Allow Flush(sync=true) not supported in DB::Open() and db_stress (#10784)

Summary:
**Context:**
https://github.com/facebook/rocksdb/pull/10698 made `Flush(sync=true)` required for` DB::Open()` (to pass the original but now deleted assertion `impl->TEST_WALBufferIsEmpty()` under `manual_wal_flush=true`, see https://github.com/facebook/rocksdb/pull/10698 summary for more ) as well as db_stress to pass.

However RocksDB users may not implement SyncWAL() (used inFlush(sync=true)). Therefore we replace such in DB::Open and db_stress in this PR and align with https://github.com/facebook/rocksdb/blob/main/db/db_impl/db_impl_open.cc#L1883-L1887 and https://github.com/facebook/rocksdb/blob/main/db_stress_tool/db_stress_test_base.cc#L847-L849

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10784

Test Plan: make check

Reviewed By: anand1976

Differential Revision: D40193354

Pulled By: anand1976

fbshipit-source-id: e80d53880799ae01bdd717641d07997d3bfe2b54

db/db_impl/db_impl_open.cc

@@ -2080,9 +2080,15 @@ Status DBImpl::Open(const DBOptions& db_options, const std::string& dbname,
                      impl);
     LogFlush(impl->immutable_db_options_.info_log);
     if (!impl->WALBufferIsEmpty()) {
-      impl->FlushWAL(true /* sync */);
+      s = impl->FlushWAL(false);
+      if (s.ok()) {
+        // Sync is needed otherwise WAL buffered data might get lost after a
+        // power reset.
+        log::Writer* log_writer = impl->logs_.back().writer;
+        s = log_writer->file()->Sync(impl->immutable_db_options_.use_fsync);
+      }
     }
-    if (!persist_options_status.ok()) {
+    if (s.ok() && !persist_options_status.ok()) {
       s = Status::IOError(
           "DB::Open() failed --- Unable to persist Options file",
           persist_options_status.ToString());

db_stress_tool/db_stress_test_base.cc

@@ -836,7 +836,7 @@ void StressTest::OperateDb(ThreadState* thread) {
       if (thread->rand.OneInOpt(FLAGS_manual_wal_flush_one_in)) {
         bool sync = thread->rand.OneIn(2) ? true : false;
         Status s = db_->FlushWAL(sync);
-        if (!s.ok()) {
+        if (!s.ok() && !(sync && s.IsNotSupported())) {
           fprintf(stderr, "FlushWAL(sync=%s) failed: %s\n",
                   (sync ? "true" : "false"), s.ToString().c_str());
         }