region_scatterer: fix the bug that could generate schedule with too few peers (tikv#4570)

close tikv#4565

Signed-off-by: HunDunDM <hundundm@gmail.com>

Author: HunDunDM

server/schedule/region_scatterer.go

@@ -273,7 +273,7 @@ func (r *RegionScatterer) Scatter(region *core.RegionInfo, group string) (*opera
 
 func (r *RegionScatterer) scatterRegion(region *core.RegionInfo, group string) *operator.Operator {
 	ordinaryFilter := filter.NewOrdinaryEngineFilter(r.name)
-	ordinaryPeers := make(map[uint64]*metapb.Peer)
+	ordinaryPeers := make(map[uint64]*metapb.Peer, len(region.GetPeers()))
 	specialPeers := make(map[string]map[uint64]*metapb.Peer)
 	// Group peers by the engine of their stores
 	for _, peer := range region.GetPeers() {
@@ -282,24 +282,36 @@ func (r *RegionScatterer) scatterRegion(region *core.RegionInfo, group string) *
 			return nil
 		}
 		if ordinaryFilter.Target(r.cluster.GetOpts(), store) {
-			ordinaryPeers[peer.GetId()] = peer
+			ordinaryPeers[peer.GetStoreId()] = peer
 		} else {
 			engine := store.GetLabelValue(filter.EngineKey)
 			if _, ok := specialPeers[engine]; !ok {
 				specialPeers[engine] = make(map[uint64]*metapb.Peer)
 			}
-			specialPeers[engine][peer.GetId()] = peer
+			specialPeers[engine][peer.GetStoreId()] = peer
 		}
 	}
 
-	targetPeers := make(map[uint64]*metapb.Peer)
-	selectedStores := make(map[uint64]struct{})
-	scatterWithSameEngine := func(peers map[uint64]*metapb.Peer, context engineContext) {
+	targetPeers := make(map[uint64]*metapb.Peer, len(region.GetPeers()))                  // StoreID -> Peer
+	selectedStores := make(map[uint64]struct{}, len(region.GetPeers()))                   // StoreID set
+	scatterWithSameEngine := func(peers map[uint64]*metapb.Peer, context engineContext) { // peers: StoreID -> Peer
 		for _, peer := range peers {
-			candidates := r.selectCandidates(region, peer.GetStoreId(), selectedStores, context)
-			newPeer := r.selectStore(group, peer, peer.GetStoreId(), candidates, context)
-			targetPeers[newPeer.GetStoreId()] = newPeer
-			selectedStores[newPeer.GetStoreId()] = struct{}{}
+			if _, ok := selectedStores[peer.GetStoreId()]; ok {
+				// It is both sourcePeer and targetPeer itself, no need to select.
+				continue
+			}
+			for {
+				candidates := r.selectCandidates(region, peer.GetStoreId(), selectedStores, context)
+				newPeer := r.selectStore(group, peer, peer.GetStoreId(), candidates, context)
+				targetPeers[newPeer.GetStoreId()] = newPeer
+				selectedStores[newPeer.GetStoreId()] = struct{}{}
+				// If the selected peer is a peer other than origin peer in this region,
+				// it is considered that the selected peer select itself.
+				// This origin peer re-selects.
+				if _, ok := peers[newPeer.GetStoreId()]; !ok || peer.GetStoreId() == newPeer.GetStoreId() {
+					break
+				}
+			}
 		}
 	}
 

server/schedule/region_scatterer_test.go

@@ -9,6 +9,7 @@ import (
 
 	. "github.com/pingcap/check"
 	"github.com/pingcap/failpoint"
+	"github.com/pingcap/kvproto/pkg/metapb"
 	"github.com/tikv/pd/pkg/mock/mockcluster"
 	"github.com/tikv/pd/server/config"
 	"github.com/tikv/pd/server/core"
@@ -474,3 +475,54 @@ func (s *testScatterRegionSuite) TestRegionFromDifferentGroups(c *C) {
 	}
 	check(scatterer.ordinaryEngine.selectedPeer)
 }
+
+// TestSelectedStores tests if the peer count has changed due to the picking strategy.
+// Ref https://github.com/tikv/pd/issues/4565
+func (s *testScatterRegionSuite) TestSelectedStores(c *C) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	opt := config.NewTestOptions()
+	tc := mockcluster.NewCluster(ctx, opt)
+	// Add 4 stores.
+	for i := uint64(1); i <= 4; i++ {
+		tc.AddRegionStore(i, 0)
+		// prevent store from being disconnected
+		tc.SetStoreLastHeartbeatInterval(i, -10*time.Minute)
+	}
+	group := "group"
+	scatterer := NewRegionScatterer(ctx, tc)
+
+	// Put a lot of regions in Store 1/2/3.
+	for i := uint64(1); i < 100; i++ {
+		region := tc.AddLeaderRegion(i+10, i%3+1, (i+1)%3+1, (i+2)%3+1)
+		peers := make(map[uint64]*metapb.Peer, 3)
+		for _, peer := range region.GetPeers() {
+			peers[peer.GetStoreId()] = peer
+		}
+		scatterer.Put(peers, i%3+1, group)
+	}
+
+	// Try to scatter a region with peer store id 2/3/4
+	for i := uint64(1); i < 20; i++ {
+		region := tc.AddLeaderRegion(i+200, i%3+2, (i+1)%3+2, (i+2)%3+2)
+		op := scatterer.scatterRegion(region, group)
+		c.Assert(isPeerCountChanged(op), IsFalse)
+	}
+}
+
+func isPeerCountChanged(op *operator.Operator) bool {
+	if op == nil {
+		return false
+	}
+	add, remove := 0, 0
+	for i := 0; i < op.Len(); i++ {
+		step := op.Step(i)
+		switch step.(type) {
+		case operator.AddPeer, operator.AddLearner, operator.AddLightPeer, operator.AddLightLearner:
+			add++
+		case operator.RemovePeer:
+			remove++
+		}
+	}
+	return add != remove
+}