Since commit f08c9fb0c6a799e3cb75ca5f763a746471625beb from PR #21726, index BlockUntilSyncedToCurrentChain behavior has been less reliable, and there has also been a race condition in the coinstatsindex_initial_sync unit test.
It seems better for BlockUntilSyncedToCurrentChain to actually wait for the last connected block to be fully processed, than to be able to return before prune locks are set, so this switches the order of m_best_block_index = block; and UpdatePruneLock statements in SetBestBlockIndex to make it more reliable.
Also since commit f08c9fb0c6a799e3cb75ca5f763a746471625beb, there has been a race condition in the coinstatsindex_initial_sync test. Before that commit, the atomic index best block pointer m_best_block_index was updated as the last step of BaseIndex::BlockConnected, so BlockUntilSyncedToCurrentChain could safely be used in tests to wait for the last BlockConnected notification to be finished before stopping and destroying the index. But after that commit, calling BlockUntilSyncedToCurrentChain is no longer sufficient, and there is a race between the test shutdown code which destroys the index object and the new code introduced in that commit calling AllowPrune() and GetName() on the index object. Reproducibility instructions for this are in #25365 (comment)
This commit fixes the coinstatsindex_initial_sync race condition, even though it will require an additional change to silence TSAN false positives, #26188, after it is fixed. So this partially addresses but does not resolve the bug reporting TSAN errors #25365.
There is no known race condition outside of test code currently, because the bitcoind Shutdown function calls FlushBackgroundCallbacks not BlockUntilSyncedToCurrentChain to safely shut down.
Co-authored-by: vasild Co-authored-by: MarcoFalke
