[v23.3.x] Fixed background apply fiber race condition in raft::state_machine_manager
#16897
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`raft::state_machine_manager` uses background apply fiber to individually apply batches to state machines which are behind the main apply fiber. When background apply fiber is active it reads and apply batches up to current committed offset. When background apply fiber is active it acquires the mutex. When mutex is acquired the main apply fiber do not consider the stm as up to date. The code was prone to very rare race condition as the background apply was finished in one continuation but the units were release in subsequent `finally` block. Normally this approach is harmless as the semaphore is waited for and it will be signaled after the `finally` fiber is executed. In `state_machine_manager` we only check if the semaphore is available, this makes the solution vulnerable to timing. Signed-off-by: Michal Maslanka <[email protected]> (cherry picked from commit df97290)
Added state machine manager test waiting for batches to be applied after each replicate. This test is designed to detect a situation in which background apply fiber finishes but managed stm is still behind the others. Signed-off-by: Michal Maslanka <[email protected]> (cherry picked from commit a55d01a)
mmaslankaprv
approved these changes
Mar 5, 2024
This was referenced Mar 13, 2024
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Backport of PR #16850