Return error when no replica available (#16886)

Signed-off-by: NCongqi Xia <congqi.xia@zilliz.com>

internal/querycoord/impl.go

@@ -1200,6 +1200,16 @@ func (qc *QueryCoord) GetShardLeaders(ctx context.Context, req *querypb.GetShard
 		shardLeaderLists = append(shardLeaderLists, shard)
 	}
 
+	// all replicas are not available
+	if len(shardLeaderLists) == 0 {
+		return &querypb.GetShardLeadersResponse{
+			Status: &commonpb.Status{
+				ErrorCode: commonpb.ErrorCode_UnexpectedError,
+				Reason:    "no replica available",
+			},
+		}, nil
+	}
+
 	log.Info("GetShardLeaders finished",
 		zap.String("role", typeutil.QueryCoordRole),
 		zap.Int64("collectionID", req.CollectionID),

internal/querycoord/impl_test.go

@@ -1683,6 +1683,14 @@ func TestGetShardLeaders(t *testing.T) {
 	}
 	assert.Equal(t, 0, totalLeaders%3)
 
+	// mock replica all down, without triggering load balance
+	queryCoord.cluster.stopNode(node1.queryNodeID)
+	queryCoord.cluster.stopNode(node2.queryNodeID)
+	queryCoord.cluster.stopNode(node3.queryNodeID)
+	resp, err = queryCoord.GetShardLeaders(ctx, getShardLeadersReq)
+	assert.NoError(t, err)
+	assert.Equal(t, commonpb.ErrorCode_UnexpectedError, resp.Status.ErrorCode)
+
 	// TODO(yah01): Disable the unit test case for now,
 	// restore it after the rebalance between replicas feature is implemented