Fix checkpoint file deletion race with avoid_unnecessary_blocking_io (#7369)

Summary:
https://github.com/facebook/rocksdb/issues/3341 guaranteed that upon return of `GetSortedWalFiles` after
`DisableFileDeletions`, all pending purges of previously obsolete WAL
files will have finished. However, the addition of
avoid_unnecessary_blocking_io in https://github.com/facebook/rocksdb/issues/5043 opened a hole in the code making
that assurance, which can lead to files to be copied for checkpoint or
backup going missing before being copied, with that option enabled.

This change patches the hole.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7369

Test Plan:
apparent fix to backups in crash test observed. Will work
on a unit test for another commit

Reviewed By: ajkr

Differential Revision: D23620258

Pulled By: pdillinger

fbshipit-source-id: bea36b461a5b719c3e3ef802f967bc3e8ae71614

HISTORY.md

@@ -10,6 +10,7 @@
 * Fix a bug when max_write_buffer_size_to_maintain is set, immutable flushed memtable destruction is delayed until the next super version is installed. A memtable is not added to delete list because of its reference hold by super version and super version doesn't switch because of empt delete list. So memory usage keeps on increasing beyond write_buffer_size + max_write_buffer_size_to_maintain.
 * Avoid converting MERGES to PUTS when allow_ingest_behind is true.
 * Fix compression dictionary sampling together with `SstFileWriter`. Previously, the dictionary would be trained/finalized immediately with zero samples. Now, the whole `SstFileWriter` file is buffered in memory and then sampled.
+* Fix a bug with `avoid_unnecessary_blocking_io=1` and creating backups (BackupEngine::CreateNewBackup) or checkpoints (Checkpoint::Create). With this setting and WAL enabled, these operations could randomly fail with non-OK status.
 
 ### New Features
 * A new option `std::shared_ptr<FileChecksumGenFactory> file_checksum_gen_factory` is added to `BackupableDBOptions`. The default value for this option is `nullptr`. If this option is null, the default backup engine checksum function (crc32c) will be used for creating, verifying, or restoring backups. If it is not null and is set to the DB custom checksum factory, the custom checksum function used in DB will also be used for creating, verifying, or restoring backups, in addition to the default checksum function (crc32c). If it is not null and is set to a custom checksum factory different than the DB custom checksum factory (which may be null), BackupEngine will return `Status::InvalidArgument()`.

db/db_filesnapshot.cc

@@ -116,7 +116,7 @@ Status DBImpl::GetSortedWalFiles(VectorLogPtr& files) {
     // long as deletions are disabled (so the below loop must terminate).
     InstrumentedMutexLock l(&mutex_);
     while (disable_delete_obsolete_files_ > 0 &&
-           pending_purge_obsolete_files_ > 0) {
+           (pending_purge_obsolete_files_ > 0 || bg_purge_scheduled_ > 0)) {
       bg_cv_.Wait();
     }
   }