Cancel PipelineExecutor properly in case of exception in spawnThreads

[Avogar]

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Very similar to #57104, it could lead to crash in fiber because RemoteQueryExecutor is not cancelled properly.

[robot-clickhouse]

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[devcrafter]

Shouldn't we rather wrap the entire executeImpl() body in try/catch and call cancel() accordingly? I find this fix too narrow

[Avogar]

But spawnThreads is the only place where we don't cancel execution in case of an exception. Other code either doesn't throw or handles exceptions correctly. Ideally, spawnThreads should not throw, but by now it's not possible: https://github.com/ClickHouse/clickhouse-core-incidents/issues/10#issuecomment-1816103879

[devcrafter]

No pressure regarding merging the fix - feel free to, but ...

But spawnThreads is the only place ...

It seems, it's a strong statement which can change overtime, and I don't see an easy way to conclude it from the code. But the function should hold the contract, i.e. - in case of exception, cancel whatever background jobs/processes it's doing. If it'd be implemented in the proposed way, then we'd not have the escalation in the first place.

[Avogar]

Actually, I just realized that there is a code above:

    SCOPE_EXIT_SAFE(
        if (!finished_flag)
        {
            finish();
            if (pool)
                pool->wait();
        }
    );

And AFAIU finished_flag can be false only in case of an exception in executeImpl(), but we should call cancel() instead of finish() in this case, it's similar to #52533. So, let's do it

[devcrafter]

Unclear if there was any reason to call only finish() here. The more I read code around, the more questions I have regarding the contracts. In general, looks ok but can't say anything in particular from my current understanding 😕