fix(syncthing): apply folder migrations with temporary API/GUI server (#10330)

Prevent the feeling that nothing is happening / it's not starting.

Signed-off-by: Jakob Borg <jakob@kastelo.net>

cmd/syncthing/main.go

@@ -479,11 +479,20 @@ func (c *serveCmd) syncthingMain() {
 		})
 	}
 
-	var tempApiAddress string
+	migratingAPICtx, migratingAPICancel := context.WithCancel(ctx)
 	if cfgWrapper.GUI().Enabled {
-		tempApiAddress = cfgWrapper.GUI().Address()
+		// Start a temporary API server during the migration. It will wait
+		// startDelay until actually starting, so that if we quickly pass
+		// through the migration steps (e.g., there was nothing to migrate)
+		// and cancel the context, it will never even start.
+		api := migratingAPI{
+			addr:       cfgWrapper.GUI().Address(),
+			startDelay: 5 * time.Second,
+		}
+		go api.Serve(migratingAPICtx)
 	}
-	if err := syncthing.TryMigrateDatabase(ctx, c.DBDeleteRetentionInterval, tempApiAddress); err != nil {
+
+	if err := syncthing.TryMigrateDatabase(ctx, c.DBDeleteRetentionInterval); err != nil {
 		slog.Error("Failed to migrate old-style database", slogutil.Error(err))
 		os.Exit(1)
 	}
@@ -494,6 +503,8 @@ func (c *serveCmd) syncthingMain() {
 		os.Exit(1)
 	}
 
+	migratingAPICancel() // we're done with the temporary API server
+
 	// Check if auto-upgrades is possible, and if yes, and it's enabled do an initial
 	// upgrade immediately. The auto-upgrade routine can only be started
 	// later after App is initialised.
@@ -1015,3 +1026,32 @@ func setConfigDataLocationsFromFlags(homeDir, confDir, dataDir string) error {
 	}
 	return nil
 }
+
+type migratingAPI struct {
+	addr       string
+	startDelay time.Duration
+}
+
+func (m migratingAPI) Serve(ctx context.Context) error {
+	srv := &http.Server{
+		Addr: m.addr,
+		Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Content-Type", "text/plain")
+			w.Write([]byte("*** Database migration in progress ***\n\n"))
+			for _, line := range slogutil.GlobalRecorder.Since(time.Time{}) {
+				line.WriteTo(w)
+			}
+		}),
+	}
+	go func() {
+		select {
+		case <-time.After(m.startDelay):
+			slog.InfoContext(ctx, "Starting temporary GUI/API during migration", slogutil.Address(m.addr))
+			err := srv.ListenAndServe()
+			slog.InfoContext(ctx, "Temporary GUI/API closed", slogutil.Address(m.addr), slogutil.Error(err))
+		case <-ctx.Done():
+		}
+	}()
+	<-ctx.Done()
+	return srv.Close()
+}