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kvserver: narrow down 'finishing a proposal with outstanding reproposal'
In cockroachdb#94633, I introduced[^1] an assertion that attempted to catch cases in which we might otherwise accidentally end up applying a proposal twice. This assertion had a false positive. I was able to reproduce the failure within ~minutes via `./experiment.sh` in cockroachdb#97173 as of 33dcdef. Better testing of these cases would be desirable. Unfortunately, while there is an abstraction over command application (`apply.Task`), most of the logic worth testing lives in `(*replicaAppBatch)` which is essentially a `*Replica` with more moving parts attached. This does not lend itself well to unit testing. I had a run[^2][^3][^4] earlier this year to make log application standalone, but then didn't have enough time to follow through. It would be desirable to do so at a later date, perhaps with the explicit goals of having interactions like the one discussion in this PR unit become testable. [^4]: cockroachdb#93309 [^3]: cockroachdb#93266 [^2]: cockroachdb#93239 [^1]: https://github.com/cockroachdb/cockroach/pull/94633/files#diff-50e458584d176deae52b20a7c04461b3e4110795c8c9a307cf7ee6696ba6bc60R238 This assertion was previously trying to assert too much at a distance and was not only incorrect, but additionally inscrutable. It was mixing up two assertions, the first one of which is sensible: If an entry is accepted, it must not be superseded by inflight proposal. If this were violated, this superseded proposal could also apply, resulting in a failure of replay protection. This assertion is now still around as a stand-alone assertion. The other half of the assertion was more confused: if an entry is rejected, it was claiming that it couldn't also be superseded. The thinking was that if a superseding log entry exists, maybe it could apply, and that would be bad since we just told the waiting client that their proposal got rejected. This reasoning is incorrect, as the following example shows. Consider the following initial situation: [lease seq is 1] log idx 99: unrelated cmd at LAI 10000, lease seq = 1 log idx 100: cmd X at LAI 10000, lease seq = 1 And next: - a new lease enters the log at idx 101 (lease seq = 2) - an identical copy of idx 100 enters the log at idx 102 - we apply idx 100, leading to superseding reproposal at idx 103 resulting in the log: [lease seq is 1] log idx 99: unrelated cmd at LAI 10000, lease seq = 1 log idx 100: cmd X at LAI 10000, lease seq = 1 log idx 101: lease seq = 2 log idx 102: (same as idx 100) log idx 103: cmd X at LAI = 20000, lease seq = 1 During application of idx 102, we get a *permanent* rejection and yet the entry is superseded (by the proposal at idx 103). This would erroneously trigger the assertion, even though this is a legal sequence of events with no detrimental outcomes: the superseding proposal will always have the same lease sequence as its superseded copies, so it will also fail. I initially tried only soften the assertion a *little bit*. Observing that the example above led to a *permanent* rejection, should we only require that a proposal (which in this assertion is always local) is not superseded if it got rejected due to its lease index (which implies that it passed the lease check)? It turns out that this is primarily an assertion on when superseded proposals are counted as "local" at this point in the code: if there were multiple copies of this rejected proposal in the current `appTask` (i.e. the current `CommittedEntries` slice handed to us for application by raft), then all copies are initially local; and a copy that successfully spawns a superseding proposal would be made non-local from that point on. On the face of it, All other copies in the same `appTask` would now hit the assertion (erroneously): they are local, they are rejected, so why don't they enter the branch? The magic ingredient is that if an entry is superseded when we handle the lease index rejection, we also unlink the proposal from it. So these never enter this path since it's not local at this point. For example, if these are the log entries to apply (all at valid lease seq): log idx 99: unrelated cmd at LAI 10000 log idx 100: cmd X at LAI 10000 log idx 101: (identical copy of idx 100) and idxs 99-101 are applied in one batch, then idx 100 would spawn a reproposal at a new lease applied index: log idx 99: unrelated cmd at LAI 10000 log idx 100: cmd X at LAI 10000 <- applied log idx 101: (identical copy of idx 100) log idx 100: cmd X at LAI 20000 <- not in current batch When we apply 101, we observe an illegal lease index, but the proposal supersedes the entry, so we mark it as non-local and don't enter the branch that contains the assertion. The above reasoning is very difficult to understand, and it happens too far removed from where the interesting state changes happen. Also, for testing purposes it is interesting to introduce "errors" in the lease applied index assignment to artificially exercise these reproposal mechanisms. In doing so, these assertions can trip because the lease applied index assigned to a reproposal might accidentally (or intentionally!) match the existing lease applied index, in which case copies of the command in the same batch now *don't* consider themselves superseded. The value of this testing outweighs the very limited benefit of this branch of the assertion. An argument could even be made that this assertion alone as negative benefit due to its complexity. We are removing it in this commit and will instead work towards simplifying the mechanisms that played a role in explaining the asssertion. Closes cockroachdb#94633. Closes cockroachdb#97347. No release note because unreleased (except perhaps in an alpha). Epic: none Release note: None
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