fix targetGroup that disappear totally

Signed-off-by: Augustin Husson <husson.augustin@gmail.com>

documentation/examples/custom-sd/adapter-usage/main.go

@@ -94,6 +94,7 @@ type discovery struct {
 	clientDatacenter string
 	tagSeparator     string
 	logger           log.Logger
+	oldSourceList    map[string]bool
 }
 
 func (d *discovery) parseServiceNodes(resp *http.Response, name string) (*targetgroup.Group, error) {
@@ -180,6 +181,8 @@ func (d *discovery) Run(ctx context.Context, ch chan<- []*targetgroup.Group) {
 		// list of targets simply because there may have been a timeout. If the service is actually
 		// gone as far as consul is concerned, that will be picked up during the next iteration of
 		// the outer loop.
+
+		newSourceList := make(map[string]bool)
 		for name := range srvs {
 			if name == "consul" {
 				continue
@@ -195,7 +198,17 @@ func (d *discovery) Run(ctx context.Context, ch chan<- []*targetgroup.Group) {
 				break
 			}
 			tgs = append(tgs, tg)
+			newSourceList[tg.Source] = true
+		}
+		// when targetGroup disappear, send an update with empty targetList
+		for key := range d.oldSourceList {
+			if !newSourceList[key] {
+				tgs = append(tgs, &targetgroup.Group{
+					Source: key,
+				})
+			}
 		}
+		d.oldSourceList = newSourceList
 		if err == nil {
 			// We're returning all Consul services as a single targetgroup.
 			ch <- tgs
@@ -216,6 +229,7 @@ func newDiscovery(conf sdConfig) (*discovery, error) {
 		refreshInterval: conf.RefreshInterval,
 		tagSeparator:    conf.TagSeparator,
 		logger:          logger,
+		oldSourceList:   make(map[string]bool),
 	}
 	return cd, nil
 }