all: Support multiple device connections (fixes #141) (#8918)

This adds the ability to have multiple concurrent connections to a single device. This is primarily useful when the network has multiple physical links for aggregated bandwidth. A single connection will never see a higher rate than a single link can give, but multiple connections are load-balanced over multiple links.

It is also incidentally useful for older multi-core CPUs, where bandwidth could be limited by the TLS performance of a single CPU core -- using multiple connections achieves concurrency in the required crypto calculations...

Co-authored-by: Simon Frei <freisim93@gmail.com>
Co-authored-by: tomasz1986 <twilczynski@naver.com>
Co-authored-by: bt90 <btom1990@googlemail.com>

lib/api/api.go

@@ -1230,6 +1230,14 @@ func (s *service) getSupportBundle(w http.ResponseWriter, r *http.Request) {
 	promhttp.Handler().ServeHTTP(wr, &http.Request{Method: http.MethodGet})
 	files = append(files, fileEntry{name: "metrics.txt", data: buf.Bytes()})
 
+	// Connection data as JSON
+	connStats := s.model.ConnectionStats()
+	if connStatsJSON, err := json.MarshalIndent(connStats, "", "  "); err != nil {
+		l.Warnln("Support bundle: failed to serialize connection-stats.json.txt", err)
+	} else {
+		files = append(files, fileEntry{name: "connection-stats.json.txt", data: connStatsJSON})
+	}
+
 	// Heap and CPU Proofs as a pprof extension
 	var heapBuffer, cpuBuffer bytes.Buffer
 	filename := fmt.Sprintf("syncthing-heap-%s-%s-%s-%s.pprof", runtime.GOOS, runtime.GOARCH, build.Version, time.Now().Format("150405")) // hhmmss
@@ -1607,7 +1615,7 @@ func (s *service) getPeerCompletion(w http.ResponseWriter, _ *http.Request) {
 	for _, folder := range s.cfg.Folders() {
 		for _, device := range folder.DeviceIDs() {
 			deviceStr := device.String()
-			if _, ok := s.model.Connection(device); ok {
+			if s.model.ConnectedTo(device) {
 				comp, err := s.model.Completion(device, folder.ID)
 				if err != nil {
 					http.Error(w, err.Error(), http.StatusInternalServerError)