region_scatter: fix a bug that PD may panic when scatter region if re…

…gion is unhealthy (#6128) (#6129)

close #6099, ref #6128

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

Co-authored-by: HunDunDM <hundundm@gmail.com>

server/schedule/region_scatterer.go

@@ -394,12 +394,14 @@ func allowLeader(fit *placement.RegionFit, peer *metapb.Peer) bool {
 	if peer.IsWitness {
 		return false
 	}
-
-	rule := fit.GetRuleFit(peer.GetId()).Rule
-	if rule.IsWitness {
+	peerFit := fit.GetRuleFit(peer.GetId())
+	if peerFit == nil || peerFit.Rule == nil {
+		return false
+	}
+	if peerFit.Rule.IsWitness {
 		return false
 	}
-	switch rule.Role {
+	switch peerFit.Rule.Role {
 	case placement.Voter, placement.Leader:
 		return true
 	}

server/schedule/region_scatterer_test.go

@@ -758,6 +758,11 @@ func TestBalanceRegion(t *testing.T) {
 		re.Equal(uint64(150), scatterer.ordinaryEngine.selectedPeer.Get(i, group))
 		re.Equal(uint64(50), scatterer.ordinaryEngine.selectedLeader.Get(i, group))
 	}
+	// Test for unhealthy region
+	// ref https://github.com/tikv/pd/issues/6099
+	region := tc.AddLeaderRegion(1500, 2, 3, 4, 6)
+	op := scatterer.scatterRegion(region, group)
+	re.False(isPeerCountChanged(op))
 }
 
 func isPeerCountChanged(op *operator.Operator) bool {