release-2.0: sql: prioritize retryable errors in synchronizeParallelStmts

[nvanbenschoten]

At the moment, parallel statement execution works by sending batches
concurrently through a single client.Txn. This make the handling of
retryable errors tricky because it's difficult to know when its safe
to prepare the transaction state for a retry. Our approach to this is
far from optimal, and relies on a mess of locking in both client.Txn
and TxnCoordSender. This works well enough to prevent anything from
seriously going wrong (#17197), but can result in some confounding error
behavior when statements operate in the context of transaction epochs
that they weren't expecting.

The ideal situation would be for all statements with a handle to a txn
to always work under the same txn epoch at a single point in time. Any
retryable error seen by these statements would be propagated up through
client.Txn without changing any state (and without yet being converted
to a HandledRetryableTxnError), and only after the statements have all
been synchronized would the retryable error be used to update the txn
and prepare for the retry attempt. This would require a change like #22615.
I've created a POC for this approach, but it is way to invasive to
cherry-pick.

So with our current state of things, we need to do a better job catching
errors caused by concurrent retries. In the past we've tried to carefully
determine which errors could be a symptom of a concurrent retry and ignore
them. I now think this was a mistake, as this process of inferring which
errors could be caused by a txn retry is fraught for failure. We now
always return retryable errors from synchronizeParallelStmts when they
exist. The reasoning for this is that if an error was a symptom of the
txn retry, it will not be present during the next txn attempt. If it was
not and instead was a legitimate query execution error, we expect to
hit it again on the next txn attempt and the behavior will mirror that
where the statement throwing the execution error was not even run before
the parallel queue hit the retryable error.

Release note: None

[cockroach-teamcity]

This change is 

[andreimatei]

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Review status: 0 of 3 files reviewed at latest revision, all discussions resolved, some commit checks pending.

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