REP-6873 Handle nonempty initial change stream #183
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tdq45gj
reviewed
Dec 5, 2025
| if !hasSavedToken { | ||
| // Usually the change stream’s initial response is empty, but sometimes | ||
| // there are events right away. We can discard those events because | ||
| // they’ve already happened, and our initial scan is yet to come. |
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Am I understanding correctly that it is safe because the initial scan enforces afterClusterTime that is after the resume token timestamp?
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Correct. If there was no saved resume token, then the time that this function returns gets used as the find queries’ initial afterClusterTime.
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Verifier has historically assumed that a change stream’s initial response will be empty (save for the resume token), but that’s not always the case.
When a nonempty initial response arrives, Verifier has been crashing because the driver’s ResumeToken() method has returned nil.
This changeset fixes Verifier to anticipate a nonempty initial response.
A generic cursor-batch-reader is added to mmongo to facilitate this. Also, the log about the start timestamp now more clearly distinguishes when the cluster time predates the resume token’s timestamp.