[BUGFIX] fix sstable double free both gc by iter and gc by queue.

src/storage/ob_pg_sstable_garbage_collector.cpp

@@ -103,6 +103,19 @@ int ObPGSSTableGarbageCollector::gc_free_sstable_by_pg_iter()
         left_recycle_cnt -= recycle_cnt;
       }
     }
+    if (OB_SUCC(ret)) {
+      while (OB_SUCC(ret) && free_sstables_queue_.size() > 0) {
+        ObLink *ptr = NULL;
+        if (OB_FAIL(free_sstables_queue_.pop(ptr))) {
+          LOG_WARN("fail to pop item", K(ret));
+        } else if (OB_ISNULL(ptr)) {
+          ret = OB_ERR_UNEXPECTED;
+          LOG_WARN("unexpected error, ptr is nullptr", K(ret), KP(ptr));
+        } else {
+          free_sstable_gc_item(static_cast<ObSSTableGCItem *>(ptr));
+        }
+      }
+    }
   }
   if (nullptr != partition_iter) {
     ObPartitionService::get_instance().revert_pg_iter(partition_iter);

src/storage/ob_sstable.cpp

@@ -3203,15 +3203,19 @@ int ObSSTable::add_macro_ref()
 
 int64_t ObSSTable::dec_ref()
 {
-  int64_t ref_cnt = ATOMIC_SAF(&ref_cnt_, 1 /* just sub 1 */);
-
-  if (0 == ref_cnt) {
+  // If current ref_cnt is 1, it should be pushed into gc queue firstly. Then, decrease reference
+  // counts. The reason is that gc by iter may be faster than gc by queue.
+  if (1 == get_ref()) {
     int ret = OB_SUCCESS;
     if (OB_FAIL(ObPGSSTableGarbageCollector::get_instance().push_sstable_into_gc_queue(key_))) {
       LOG_WARN("fail to push sstable into gc queue", K(ret), K(key_));
     }
   }
 
+  int64_t ref_cnt = ATOMIC_SAF(&ref_cnt_, 1 /* just sub 1 */);
+  if (OB_UNLIKELY(ref_cnt < 0)) {
+    LOG_ERROR("Unexpected ref cnt of sstable", K(ref_cnt), K(key_));
+  }
   return ref_cnt;
 }