Merge #13023: Fix some concurrency issues in ActivateBestChain()

dd435ad Add unit tests for signals generated by ProcessNewBlock() (Jesse Cohen)
a3ae8e6 Fix concurrency-related bugs in ActivateBestChain (Jesse Cohen)
ecc3c4a Do not unlock cs_main in ABC unless we've actually made progress. (Matt Corallo)

Pull request description:

  Originally this PR was just to add tests around concurrency in block validation - those tests seem to have uncovered another bug in ActivateBestChain - this now fixes that bug and adds tests.

  ActivateBestChain (invoked after a new block is validated) proceeds in steps - acquiring and releasing cs_main while incrementally disconnecting and connecting blocks to sync to the most work chain known (FindMostWorkChain()). Every time cs_main is released the result of FindMostWorkChain() can change - but currently that value is cached across acquisitions of cs_main and only refreshed when an invalid chain is explored. It needs to be refreshed every time cs_main is reacquired. The test added in 6094ce7304 will occasionally fail without the commit fixing this issue 26bfdbaddb

  Original description below
  --

  After a bug discovered where UpdatedBlockTip() notifications could be triggered out of order (#12978), these unit tests check certain invariants about these signals.

  The scheduler test asserts that a SingleThreadedSchedulerClient processes callbacks fully and sequentially.

  The block validation test generates a random chain and calls ProcessNewBlock from multiple threads at random and in parallel. ValidationInterface callbacks verify that the ordering of BlockConnected BlockDisconnected and UpdatedBlockTip events occur as expected.

Tree-SHA512: 4102423a03d2ea28580c7a70add8a6bdb22ef9e33b107c3aadef80d5af02644cdfaae516c44933924717599c81701e0b96fbf9cf38696e9e41372401a5ee1f3c

src/validationinterface.h

@@ -61,7 +61,11 @@ protected:
      */
     ~CValidationInterface() = default;
     /**
-     * Notifies listeners of updated block chain tip
+     * Notifies listeners when the block chain tip advances.
+     *
+     * When multiple blocks are connected at once, UpdatedBlockTip will be called on the final tip
+     * but may not be called on every intermediate tip. If the latter behavior is desired,
+     * subscribe to BlockConnected() instead.
      *
      * Called on a background thread.
      */