WIP on fixing the cancellation #74121
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… for FlowStream RPC"" This reverts commit 48dd74c.
In `flowCtxCancel` scenario there are two possible errors that might be returned to the reader depending on the exact sequence of events: - `QueryCanceledError` is used when the flow ctx cancellation is observed before the stream arrived - wrapped `context.Canceled` error is used when the inbox handler goroutine notices the cancellation first and ungracefully shuts down the stream. Previously, we assumed that only the latter could occur, and we allow for either. Release note: None
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This commit fixes a long standing bug in the distributed vectorized query shutdown where in case of a graceful completion of the flow on one node, we might get an error on another node resulting in the ungraceful termination of the query. This was caused by the fact that on remote nodes the last outbox to exit would cancel the flow context; however, when instantiating `FlowStream` RPC the outboxes used a child context of the flow context, so that "graceful" cancellation of the flow context would cause the inbox to get an ungraceful termination of the gRPC stream. As a result, the whole query could get "context canceled" error. I believe this bug was introduced by me over two years ago because I didn't fully understand how the shutdown should work, and in particular I was missing that when an inbox observes the flow context cancellation, it should terminate the `FlowStream` RPC ungracefully in order to propagate the ungracefullness to the other side of the stream. This shortcoming was fixed in the previous commit. The shutdown protocol is now as follows: - on the gateway node we keep on canceling the flow context at the very end of the query execution. It doesn't matter whether the query resulted in an error or not, and doing so allows us to ensure that everything exits on the gateway node. This behavior is already present. - due to the fix in a previous commit, that flow context cancellation terminates ungracefully all still open gRPC streams for `FlowStream` RPC for which the gateway node is the inbox host. - the outboxes on the remote nodes get the ungraceful termination and cancel the flow context of their hosts. This, in turn, would trigger propagation of the ungraceful termination on other gRPC streams, etc. Release note (bug fix): Previously, CockroachDB could return a spurious "context canceled" error for a query that actually succeeded in extremely rare cases, and this has now been fixed.
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