checker: fix the issue that a region does not merge to the sibling wi…

…th smaller size (#1723) (#1726)

Signed-off-by: disksing <i@disksing.com>

server/schedule/merge_checker.go

@@ -100,12 +100,9 @@ func (m *MergeChecker) Check(region *core.RegionInfo) []*Operator {
 
 	prev, next := m.cluster.GetAdjacentRegions(region)
 
-	var target *core.RegionInfo
-	targetNext := m.checkTarget(region, next, target)
-	target = m.checkTarget(region, prev, target)
-	if target != targetNext && m.cluster.GetEnableOneWayMerge() {
-		checkerCounter.WithLabelValues("merge_checker", "skip_left").Inc()
-		target = targetNext
+	target := m.checkTarget(region, next, nil)
+	if !m.cluster.GetEnableOneWayMerge() {
+		target = m.checkTarget(region, prev, target)
 	}
 
 	if target == nil {

server/schedule/merge_checker_test.go

@@ -128,13 +128,25 @@ func (s *testMergeCheckerSuite) TestBasic(c *C) {
 		op.startTime = op.startTime.Add(-RegionOperatorWaitTime - time.Second)
 		c.Assert(op.IsTimeout(), IsTrue)
 	}
+	// Check merge with previous region.
+	c.Assert(ops[0].RegionID(), Equals, s.regions[2].GetID())
+	c.Assert(ops[1].RegionID(), Equals, s.regions[1].GetID())
 
 	// Enable one way merge
 	s.cluster.EnableOneWayMerge = true
 	ops = s.mc.Check(s.regions[2])
 	c.Assert(ops, IsNil)
 	s.cluster.EnableOneWayMerge = false
 
+	// Make up peers for next region.
+	s.regions[3] = s.regions[3].Clone(core.WithAddPeer(&metapb.Peer{Id: 110, StoreId: 1}), core.WithAddPeer(&metapb.Peer{Id: 111, StoreId: 2}))
+	s.cluster.PutRegion(s.regions[3])
+	ops = s.mc.Check(s.regions[2])
+	c.Assert(ops, NotNil)
+	// Now it merges to next region.
+	c.Assert(ops[0].RegionID(), Equals, s.regions[2].GetID())
+	c.Assert(ops[1].RegionID(), Equals, s.regions[3].GetID())
+
 	// Skip recently split regions.
 	s.mc.RecordRegionSplit(s.regions[2].GetID())
 	ops = s.mc.Check(s.regions[2])