cmd/syncthing: Allow custom event subscriptions (fixes #1879)

This adds a parameter "events" to the /rest/events endpoint. It should
be a comma separated list of the events the consumer is interested in.
When not given it defaults to the current set of events, so it's
backwards compatible.

The API service then manages subscriptions, creating them as required
for each requested event mask. Old subscriptions are not "garbage
collected" - it's assumed that in normal usage the set of event
subscriptions will be small enough. Possibly lower than before, as we
will not set up the disk event subscription unless it's actually used.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4092

cmd/syncthing/gui.go

@@ -45,6 +45,12 @@ var (
 	startTime = time.Now()
 )
 
+const (
+	defaultEventMask   = events.AllEvents &^ events.LocalChangeDetected &^ events.RemoteChangeDetected
+	diskEventMask      = events.LocalChangeDetected | events.RemoteChangeDetected
+	eventSubBufferSize = 1000
+)
+
 type apiService struct {
 	id                 protocol.DeviceID
 	cfg                configIntf
@@ -52,8 +58,8 @@ type apiService struct {
 	httpsKeyFile       string
 	statics            *staticsServer
 	model              modelIntf
-	eventSub           events.BufferedSubscription
-	diskEventSub       events.BufferedSubscription
+	eventSubs          map[events.EventType]events.BufferedSubscription
+	eventSubsMut       sync.Mutex
 	discoverer         discover.CachingMux
 	connectionsService connectionsIntf
 	fss                *folderSummaryService
@@ -114,16 +120,19 @@ type connectionsIntf interface {
 	Status() map[string]interface{}
 }
 
-func newAPIService(id protocol.DeviceID, cfg configIntf, httpsCertFile, httpsKeyFile, assetDir string, m modelIntf, eventSub events.BufferedSubscription, diskEventSub events.BufferedSubscription, discoverer discover.CachingMux, connectionsService connectionsIntf, errors, systemLog logger.Recorder) *apiService {
+func newAPIService(id protocol.DeviceID, cfg configIntf, httpsCertFile, httpsKeyFile, assetDir string, m modelIntf, defaultSub, diskSub events.BufferedSubscription, discoverer discover.CachingMux, connectionsService connectionsIntf, errors, systemLog logger.Recorder) *apiService {
 	service := &apiService{
-		id:                 id,
-		cfg:                cfg,
-		httpsCertFile:      httpsCertFile,
-		httpsKeyFile:       httpsKeyFile,
-		statics:            newStaticsServer(cfg.GUI().Theme, assetDir),
-		model:              m,
-		eventSub:           eventSub,
-		diskEventSub:       diskEventSub,
+		id:            id,
+		cfg:           cfg,
+		httpsCertFile: httpsCertFile,
+		httpsKeyFile:  httpsKeyFile,
+		statics:       newStaticsServer(cfg.GUI().Theme, assetDir),
+		model:         m,
+		eventSubs: map[events.EventType]events.BufferedSubscription{
+			defaultEventMask: defaultSub,
+			diskEventMask:    diskSub,
+		},
+		eventSubsMut:       sync.NewMutex(),
 		discoverer:         discoverer,
 		connectionsService: connectionsService,
 		systemConfigMut:    sync.NewMutex(),
@@ -234,7 +243,7 @@ func (s *apiService) Serve() {
 	getRestMux.HandleFunc("/rest/db/need", s.getDBNeed)                          // folder [perpage] [page]
 	getRestMux.HandleFunc("/rest/db/status", s.getDBStatus)                      // folder
 	getRestMux.HandleFunc("/rest/db/browse", s.getDBBrowse)                      // folder [prefix] [dirsonly] [levels]
-	getRestMux.HandleFunc("/rest/events", s.getIndexEvents)                      // [since] [limit] [timeout]
+	getRestMux.HandleFunc("/rest/events", s.getIndexEvents)                      // [since] [limit] [timeout] [events]
 	getRestMux.HandleFunc("/rest/events/disk", s.getDiskEvents)                  // [since] [limit] [timeout]
 	getRestMux.HandleFunc("/rest/stats/device", s.getDeviceStats)                // -
 	getRestMux.HandleFunc("/rest/stats/folder", s.getFolderStats)                // -
@@ -1011,11 +1020,14 @@ func (s *apiService) postDBIgnores(w http.ResponseWriter, r *http.Request) {
 
 func (s *apiService) getIndexEvents(w http.ResponseWriter, r *http.Request) {
 	s.fss.gotEventRequest()
-	s.getEvents(w, r, s.eventSub)
+	mask := s.getEventMask(r.URL.Query().Get("events"))
+	sub := s.getEventSub(mask)
+	s.getEvents(w, r, sub)
 }
 
 func (s *apiService) getDiskEvents(w http.ResponseWriter, r *http.Request) {
-	s.getEvents(w, r, s.diskEventSub)
+	sub := s.getEventSub(diskEventMask)
+	s.getEvents(w, r, sub)
 }
 
 func (s *apiService) getEvents(w http.ResponseWriter, r *http.Request, eventSub events.BufferedSubscription) {
@@ -1047,6 +1059,31 @@ func (s *apiService) getEvents(w http.ResponseWriter, r *http.Request, eventSub
 	sendJSON(w, evs)
 }
 
+func (s *apiService) getEventMask(evs string) events.EventType {
+	eventMask := defaultEventMask
+	if evs != "" {
+		eventList := strings.Split(evs, ",")
+		eventMask = 0
+		for _, ev := range eventList {
+			eventMask |= events.UnmarshalEventType(strings.TrimSpace(ev))
+		}
+	}
+	return eventMask
+}
+
+func (s *apiService) getEventSub(mask events.EventType) events.BufferedSubscription {
+	s.eventSubsMut.Lock()
+	bufsub, ok := s.eventSubs[mask]
+	if !ok {
+		evsub := events.Default.Subscribe(mask)
+		bufsub = events.NewBufferedSubscription(evsub, eventSubBufferSize)
+		s.eventSubs[mask] = bufsub
+	}
+	s.eventSubsMut.Unlock()
+
+	return bufsub
+}
+
 func (s *apiService) getSystemUpgrade(w http.ResponseWriter, r *http.Request) {
 	if noUpgradeFromEnv {
 		http.Error(w, upgrade.ErrUpgradeUnsupported.Error(), 500)