api, gui: Prevent connection issues due to unsupported-upgrade (fixes #8569) (#8586)

There are some situations where an upgrade wouldn't be supported, even though the noUpgrade bool isn't set. So when handling the errors that are caused by this, when attempting an upgrade, it shouldn't lead to some sort of offline-message/restart/warning/etc...

I added some checks on specific errors related to this and return a 501 (Not Implemented) response instead, in case of an "UpgradeUnsupported"-error. Additionally, on the GUI-side, the 501-response is now not to be considered an error to act upon.

2 files changed, 30 insertions, 22 deletions

diff --git a/gui/default/syncthing/core/syncthingController.js b/gui/default/syncthing/core/syncthingController.js
index 49d9d446e8..1280145f59 100755
--- a/gui/default/syncthing/core/syncthingController.js
+++ b/gui/default/syncthing/core/syncthingController.js
@@ -183,8 +183,9 @@ angular.module('syncthing.core')
                 if (arg.status === 0) {
                     // A network error, not an HTTP error
                     $scope.$emit(Events.OFFLINE);
-                } else if (arg.status >= 400 && arg.status <= 599) {
-                    // A genuine HTTP error
+                } else if (arg.status >= 400 && arg.status <= 599 && arg.status != 501) {
+                    // A genuine HTTP error. 501/NotImplemented is considered intentional 
+                    // and not an error which we need to act upon.
                     $('#networkError').modal('hide');
                     $('#restarting').modal('hide');
                     $('#shutdown').modal('hide');
diff --git a/lib/api/api.go b/lib/api/api.go
index f24a974112..5ca8f52152 100644
--- a/lib/api/api.go
+++ b/lib/api/api.go
@@ -864,7 +864,7 @@ func (s *service) getDBRemoteNeed(w http.ResponseWriter, r *http.Request) {
 	device := qs.Get("device")
 	deviceID, err := protocol.DeviceIDFromString(device)
 	if err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 
@@ -1017,7 +1017,7 @@ func (s *service) postSystemReset(w http.ResponseWriter, r *http.Request) {
 
 	if len(folder) > 0 {
 		if _, ok := s.cfg.Folders()[folder]; !ok {
-			http.Error(w, "Invalid folder ID", 500)
+			http.Error(w, "Invalid folder ID", http.StatusInternalServerError)
 			return
 		}
 	}
@@ -1291,7 +1291,7 @@ func (s *service) getReport(w http.ResponseWriter, r *http.Request) {
 		version = val
 	}
 	if r, err := s.urService.ReportDataPreview(context.TODO(), version); err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	} else {
 		sendJSON(w, r)
@@ -1316,7 +1316,7 @@ func (s *service) getDBIgnores(w http.ResponseWriter, r *http.Request) {
 
 	lines, patterns, err := s.model.LoadIgnores(folder)
 	if err != nil && !ignore.IsParseError(err) {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 
@@ -1333,20 +1333,20 @@ func (s *service) postDBIgnores(w http.ResponseWriter, r *http.Request) {
 	bs, err := io.ReadAll(r.Body)
 	r.Body.Close()
 	if err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 
 	var data map[string][]string
 	err = json.Unmarshal(bs, &data)
 	if err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 
 	err = s.model.SetIgnores(qs.Get("folder"), data["ignore"])
 	if err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 
@@ -1424,13 +1424,13 @@ func (s *service) getEventSub(mask events.EventType) events.BufferedSubscription
 
 func (s *service) getSystemUpgrade(w http.ResponseWriter, _ *http.Request) {
 	if s.noUpgrade {
-		http.Error(w, upgrade.ErrUpgradeUnsupported.Error(), http.StatusServiceUnavailable)
+		http.Error(w, upgrade.ErrUpgradeUnsupported.Error(), http.StatusNotImplemented)
 		return
 	}
 	opts := s.cfg.Options()
 	rel, err := upgrade.LatestRelease(opts.ReleasesURL, build.Version, opts.UpgradeToPreReleases)
 	if err != nil {
-		http.Error(w, err.Error(), 500)
+		httpError(w, err)
 		return
 	}
 	res := make(map[string]interface{})
@@ -1472,8 +1472,7 @@ func (s *service) postSystemUpgrade(w http.ResponseWriter, _ *http.Request) {
 	opts := s.cfg.Options()
 	rel, err := upgrade.LatestRelease(opts.ReleasesURL, build.Version, opts.UpgradeToPreReleases)
 	if err != nil {
-		l.Warnln("getting latest release:", err)
-		http.Error(w, err.Error(), 500)
+		httpError(w, err)
 		return
 	}
 
@@ -1481,7 +1480,7 @@ func (s *service) postSystemUpgrade(w http.ResponseWriter, _ *http.Request) {
 		err = upgrade.To(rel)
 		if err != nil {
 			l.Warnln("upgrading:", err)
-			http.Error(w, err.Error(), 500)
+			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
 
@@ -1528,7 +1527,7 @@ func (s *service) makeDevicePauseHandler(paused bool) http.HandlerFunc {
 		if msg != "" {
 			http.Error(w, msg, status)
 		} else if err != nil {
-			http.Error(w, err.Error(), 500)
+			http.Error(w, err.Error(), http.StatusInternalServerError)
 		}
 	}
 }
@@ -1540,7 +1539,7 @@ func (s *service) postDBScan(w http.ResponseWriter, r *http.Request) {
 		subs := qs["sub"]
 		err := s.model.ScanFolderSubdirs(folder, subs)
 		if err != nil {
-			http.Error(w, err.Error(), 500)
+			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
 		nextStr := qs.Get("next")
@@ -1551,7 +1550,7 @@ func (s *service) postDBScan(w http.ResponseWriter, r *http.Request) {
 	} else {
 		errors := s.model.ScanFolders()
 		if len(errors) > 0 {
-			http.Error(w, "Error scanning folders", 500)
+			http.Error(w, "Error scanning folders", http.StatusInternalServerError)
 			sendJSON(w, errors)
 			return
 		}
@@ -1571,7 +1570,7 @@ func (*service) getQR(w http.ResponseWriter, r *http.Request) {
 	var text = qs.Get("text")
 	code, err := qr.Encode(text, qr.M)
 	if err != nil {
-		http.Error(w, "Invalid", 500)
+		http.Error(w, "Invalid", http.StatusInternalServerError)
 		return
 	}
 
@@ -1612,7 +1611,7 @@ func (s *service) getFolderVersions(w http.ResponseWriter, r *http.Request) {
 	qs := r.URL.Query()
 	versions, err := s.model.GetFolderVersions(qs.Get("folder"))
 	if err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 	sendJSON(w, versions)
@@ -1624,20 +1623,20 @@ func (s *service) postFolderVersionsRestore(w http.ResponseWriter, r *http.Reque
 	bs, err := io.ReadAll(r.Body)
 	r.Body.Close()
 	if err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 
 	var versions map[string]time.Time
 	err = json.Unmarshal(bs, &versions)
 	if err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 
 	ferr, err := s.model.RestoreFolderVersions(qs.Get("folder"), versions)
 	if err != nil {
-		http.Error(w, err.Error(), 500)
+		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
 	sendJSON(w, errorStringMap(ferr))
@@ -2015,3 +2014,11 @@ func isFolderNotFound(err error) bool {
 	}
 	return false
 }
+
+func httpError(w http.ResponseWriter, err error) {
+	if errors.Is(err, upgrade.ErrUpgradeUnsupported) {
+		http.Error(w, upgrade.ErrUpgradeUnsupported.Error(), http.StatusNotImplemented)
+	} else {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+	}
+}