Fix assert(cfd->imm()->NumNotFlushed() > 0) in FlushMemtable (#7744)

Summary:
In current code base, in FlushMemtable, when `(Flush_reason == FlushReason::kErrorRecoveryRetryFlush && (!cfd->mem()->IsEmpty() || !cached_recoverable_state_empty_.load()))`, we assert that cfd->imm()->NumNotFlushed() > 0. However, there are some corner cases that can fail this assert: 1) if there are multiple CFs, some CF has immutable memtable, some CFs don't. In ResumeImpl, all CFs will call FlushMemtable, which will hit the assert. 2) Regular flush is scheduled and running, the resume thread is waiting. New KVs are inserted and SchedulePendingFlush is called. Regular flush will continue call MaybeScheduleFlushAndCompaction until all the immutable memtables are flushed. When regular flush ends and auto resume thread starts to schedule new flushes, cfd->imm()->NumNotFlushed() can be 0.

Remove the assert and added the comments.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7744

Test Plan: make check and pass the stress test

Reviewed By: riversand963

Differential Revision: D25340573

Pulled By: zhichao-cao

fbshipit-source-id: eac357bdace660247c197f01a9ff6857e3c97672

db/db_impl/db_impl_compaction_flush.cc

@@ -1765,12 +1765,16 @@ Status DBImpl::FlushMemTable(ColumnFamilyData* cfd,
     }
     WaitForPendingWrites();
 
-    if (!cfd->mem()->IsEmpty() || !cached_recoverable_state_empty_.load()) {
-      if (flush_reason != FlushReason::kErrorRecoveryRetryFlush) {
-        s = SwitchMemtable(cfd, &context);
-      } else {
-        assert(cfd->imm()->NumNotFlushed() > 0);
-      }
+    if (flush_reason != FlushReason::kErrorRecoveryRetryFlush &&
+        (!cfd->mem()->IsEmpty() || !cached_recoverable_state_empty_.load())) {
+      // Note that, when flush reason is kErrorRecoveryRetryFlush, during the
+      // auto retry resume, we want to avoid creating new small memtables.
+      // Therefore, SwitchMemtable will not be called. Also, since ResumeImpl
+      // will iterate through all the CFs and call FlushMemtable during auto
+      // retry resume, it is possible that in some CFs,
+      // cfd->imm()->NumNotFlushed() = 0. In this case, so no flush request will
+      // be created and scheduled, status::OK() will be returned.
+      s = SwitchMemtable(cfd, &context);
     }
     const uint64_t flush_memtable_id = port::kMaxUint64;
     if (s.ok()) {