Commits on Feb 8, 2022

1. kv: support server-side refreshes of uncertainty errors …

   Extracted from cockroachdb#73732, with relevant comments addressed.
   
   This commit adds support for server-side refreshes of
   `ReadWithinUncertaintyIntervalError`s. This serves as a performance optimization
   for transactional requests, which now benefit from this new capability to
   refresh away `ReadWithinUncertaintyIntervalErrors` early in their transaction,
   before they've accumulated any refresh spans. There's some complexity around
   supporting this form of server-side retry, because it must be done above
   latching, instead of below. However, the recent refactoring in cockroachdb#73557 has made
   this possible to support cleanly.
   
   Specifically, we now handle `ReadWithinUncertaintyIntervalError` as a concurrency
   error in the `executeWithConcurrencyRetries` retry loop. This is different from
   other server-side retries, which are hit during writes and can be handled without
   releasing latches. This difference stems from the difference in how read and write
   latches behave. Write latches protect their MVCC timestamp and any later time.
   Meanwhile, read latches protect their MVCC timestamp and any earlier time. This
   means that a request holding read latches that hits an uncertainty error can't
   refresh without dropping those latches and acquiring new ones.
   
   This is also a prerequisite to giving non-transactional requests uncertainty
   intervals (cockroachdb#73732), because we don't want ReadWithinUncertaintyIntervalErrors to
   reach the client for non-transactional requests. Conveniently, because
   non-transactional requests are always scoped to a single-range, those that hit
   uncertainty errors will always be able to retry on the server, so these errors
   will never bubble up to the client that initiated the request.
   
   Release note (performance improvement): Certain forms of automatically retried
   "read uncertainty" errors are now retried more efficiently, avoiding a network
   round trip.

   

   nvanbenschoten committed Feb 8, 2022
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