topology_watcher: Allow tablets to reuse old tablet addresses.

go/vt/discovery/fake_healthcheck.go

@@ -21,6 +21,7 @@ import (
 	"sync"
 
 	"vitess.io/vitess/go/vt/topo"
+	"vitess.io/vitess/go/vt/topo/topoproto"
 	"vitess.io/vitess/go/vt/vttablet/queryservice"
 	"vitess.io/vitess/go/vt/vttablet/sandboxconn"
 
@@ -104,6 +105,17 @@ func (fhc *FakeHealthCheck) RemoveTablet(tablet *topodatapb.Tablet) {
 	fhc.mu.Lock()
 	defer fhc.mu.Unlock()
 	key := TabletToMapKey(tablet)
+	item, ok := fhc.items[key]
+	if !ok {
+		return
+	}
+	// Make sure the key still corresponds to the tablet we want to delete.
+	// If it doesn't match, we should do nothing. The tablet we were asked to
+	// delete is already gone, and some other tablet is using the key
+	// (host:port) that the original tablet used to use, which is fine.
+	if !topoproto.TabletAliasEqual(tablet.Alias, item.ts.Tablet.Alias) {
+		return
+	}
 	delete(fhc.items, key)
 }
 

go/vt/discovery/healthcheck.go

@@ -688,6 +688,17 @@ func (hc *HealthCheckImpl) deleteConn(tablet *topodatapb.Tablet) {
 	if !ok {
 		return
 	}
+	// Make sure the key still corresponds to the tablet we want to delete.
+	// If it doesn't match, we should do nothing. The tablet we were asked to
+	// delete is already gone, and some other tablet is using the key
+	// (host:port) that the original tablet used to use, which is fine.
+	if !topoproto.TabletAliasEqual(tablet.Alias, th.latestTabletStats.Tablet.Alias) {
+		return
+	}
+	hc.deleteConnLocked(key, th)
+}
+
+func (hc *HealthCheckImpl) deleteConnLocked(key string, th *tabletHealth) {
 	th.latestTabletStats.Up = false
 	th.cancelFunc()
 	delete(hc.addrToHealth, key)
@@ -733,10 +744,18 @@ func (hc *HealthCheckImpl) AddTablet(tablet *topodatapb.Tablet, name string) {
 		hc.mu.Unlock()
 		return
 	}
-	if _, ok := hc.addrToHealth[key]; ok {
-		hc.mu.Unlock()
-		log.Warningf("adding duplicate tablet %v for %v: %+v", name, tablet.Alias.Cell, tablet)
-		return
+	if th, ok := hc.addrToHealth[key]; ok {
+		// Something already exists at this key.
+		// If it's the same tablet, something is wrong.
+		if topoproto.TabletAliasEqual(th.latestTabletStats.Tablet.Alias, tablet.Alias) {
+			hc.mu.Unlock()
+			log.Warningf("refusing to add duplicate tablet %v for %v: %+v", name, tablet.Alias.Cell, tablet)
+			return
+		}
+		// If it's a different tablet, then we trust this new tablet that claims
+		// it has taken over the host:port that the old tablet used to be on.
+		// Remove the old tablet to clear the way.
+		hc.deleteConnLocked(key, th)
 	}
 	hc.addrToHealth[key] = &tabletHealth{
 		cancelFunc:        cancelFunc,
@@ -752,15 +771,13 @@ func (hc *HealthCheckImpl) AddTablet(tablet *topodatapb.Tablet, name string) {
 // RemoveTablet removes the tablet, and stops the health check.
 // It does not block.
 func (hc *HealthCheckImpl) RemoveTablet(tablet *topodatapb.Tablet) {
-	go hc.deleteConn(tablet)
+	hc.deleteConn(tablet)
 }
 
 // ReplaceTablet removes the old tablet and adds the new tablet.
 func (hc *HealthCheckImpl) ReplaceTablet(old, new *topodatapb.Tablet, name string) {
-	go func() {
-		hc.deleteConn(old)
-		hc.AddTablet(new, name)
-	}()
+	hc.deleteConn(old)
+	hc.AddTablet(new, name)
 }
 
 // WaitForInitialStatsUpdates waits until all tablets added via AddTablet() call

go/vt/discovery/topology_watcher_test.go

@@ -234,6 +234,56 @@ func checkWatcher(t *testing.T, cellTablets, refreshKnownTablets bool) {
 		t.Errorf("fhc.GetAllTablets() = %+v; want %v => %+v", allTablets, key, tablet2)
 	}
 
+	// Both tablets restart on different hosts.
+	// tablet2 happens to land on the host:port that tablet 1 used to be on.
+	// This can only be tested when we refresh known tablets.
+	if refreshKnownTablets {
+		origTablet := *tablet
+		origTablet2 := *tablet2
+
+		if _, err := ts.UpdateTabletFields(context.Background(), tablet2.Alias, func(t *topodatapb.Tablet) error {
+			t.Hostname = tablet.Hostname
+			t.PortMap = tablet.PortMap
+			tablet2 = t
+			return nil
+		}); err != nil {
+			t.Fatalf("UpdateTabletFields failed: %v", err)
+		}
+		if _, err := ts.UpdateTabletFields(context.Background(), tablet.Alias, func(t *topodatapb.Tablet) error {
+			t.Hostname = "host3"
+			tablet = t
+			return nil
+		}); err != nil {
+			t.Fatalf("UpdateTabletFields failed: %v", err)
+		}
+		tw.loadTablets()
+		counts = checkOpCounts(t, tw, counts, map[string]int64{"ListTablets": 1, "GetTablet": 2, "ReplaceTablet": 2})
+		allTablets = fhc.GetAllTablets()
+		key2 := TabletToMapKey(tablet2)
+		if _, ok := allTablets[key2]; !ok {
+			t.Fatalf("tablet was lost because it's reusing an address recently used by another tablet: %v", key2)
+		}
+
+		// Change tablets back to avoid altering later tests.
+		if _, err := ts.UpdateTabletFields(context.Background(), tablet2.Alias, func(t *topodatapb.Tablet) error {
+			t.Hostname = origTablet2.Hostname
+			t.PortMap = origTablet2.PortMap
+			tablet2 = t
+			return nil
+		}); err != nil {
+			t.Fatalf("UpdateTabletFields failed: %v", err)
+		}
+		if _, err := ts.UpdateTabletFields(context.Background(), tablet.Alias, func(t *topodatapb.Tablet) error {
+			t.Hostname = origTablet.Hostname
+			tablet = t
+			return nil
+		}); err != nil {
+			t.Fatalf("UpdateTabletFields failed: %v", err)
+		}
+		tw.loadTablets()
+		counts = checkOpCounts(t, tw, counts, map[string]int64{"ListTablets": 1, "GetTablet": 2, "ReplaceTablet": 2})
+	}
+
 	// Remove the tablet and check that it is detected as being gone.
 	if err := ts.DeleteTablet(context.Background(), tablet.Alias); err != nil {
 		t.Fatalf("DeleteTablet failed: %v", err)