Commits on Feb 10, 2020

1. kv: refresh less and retry more …

   Before this patch, when the DistSender would split a batch into multiple
   sub-batches and one of the sub-batches fails, it would collect responses
   for the successful ones and return them together with the error. This
   used to be pretty important before we had write idempotency, because it
   allowed the span refresher to only retry an EndTxn without also retring
   other writes in that batch (which would have failed).
   
   Since we've gotten idempotency in the meantime, we can retry those other
   writes. In fact, it's arguably better to do it: there's a tradeoff
   between refreshing and retrying. Currently the span refresher needs to
   refresh the read spans of the successful sub-batches, which refresh is
   at risk of failing under contention.
   
   This patch makes the span refresher retry the whole batch without
   considering partial successes. With this patch, refreshing the partial
   successes is no longer needed because we'll retry those requests. In
   other words, we'll refresh less and retry more.
   
   The existing policy of refreshing more and retrying less will start to be
   applied inconsistenly with cockroachdb#44654, where we start refreshing when the
   client sees a WriteTooOld flag - but we're forced to refresh the whole
   batch.
   
   Besides the rationalizations above, this patch allows us to simplify
   code by not having to deal with both responses and errors. We can thus
   get rid of the enthralling comment on the client.Sender.Send() stating:
   "
   // The contract about whether both a response and an error can be
   // returned varies between layers.
   "
   
   Release note: None

   

   andreimatei committed Feb 10, 2020
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