lightning: release memory pool when external engine close (#48537)

close pingcap/tidb#48538

br/pkg/lightning/backend/external/engine.go

@@ -379,7 +379,22 @@ func (e *Engine) SplitRanges(
 }
 
 // Close implements common.Engine.
-func (e *Engine) Close() error { return nil }
+func (e *Engine) Close() error {
+	if e.bufPool != nil {
+		e.bufPool.Destroy()
+		e.bufPool = nil
+	}
+	return nil
+}
+
+// Reset resets the memory buffer pool.
+func (e *Engine) Reset() error {
+	if e.bufPool != nil {
+		e.bufPool.Destroy()
+		e.bufPool = membuf.NewPool()
+	}
+	return nil
+}
 
 // MemoryIngestData is the in-memory implementation of IngestData.
 type MemoryIngestData struct {

br/pkg/lightning/backend/local/local.go

@@ -1734,6 +1734,11 @@ func (local *Backend) ResetEngine(ctx context.Context, engineUUID uuid.UUID) err
 	// the only way to reset the engine + reclaim the space is to delete and reopen it 🤷
 	localEngine := local.lockEngine(engineUUID, importMutexStateClose)
 	if localEngine == nil {
+		if engineI, ok := local.externalEngine[engineUUID]; ok {
+			extEngine := engineI.(*external.Engine)
+			return extEngine.Reset()
+		}
+
 		log.FromContext(ctx).Warn("could not find engine in cleanupEngine", zap.Stringer("uuid", engineUUID))
 		return nil
 	}
@@ -1767,6 +1772,11 @@ func (local *Backend) CleanupEngine(ctx context.Context, engineUUID uuid.UUID) e
 	localEngine := local.lockEngine(engineUUID, importMutexStateClose)
 	// release this engine after import success
 	if localEngine == nil {
+		if extEngine, ok := local.externalEngine[engineUUID]; ok {
+			retErr := extEngine.Close()
+			delete(local.externalEngine, engineUUID)
+			return retErr
+		}
 		log.FromContext(ctx).Warn("could not find engine in cleanupEngine", zap.Stringer("uuid", engineUUID))
 		return nil
 	}