ring: return an error in pipeline when all shards are down.

ring.go

@@ -298,6 +298,9 @@ func (pipe *RingPipeline) Exec() (cmds []Cmder, retErr error) {
 		name := pipe.ring.hash.Get(hashKey(cmd.clusterKey()))
 		if name == "" {
 			cmd.setErr(errRingShardsDown)
+			if retErr == nil {
+				retErr = errRingShardsDown
+			}
 			continue
 		}
 		cmdsMap[name] = append(cmdsMap[name], cmd)

ring_test.go

@@ -94,6 +94,15 @@ var _ = Describe("Redis ring", func() {
 	})
 
 	Describe("pipelining", func() {
+		It("returns an error when all shards are down", func() {
+			ring := redis.NewRing(&redis.RingOptions{})
+			_, err := ring.Pipelined(func(pipe *redis.RingPipeline) error {
+				pipe.Ping()
+				return nil
+			})
+			Expect(err).To(MatchError("redis: all ring shards are down"))
+		})
+
 		It("uses both shards", func() {
 			pipe := ring.Pipeline()
 			for i := 0; i < 100; i++ {