Summary:
Currently, when a WAL becomes obsolete after flushing, if VersionSet::WalSet does not contain the WAL, we do not track the WAL obsoletion event in MANIFEST.
But consider this case:
* WAL 10 is synced, a VersionEdit is LogAndApplied to MANIFEST to log this WAL addition event, but the VersionEdit is not applied to WalSet yet since its corresponding ManifestWriter is still pending in the write queue;
* Since the above ManifestWriter is blocking, the LogAndApply will block on a conditional variable and release the db mutex, so another LogAndApply can proceed to enqueue other VersionEdits concurrently;
* Now flush happens, and WAL 10 becomes obsolete, although WalSet does not contain WAL 10 yet, we should call LogAndApply to enqueue a VersionEdit to indicate the obsoletion of WAL 10;
* otherwise, when the queued edit indicating WAL 10 addition is logged to MANIFEST, and DB crashes and reopens, the WAL 10 might have been removed from disk, but it still exists in MANIFEST.
This PR changes the behavior to: always `LogAndApply` any WAL addition or obsoletion event, without considering the order issues caused by concurrency, but when applying the edits to `WalSet`, do not add the WALs if they are already obsolete. In this approach, the logical events of WAL addition and obsoletion are always tracked in MANIFEST, so we can inspect the MANIFEST and know all the previous WAL events, but we choose to ignore certain events due to the concurrency issues such as the case above, or the case in https://github.com/facebook/rocksdb/pull/7725.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7759
Test Plan: make check
Reviewed By: pdillinger
Differential Revision: D25423089
Pulled By: cheng-chang
fbshipit-source-id: 9cb9a7fbc1875bf954f2a42f9b6cfd6d49a7b21c
efe827baf0