store: fix possible index out of range in (*RegionStore).kvPeer() (#22319)

store/tikv/region_cache.go

@@ -174,7 +174,7 @@ func (r *RegionStore) kvPeer(seed uint32) AccessIndex {
 	if len(candidates) == 0 {
 		return r.workTiKVIdx
 	}
-	return candidates[int32(seed)%int32(len(candidates))]
+	return candidates[seed%uint32(len(candidates))]
 }
 
 // init initializes region after constructed.

store/tikv/region_cache_test.go

@@ -128,6 +128,7 @@ func (s *testRegionCacheSuite) TestSimple(c *C) {
 	c.Assert(r.GetID(), Equals, s.region1)
 	c.Assert(s.getAddr(c, []byte("a"), kv.ReplicaReadLeader, 0), Equals, s.storeAddr(s.store1))
 	c.Assert(s.getAddr(c, []byte("a"), kv.ReplicaReadFollower, seed), Equals, s.storeAddr(s.store2))
+	c.Assert(s.getAddr(c, []byte("a"), kv.ReplicaReadMixed, 1<<31+1), Equals, s.storeAddr(s.store2))
 	s.checkCache(c, 1)
 	c.Assert(r.GetMeta(), DeepEquals, r.meta)
 	c.Assert(r.GetLeaderPeerID(), Equals, r.meta.Peers[r.getStore().workTiKVIdx].Id)