Deflake DBWriteTest.LockWALInEffect (#11382)

Summary:
This test exhibited the following flaky failure:

```
db/db_write_test.cc:653: Failure
db_->Resume()
Corruption: Not active
```

I was able to repro it by applying the following patch to coerce a specific race condition:

```
 diff --git a/db/db_write_test.cc b/db/db_write_test.cc
index d82c57376..775ba3cde 100644
 --- a/db/db_write_test.cc
+++ b/db/db_write_test.cc
@@ -636,6 +636,10 @@ TEST_P(DBWriteTest, LockWALInEffect) {
   ASSERT_TRUE(dbfull()->WALBufferIsEmpty());
   ASSERT_OK(db_->UnlockWAL());

+  // Test thread: sleep interval: [0, 3)
+  // In this interval, the file system is active
+  sleep(3);
+
   // Fail the WAL flush if applicable
   fault_fs->SetFilesystemActive(false);
   Status s = Put("key2", "value");
@@ -649,6 +653,11 @@ TEST_P(DBWriteTest, LockWALInEffect) {
     ASSERT_OK(db_->LockWAL());
     ASSERT_OK(db_->UnlockWAL());
   }
+
+  // Test thread: sleep interval: [3, 6)
+  // In this interval, the file system is inactive
+  sleep(3);
+
   fault_fs->SetFilesystemActive(true);
   ASSERT_OK(db_->Resume());
   // Writes should work again
 diff --git a/db/flush_job.cc b/db/flush_job.cc
index 8193f594f..602ee2c9f 100644
 --- a/db/flush_job.cc
+++ b/db/flush_job.cc
@@ -979,6 +979,10 @@ Status FlushJob::WriteLevel0Table() {
           DirFsyncOptions(DirFsyncOptions::FsyncReason::kNewFileSynced));
     }
     TEST_SYNC_POINT_CALLBACK("FlushJob::WriteLevel0Table", &mems_);
+    // Flush thread: sleep interval: [0, 4)
+    // Upon awakening, the file system will be inactive. Then the MANIFEST
+    // update will fail.
+    sleep(4);
     db_mutex_->Lock();
   }
   base_->Unref();
```

The fix for this scenario is explained in the code change.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11382

Reviewed By: cbi42

Differential Revision: D45027632

Pulled By: ajkr

fbshipit-source-id: 6bfa35a5781c0c080fb74e13f2b2c9f871f7effb

db/db_write_test.cc

@@ -636,6 +636,11 @@ TEST_P(DBWriteTest, LockWALInEffect) {
   ASSERT_TRUE(dbfull()->WALBufferIsEmpty());
   ASSERT_OK(db_->UnlockWAL());
 
+  // The above `TEST_SwitchWAL()` triggered a flush. That flush needs to finish
+  // before we make the filesystem inactive, otherwise the flush might hit an
+  // unrecoverable error (e.g., failed MANIFEST update).
+  ASSERT_OK(dbfull()->TEST_WaitForFlushMemTable(nullptr));
+
   // Fail the WAL flush if applicable
   fault_fs->SetFilesystemActive(false);
   Status s = Put("key2", "value");