Fix race between test and recovery flush switch memtable (#12884)

Summary:
As titled, to fix this type of data race:
https://github.com/facebook/rocksdb/actions/runs/10066814221/job/27829003372?pr=12882

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12884

Test Plan:
COMPILE_WITH_TSAN=1 make -j10 db_wal_test
./db_wal_test --gtest_filter=DBWALTest.RecoveryFlushSwitchWALOnEmptyMemtable --gtest_repeat=100

Reviewed By: anand1976

Differential Revision: D60197834

Pulled By: jowlyzhang

fbshipit-source-id: 89524cdb4d17a1b647295bcccf5eb2d7d425bc6a

db/db_wal_test.cc

@@ -2765,6 +2765,13 @@ TEST_F(DBWALTest, RecoveryFlushSwitchWALOnEmptyMemtable) {
   options.avoid_flush_during_shutdown = true;
   DestroyAndReopen(options);
 
+  // Make sure the memtable switch in recovery flush happened after test checks
+  // the memtable is empty.
+  SyncPoint::GetInstance()->LoadDependency(
+      {{"DBWALTest.RecoveryFlushSwitchWALOnEmptyMemtable:"
+        "AfterCheckMemtableEmpty",
+        "RecoverFromRetryableBGIOError:BeforeStart"}});
+  SyncPoint::GetInstance()->EnableProcessing();
   fault_fs->SetThreadLocalErrorContext(
       FaultInjectionIOType::kMetadataWrite, 7 /* seed*/, 1 /* one_in */,
       true /* retryable */, false /* has_data_loss*/);
@@ -2781,6 +2788,10 @@ TEST_F(DBWALTest, RecoveryFlushSwitchWALOnEmptyMemtable) {
                   ->mem()
                   ->IsEmpty());
   ASSERT_EQ("NOT_FOUND", Get("k"));
+  TEST_SYNC_POINT(
+      "DBWALTest.RecoveryFlushSwitchWALOnEmptyMemtable:"
+      "AfterCheckMemtableEmpty");
+  SyncPoint::GetInstance()->DisableProcessing();
 
   fault_fs->DisableThreadLocalErrorInjection(
       FaultInjectionIOType::kMetadataWrite);