kv: compare MVCC GC threshold against Refresh{Range}Request.RefreshFrom#75624
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Noticed by Sumeer in cockroachdb#74628. A Refresh request needs to observe all MVCC versions between its exclusive RefreshFrom time and its inclusive RefreshTo time. If it were to permit MVCC GC between these times then it could miss conflicts that should cause the refresh to fail. This could in turn lead to violations of serializability. For example: ``` txn1 reads value k1@10 txn2 deletes (tombstones) k1@15 mvcc gc @ 20 clears versions k1@10 and k1@15 txn1 refreshes @ 25, sees no value between (10, 25], refresh successful ``` In the example, the refresh erroneously succeeds because the request is permitted to evaluate after part of the MVCC history it needs to read has been GCed. By considering the RefreshFrom time to be the earliest active timestamp of the request, we avoid this hazard. Instead of being allowed to evaluate, the refresh request in the example would have hit a BatchTimestampBeforeGCError.
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Noticed by Sumeer in #74628.
A Refresh request needs to observe all MVCC versions between its
exclusive RefreshFrom time and its inclusive RefreshTo time. If it were
to permit MVCC GC between these times then it could miss conflicts that
should cause the refresh to fail. This could in turn lead to violations
of serializability. For example:
In the example, the refresh erroneously succeeds because the request is
permitted to evaluate after part of the MVCC history it needs to read
has been GCed. By considering the RefreshFrom time to be the earliest
active timestamp of the request, we avoid this hazard. Instead of being
allowed to evaluate, the refresh request in the example would have hit
a BatchTimestampBeforeGCError.