Fix partition's gc (#27816)

Signed-off-by: longjiquan <jiquan.long@zilliz.com>

internal/metastore/kv/rootcoord/kv_catalog.go

@@ -523,6 +523,11 @@ func dropPartition(collMeta *pb.CollectionInfo, partitionID typeutil.UniqueID) {
 
 func (kc *Catalog) DropPartition(ctx context.Context, dbID int64, collectionID typeutil.UniqueID, partitionID typeutil.UniqueID, ts typeutil.Timestamp) error {
 	collMeta, err := kc.loadCollection(ctx, dbID, collectionID, ts)
+	if errors.Is(err, merr.ErrCollectionNotFound) {
+		// collection's gc happened before partition's.
+		return nil
+	}
+
 	if err != nil {
 		return err
 	}

internal/metastore/kv/rootcoord/kv_catalog_test.go

@@ -666,10 +666,9 @@ func TestCatalog_DropPartitionV2(t *testing.T) {
 	t.Run("failed to load collection", func(t *testing.T) {
 		ctx := context.Background()
 
-		snapshot := kv.NewMockSnapshotKV()
-		snapshot.LoadFunc = func(key string, ts typeutil.Timestamp) (string, error) {
-			return "", errors.New("mock")
-		}
+		snapshot := mocks.NewSnapShotKV(t)
+		snapshot.On("Load",
+			mock.Anything, mock.Anything).Return("not in codec format", nil)
 
 		kc := Catalog{Snapshot: snapshot}
 
@@ -677,6 +676,19 @@ func TestCatalog_DropPartitionV2(t *testing.T) {
 		assert.Error(t, err)
 	})
 
+	t.Run("failed to load collection, no key found", func(t *testing.T) {
+		ctx := context.Background()
+
+		snapshot := mocks.NewSnapShotKV(t)
+		snapshot.On("Load",
+			mock.Anything, mock.Anything).Return("", merr.WrapErrIoKeyNotFound("partition"))
+
+		kc := Catalog{Snapshot: snapshot}
+
+		err := kc.DropPartition(ctx, 0, 100, 101, 0)
+		assert.NoError(t, err)
+	})
+
 	t.Run("partition version after 210", func(t *testing.T) {
 		ctx := context.Background()