Fix checkpoint stuck (#7921)

Summary:
## 1. Bug description:

When RocksDB Checkpoint, it may be stuck in `WaitUntilFlushWouldNotStallWrites` method.

## 2. Simple analysis of the reasons:

### 2.1 Configuration parameters:

```yaml
Compaction Style : Universal

max_write_buffer_number : 4
min_write_buffer_number_to_merge : 3
```

Checkpoint is usually very fast. When the Checkpoint is executed, `WaitUntilFlushWouldNotStallWrites` is called. If there are 2 Immutable MemTables, which are less than `min_write_buffer_number_to_merge`, they will not be flushed. But will enter this code.

```c++
// method: GetWriteStallConditionAndCause
if (mutable_cf_options.max_write_buffer_number> 3 &&
              num_unflushed_memtables >=
                  mutable_cf_options.max_write_buffer_number-1) {
     return {WriteStallCondition::kDelayed, WriteStallCause::kMemtableLimit};
}
```

code link: fbed72f03c/db/column_family.cc (L847)

Checkpoint thought there was a FlushJob, but it didn't. So will always wait.

### 2.2 solution:

Increase the restriction: the `number of Immutable MemTable` >= `min_write_buffer_number_to_merge will wait`.

If there are other better solutions, you can correct me.

### 2.3 Code that can reproduce the problem:

https://github.com/1996fanrui/fanrui-learning/blob/flink-1.12/module-java/src/main/java/com/dream/rocksdb/RocksDBCheckpointStuck.java

## 3. Interesting point

This bug will be triggered only when `the number of sorted runs >= level0_file_num_compaction_trigger`.

Because there is a break in WaitUntilFlushWouldNotStallWrites.

```c++
if (cfd->imm()->NumNotFlushed() <
        cfd->ioptions()->min_write_buffer_number_to_merge &&
    vstorage->l0_delay_trigger_count() <
        mutable_cf_options.level0_file_num_compaction_trigger) {
  break;
}
```

code link: fbed72f03c/db/db_impl/db_impl_compaction_flush.cc (L1974)

Universal may have `l0_delay_trigger_count() >= level0_file_num_compaction_trigger`, so this bug is triggered.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7921

Reviewed By: jay-zhuang

Differential Revision: D26900559

Pulled By: ajkr

fbshipit-source-id: 133c1252dad7393753f04a47590b68c7d8e670df

db/column_family.cc

@@ -835,7 +835,8 @@ std::pair<WriteStallCondition, ColumnFamilyData::WriteStallCause>
 ColumnFamilyData::GetWriteStallConditionAndCause(
     int num_unflushed_memtables, int num_l0_files,
     uint64_t num_compaction_needed_bytes,
-    const MutableCFOptions& mutable_cf_options) {
+    const MutableCFOptions& mutable_cf_options,
+    const ImmutableCFOptions& immutable_cf_options) {
   if (num_unflushed_memtables >= mutable_cf_options.max_write_buffer_number) {
     return {WriteStallCondition::kStopped, WriteStallCause::kMemtableLimit};
   } else if (!mutable_cf_options.disable_auto_compactions &&
@@ -849,7 +850,9 @@ ColumnFamilyData::GetWriteStallConditionAndCause(
             WriteStallCause::kPendingCompactionBytes};
   } else if (mutable_cf_options.max_write_buffer_number > 3 &&
              num_unflushed_memtables >=
-                 mutable_cf_options.max_write_buffer_number - 1) {
+                 mutable_cf_options.max_write_buffer_number - 1 &&
+             num_unflushed_memtables - 1 >=
+                 immutable_cf_options.min_write_buffer_number_to_merge) {
     return {WriteStallCondition::kDelayed, WriteStallCause::kMemtableLimit};
   } else if (!mutable_cf_options.disable_auto_compactions &&
              mutable_cf_options.level0_slowdown_writes_trigger >= 0 &&
@@ -877,7 +880,8 @@ WriteStallCondition ColumnFamilyData::RecalculateWriteStallConditions(
 
     auto write_stall_condition_and_cause = GetWriteStallConditionAndCause(
         imm()->NumNotFlushed(), vstorage->l0_delay_trigger_count(),
-        vstorage->estimated_compaction_needed_bytes(), mutable_cf_options);
+        vstorage->estimated_compaction_needed_bytes(), mutable_cf_options,
+        *ioptions());
     write_stall_condition = write_stall_condition_and_cause.first;
     auto write_stall_cause = write_stall_condition_and_cause.second;
 

db/column_family.h

@@ -475,9 +475,11 @@ class ColumnFamilyData {
     kPendingCompactionBytes,
   };
   static std::pair<WriteStallCondition, WriteStallCause>
-  GetWriteStallConditionAndCause(int num_unflushed_memtables, int num_l0_files,
-                                 uint64_t num_compaction_needed_bytes,
-                                 const MutableCFOptions& mutable_cf_options);
+  GetWriteStallConditionAndCause(
+      int num_unflushed_memtables, int num_l0_files,
+      uint64_t num_compaction_needed_bytes,
+      const MutableCFOptions& mutable_cf_options,
+      const ImmutableCFOptions& immutable_cf_options);
 
   // Recalculate some small conditions, which are changed only during
   // compaction, adding new memtable and/or

db/db_impl/db_impl_compaction_flush.cc

@@ -2061,12 +2061,12 @@ Status DBImpl::WaitUntilFlushWouldNotStallWrites(ColumnFamilyData* cfd,
       // check whether one extra immutable memtable or an extra L0 file would
       // cause write stalling mode to be entered. It could still enter stall
       // mode due to pending compaction bytes, but that's less common
-      write_stall_condition =
-          ColumnFamilyData::GetWriteStallConditionAndCause(
-              cfd->imm()->NumNotFlushed() + 1,
-              vstorage->l0_delay_trigger_count() + 1,
-              vstorage->estimated_compaction_needed_bytes(), mutable_cf_options)
-              .first;
+      write_stall_condition = ColumnFamilyData::GetWriteStallConditionAndCause(
+                                  cfd->imm()->NumNotFlushed() + 1,
+                                  vstorage->l0_delay_trigger_count() + 1,
+                                  vstorage->estimated_compaction_needed_bytes(),
+                                  mutable_cf_options, *cfd->ioptions())
+                                  .first;
     } while (write_stall_condition != WriteStallCondition::kNormal);
   }
   return Status::OK();