region_cache: allow pd follower handle region api (#1072)

Signed-off-by: Cabinfever_B <cabinfeveroier@gmail.com>

internal/locate/region_cache.go

@@ -1117,7 +1117,7 @@ func (c *RegionCache) findRegionByKey(bo *retry.Backoffer, key []byte, isEndKey
 	r = c.searchCachedRegion(key, isEndKey)
 	if r == nil {
 		// load region when it is not exists or expired.
-		lr, err := c.loadRegion(bo, key, isEndKey)
+		lr, err := c.loadRegion(bo, key, isEndKey, pd.WithAllowFollowerHandle())
 		if err != nil {
 			// no region data, return error if failure.
 			return nil, err
@@ -1125,8 +1125,20 @@ func (c *RegionCache) findRegionByKey(bo *retry.Backoffer, key []byte, isEndKey
 		logutil.Eventf(bo.GetCtx(), "load region %d from pd, due to cache-miss", lr.GetID())
 		r = lr
 		c.mu.Lock()
-		c.insertRegionToCache(r, true, true)
+		stale := !c.insertRegionToCache(r, true, true)
 		c.mu.Unlock()
+		// just retry once, it won't bring much overhead.
+		if stale {
+			lr, err = c.loadRegion(bo, key, isEndKey)
+			if err != nil {
+				// no region data, return error if failure.
+				return nil, err
+			}
+			r = lr
+			c.mu.Lock()
+			c.insertRegionToCache(r, true, true)
+			c.mu.Unlock()
+		}
 	} else if r.checkNeedReloadAndMarkUpdated() {
 		// load region when it be marked as need reload.
 		lr, err := c.loadRegion(bo, key, isEndKey)
@@ -1673,7 +1685,7 @@ func filterUnavailablePeers(region *pd.Region) {
 // loadRegion loads region from pd client, and picks the first peer as leader.
 // If the given key is the end key of the region that you want, you may set the second argument to true. This is useful
 // when processing in reverse order.
-func (c *RegionCache) loadRegion(bo *retry.Backoffer, key []byte, isEndKey bool) (*Region, error) {
+func (c *RegionCache) loadRegion(bo *retry.Backoffer, key []byte, isEndKey bool, opts ...pd.GetRegionOption) (*Region, error) {
 	ctx := bo.GetCtx()
 	if span := opentracing.SpanFromContext(ctx); span != nil && span.Tracer() != nil {
 		span1 := span.Tracer().StartSpan("loadRegion", opentracing.ChildOf(span.Context()))
@@ -1683,6 +1695,7 @@ func (c *RegionCache) loadRegion(bo *retry.Backoffer, key []byte, isEndKey bool)
 
 	var backoffErr error
 	searchPrev := false
+	opts = append(opts, pd.WithBuckets())
 	for {
 		if backoffErr != nil {
 			err := bo.Backoff(retry.BoPDRPC, backoffErr)
@@ -1694,9 +1707,9 @@ func (c *RegionCache) loadRegion(bo *retry.Backoffer, key []byte, isEndKey bool)
 		var reg *pd.Region
 		var err error
 		if searchPrev {
-			reg, err = c.pdClient.GetPrevRegion(ctx, key, pd.WithBuckets())
+			reg, err = c.pdClient.GetPrevRegion(ctx, key, opts...)
 		} else {
-			reg, err = c.pdClient.GetRegion(ctx, key, pd.WithBuckets())
+			reg, err = c.pdClient.GetRegion(ctx, key, opts...)
 		}
 		metrics.LoadRegionCacheHistogramWhenCacheMiss.Observe(time.Since(start).Seconds())
 		if err != nil {
@@ -1848,7 +1861,7 @@ func (c *RegionCache) scanRegions(bo *retry.Backoffer, startKey, endKey []byte,
 			}
 		}
 		start := time.Now()
-		regionsInfo, err := c.pdClient.ScanRegions(ctx, startKey, endKey, limit)
+		regionsInfo, err := c.pdClient.ScanRegions(ctx, startKey, endKey, limit, pd.WithAllowFollowerHandle())
 		metrics.LoadRegionCacheHistogramWithRegions.Observe(time.Since(start).Seconds())
 		if err != nil {
 			if apicodec.IsDecodeError(err) {

internal/locate/region_cache_test.go

@@ -1952,6 +1952,40 @@ func (s *testRegionCacheWithDelaySuite) TestStaleGetRegion() {
 	s.Equal([]byte("b"), r.meta.EndKey)
 }
 
+func (s *testRegionCacheWithDelaySuite) TestFollowerGetStaleRegion() {
+	var delay uatomic.Bool
+	pdCli3 := &CodecPDClient{mocktikv.NewPDClient(s.cluster, mocktikv.WithDelay(&delay)), apicodec.NewCodecV1(apicodec.ModeTxn)}
+	followerDelayCache := NewRegionCache(pdCli3)
+
+	delay.Store(true)
+	var wg sync.WaitGroup
+	wg.Add(1)
+	var final *Region
+	go func() {
+		var err error
+		// followerDelayCache is empty now, so it will go follower.
+		final, err = followerDelayCache.findRegionByKey(s.bo, []byte("z"), false)
+		s.NoError(err)
+		wg.Done()
+	}()
+	time.Sleep(30 * time.Millisecond)
+	delay.Store(false)
+	r, err := followerDelayCache.findRegionByKey(s.bo, []byte("y"), false)
+	s.NoError(err)
+	newPeersIDs := s.cluster.AllocIDs(1)
+	s.cluster.Split(r.GetID(), s.cluster.AllocID(), []byte("z"), newPeersIDs, newPeersIDs[0])
+	r.invalidate(Other)
+	r, err = followerDelayCache.findRegionByKey(s.bo, []byte("y"), false)
+	s.NoError(err)
+	s.Equal([]byte("z"), r.meta.EndKey)
+
+	// no need to retry because
+	wg.Wait()
+	s.Equal([]byte("z"), final.meta.StartKey)
+
+	followerDelayCache.Close()
+}
+
 func generateKeyForSimulator(id int, keyLen int) []byte {
 	k := make([]byte, keyLen)
 	copy(k, fmt.Sprintf("%010d", id))