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The main intention here is to simplify the waiter/notifier pattern around PrimaryOnlyService::_rebuildInstances (defined here). In particular, callers to PrimaryOnlyService::getOrCreateInstance, PrimaryOnlyService::lookupInstance, and PrimaryOnlyService::getAllInstances block on _rebuildCV using the following:
opCtx->waitForConditionOrInterrupt(_rebuildCV, lk, [this]() { return _state != State::kRebuilding; });
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However, PrimaryOnlyService::_rebuildInstances may call notify_all on this condition variable (i.e., _rebuildCV) even if there's a change in term (example):
...
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stdx::lock_guard lk(_mutex);
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if (_state != State::kRebuilding || _term != term) {
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_rebuildCV.notify_all();
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return;
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}
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...
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We should simplify/clarify this code and the logic around notifying threads that await completion of PrimaryOnlyService::_rebuildInstances.
Acceptance criteria: clarify when a thread would block on _rebuildCV, what are the events that would stop this wait, and what's the expected behavior for each observed event. Then, modify the code to align with the findings.
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