Commits on Aug 31, 2023

1. ZOOKEEPER-4655: Communicate the Zxid that triggered a WatchEvent to f… …

   …ire (apache#1950)
   
   * Fix a race condition in WatcherCleanerTest.testDeadWatcherMetrics
   
   Because the metrics were updated _after_ the listener is invoked, the listener does not always see
   the fresh metric value. This fixes it so that the test waits for the value to become what we expect.
   
   * Leverage an existing method and refactor the rest of the code to match
   
   Since there was an existing waitFor method in ZKTestCase, along with an existing implementation of a
   waitForMetric LearnerMetricsTest, this commit moves waitForMetric to ZKTestCase and refactors the
   metric-related usages of waitFor.
   
   * Communicate the Zxid that triggered a WatchEvent to fire
   
   With the recent addition of persistent watches, many doors have opened up to significantly more
   performant and intuitive local caches of remote state, but the actual implementation can be
   difficult because to cache data locally, one needs to execute the following steps:
   
   1. Set the watch
   2. Bootstrap the watched subtree
   3. Catch up on the events that fired during the bootstrap
   
   The issue is it's now very difficult to deduplicate and sanely resolve the remote state during step
   3 because it's unknown whether an event arrived during the bootstrap or after. For example,
   imagine that between steps 1 and 2, a node /a was deleted then re-created. By the time step 3 is
   executed, there will be a NodeDeleted event queued up followed by a NodeCreated, causing at best a
   double read (one from the bootstrap, one from the NodeCreated) or at worst some data inconsistencies
   in the local cache.
   
   This change sets the Zxid in the response header whenever the watch event type is NodeCreated,
   NodeDeleted, NodeDataChanged or NodeChildrenChanged.

   

   PapaCharlie authored and anurag-harness committed Aug 31, 2023
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