SERVER-21311 fixing crash on shutdown in RocksRecoveryUnit::dbRelease…

…Snapshot

Problem description:
The crash was caused by dangling pointer to RocksRecoveryUnit stored in the instance of RocksSnapshotManager::SnapshotHolder class.

Crash scenario:
- execute administrative 'makeSnapshot' command. This will create instance of RocksSnapshotManager::SnapshotHolder which is stored in RocksSnapshotManager's snapshots map.
- SnapshotHolder has raw pointer to RocksRecoveryUnit instance which is destroyed at the end of 'makeSnapshot' command handler
- destructor of SnapshotHolder is called on shutdown when snapshot maps are cleared in RocksSnapshotManager::dropAllSnapshots
- destructor of SnapshotHolder tries to release snapshot using stored pointer to RocksRecoveryUnit instance which was destroyed earlier

Fix description:
Instead of storing RocksRecoveryUnit pointer in SnapshotHolder we will store pointer to rocksdb::DB instance.
Thus we will be able to call rocks::DB::ReleaseSnapshot() from SnapshotHolder's destructor and SnapshotHolder
will be independent of RocksRecoveryUnit lifetime.

src/rocks_recovery_unit.h

@@ -160,6 +160,8 @@ namespace mongo {
 
         void setCommittedSnapshot(const rocksdb::Snapshot* committedSnapshot);
 
+        rocksdb::DB* getDB() const { return _db; }
+
     private:
         void _releaseSnapshot();
 

src/rocks_snapshot_manager.cpp

@@ -102,14 +102,15 @@ namespace mongo {
 
     RocksSnapshotManager::SnapshotHolder::SnapshotHolder(OperationContext* opCtx, uint64_t name_) {
         name = name_;
-        ru = RocksRecoveryUnit::getRocksRecoveryUnit(opCtx);
-        snapshot = ru->getPreparedSnapshot();
+        auto rru = RocksRecoveryUnit::getRocksRecoveryUnit(opCtx);
+        snapshot = rru->getPreparedSnapshot();
+        db = rru->getDB();
     }
 
     RocksSnapshotManager::SnapshotHolder::~SnapshotHolder() {
         if (snapshot != nullptr) {
-            invariant(ru != nullptr);
-            ru->dbReleaseSnapshot(snapshot);
+            invariant(db != nullptr);
+            db->ReleaseSnapshot(snapshot);
         }
     }
 

src/rocks_snapshot_manager.h

@@ -48,7 +48,7 @@ class RocksSnapshotManager final : public SnapshotManager {
     struct SnapshotHolder {
         uint64_t name;
         const rocksdb::Snapshot* snapshot;
-        RocksRecoveryUnit* ru;
+        rocksdb::DB* db;
         SnapshotHolder(OperationContext* opCtx, uint64_t name_);
         ~SnapshotHolder();
     };