Commits on May 4, 2020

1. kv: declare write access to AbortSpan on all aborting EndTxn reqs …

   Fixes cockroachdb#43707.
   Fixes cockroachdb#48046.
   Fixes cockroachdb#48189.
   
   Part of the change made by cockroachdb#42765 was to clear AbortSpan entries on
   non-poisoning, aborting EndTxn requests. Specifically, this change was
   made in 1328787. The change forgot to update the corresponding span
   declaration logic to reflect the fact that we were now writing to the
   AbortSpan in cases where we previously weren't.
   
   This was triggering an assertion in race builds that tried to catch this
   kind of undeclared span access. The assertion failure was very rare
   because it required the following conditions to all be met:
   1. running a test with the race detector enabled
   2. a txn (A) must have been aborted by another txn (B)
   3. txn B must have cleared an intent on txn A's transaction record range
   4. txn A must have noticed and issued a non-poisoning EndTxn(ABORT)
   
   We should backport this when we get a change (once v20.1.0 has
   stabilized), but I don't expect that this could actually cause any
   issues. The AbortSpan update was strictly a matter of performance and we
   should never be racing with another request that is trying to read the
   same AbortSpan entry.

   

   nvanbenschoten committed May 4, 2020
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