Fix sync client dropping lines#960
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@simolus3 Do you know when the issue has been introduced? Just wondering if it has always been present if the Rust sync client was used, or if it was a regression introduced at some point. |
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stevensJourney
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May 13, 2026
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The isolated demos check fails because of pnpm11. There's an open PR for that, I'll merge this with failing checks because we likely want to release this first. |
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May 14, 2026
Backports powersync-ja/powersync-js#960. Without it, ~1 in 3 local writes triggers a checksum mismatch and a full bucket wipe (see #954). patch-package applies the diff on every install so CI ships it too; remove once a release with the upstream fix is published.
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A race condition in
injectable(used by the Rust sync client to forward sync lines along with local events like a completed upload) could cause sync lines to be ignored. In particular, a local write + sync caused events where thedataline in acheckpoint_diff,data,checkpoint_completegot dropped, causing checksum verification errors.The issue is caused by this sequence of events:
injectablestream, causing it to fetch a sync line from the source upstream (waiteris set,pendingSourceEventis null).inject, making the initial event complete (waiteris now null).next()again.pendingSourceEventis still null because the initial source event has not completed, so we try to callsource.next()again. Async iterators aren't meant to be used concurrently like this, andresolveWaitermay loose events since there is only one slot for a pending source event once the two complete.The fix is to not call
source.next()again, I've manually verified this in Steven's convex demo where this reproduces consistently.Product visibility
This doesn't change anything that needs to be documented, but I'm adding the label due to the severity of the issue and in case we get addititional reports for this.
AI usage disclaimer
The issue was found by Claude after manual testing; it also generated the fix and test. I've manually verified the fix in the convex demo.
Closes #954.