fix HCLRevision mismatch on zero logical clock #2122
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While developing the test-case for `EmitImmediatelyStrategy` with CRDB, I detected something I wasn't expecting: when comparing the revision out of the datastore write, and what came out of Watch API, the revisions were different. We use `SHOW COMMIT TIMESTAMP` to retrieve a CRDB [transaction timestamp] (cockroachdb/cockroach#80848). The value coming out of the `update` field in change streams had the same value, but the difference was the notation: one included the logical clock, the other didn't, but logical clocks were the same (zero): - revision generated out of the transaction in `readTransactionCommitRev` uses `NewForHLC(hlcNow)`, which takes a `Decimal`. This in turn calls `decimal.String()`, which returns a number stripped out of the decimal part if it's zero. - revision obtained out of the changefeeds uses `revisions.HLCRevisionFromString (details.Updated)`, and `details.Updated` always comes with the decimal part, even when it's zero Both timestamps were the same, but had a different string representation, and led to a different `HCLRevision` logical lock, hence `.Equal()` method failing.
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we don't expect it to be possible to have CRDB generate 4B conflicts
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While working on #2120, I detected something I wasn't expecting: when comparing the revision out of the datastore write, and what came out of Watch API, the revisions were different.
We use
SHOW COMMIT TIMESTAMPto retrieve a CRDB transaction timestamp. The value coming out of theupdatefield in change streams had the same value, but the difference was the notation: one included the logical clock, the other didn't, but logical clocks were the same (zero):readTransactionCommitRevusesNewForHLC(hlcNow), which takes aDecimal. This in turn callsdecimal.String(), which returns a number stripped out of the decimal part if it's zero.revisions.HLCRevisionFromString(details.Updated), anddetails.Updatedalways comes with the decimal part, even when it's zeroBoth timestamps were the same, but had a different string representation, and led to a different
HCLRevisionlogical lock, hence.Equal()method failing.The problem originates in the use of the offset to represent the logical part of the clock. The representation of "zero" wasn't being used consistently, and in some parts of the code the revision was initialized with logical clock of zero, when it needed the offset padding.
HLCRevision, that is, which only applies to CRDB). This means caches that append the revision to keys will be longer.