanges` so the decision to build is taken once the scan has run, and return the previous result when the updated plan has no work. Only a successful previous result can be reused. The scan answers whether anything changed since the last build, not whether the last result is still valid; after a failure those differ, and under `--only-check` a restored output would never be re-verified. The updated plan is kept rather than discarded, so the content read during the scan is what the next change is compared against.
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September 22, 2026 12:09
jensjoha
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BuildSeries.runalways ran a build, even when the scan of changed files found nothing that could affect any output. A file can be written without changing its content, and one logical write can produce more than one filesystem watch event, sowatchandservehit this case routinely and respond with a build that reportswrote 0 outputs.Move
buildLog.nextBuildafterupdateForFileChangesso the decision to build is taken once the scan has run, and return the previous result when the updated plan has no work.Only a successful previous result can be reused. The scan answers whether anything changed since the last build, not whether the last result is still valid; after a failure those differ, and under
--only-checka restored output would never be re-verified.The updated plan is kept rather than discarded, so the content read during the scan is what the next change is compared against.
Fixes some test flakes that happened under load.