Fix data race in WalManager (#12439)

Summary:
Crash tests were failing due to data race in accessing `purge_wal_files_last_run_`. This PR changes it to atomic.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12439

Test Plan:
- existing UT
- not able to repro with `python3 tools/db_crashtest.py whitebox --simple --max_key=25000000 --WAL_ttl_seconds=1` and TSAN yet, will monitor internal crash tests

Reviewed By: anand1976

Differential Revision: D54920817

Pulled By: cbi42

fbshipit-source-id: 80ee026b1785ad5dba11295ed35c88889df5f5a6

db/wal_manager.cc

@@ -158,11 +158,14 @@ void WalManager::PurgeObsoleteWALFiles() {
           ? std::min(kDefaultIntervalToDeleteObsoleteWAL,
                      std::max(uint64_t{1}, db_options_.WAL_ttl_seconds / 2))
           : kDefaultIntervalToDeleteObsoleteWAL;
-  if (purge_wal_files_last_run_ + time_to_check > now_seconds) {
+  uint64_t old_last_run_time = purge_wal_files_last_run_.LoadRelaxed();
+  do {
+    if (old_last_run_time + time_to_check > now_seconds) {
+      // last run is recent enough, no need to purge
       return;
     }
-
-  purge_wal_files_last_run_ = now_seconds;
+  } while (!purge_wal_files_last_run_.CasWeakRelaxed(
+      /*expected=*/old_last_run_time, /*desired=*/now_seconds));
 
   std::string archival_dir = ArchivalDirectory(wal_dir_);
   std::vector<std::string> files;

db/wal_manager.h

@@ -25,6 +25,7 @@
 #include "rocksdb/status.h"
 #include "rocksdb/transaction_log.h"
 #include "rocksdb/types.h"
+#include "util/atomic.h"
 
 namespace ROCKSDB_NAMESPACE {
 
@@ -118,7 +119,7 @@ class WalManager {
   port::Mutex read_first_record_cache_mutex_;
 
   // last time when PurgeObsoleteWALFiles ran.
-  uint64_t purge_wal_files_last_run_;
+  RelaxedAtomic<uint64_t> purge_wal_files_last_run_;
 
   bool seq_per_batch_;
 

unreleased_history/bug_fixes/data_race_in_wal_manager.md

@@ -0,0 +1 @@
+* Fixed a data race in WalManager that may affect how frequent PurgeObsoleteWALFiles() runs.