🎨 Fix network serving TLS on mobile devices (#17119)

* Until now, the TLS would only work via the fixed port proxy, which
   isn't used on mobile devices.
 * Move the logic for the multiplexer out of the fixed port logic
 * Use the newly moved multiplexer logic for the regular server as well,
   whenever the fixed port and the server port match.

kernel/util/cmux.go

@@ -0,0 +1,52 @@
+package util
+
+import (
+	"crypto/tls"
+	"errors"
+	"net"
+	"net/http"
+
+	"github.com/siyuan-note/logging"
+	"github.com/soheilhy/cmux"
+)
+
+func ServeMultiplexed(ln net.Listener, handler http.Handler, certPath, keyPath string, httpServer *http.Server) error {
+	m := cmux.New(ln)
+
+	tlsL := m.Match(cmux.TLS())
+	httpL := m.Match(cmux.Any())
+
+	cert, err := tls.LoadX509KeyPair(certPath, keyPath)
+	if err != nil {
+		logging.LogErrorf("failed to load TLS cert for multiplexing: %s", err)
+		return err
+	}
+
+	tlsConfig := &tls.Config{
+		Certificates: []tls.Certificate{cert},
+	}
+
+	tlsListener := tls.NewListener(tlsL, tlsConfig)
+
+	if httpServer == nil {
+		httpServer = &http.Server{Handler: handler}
+	} else {
+		httpServer.Handler = handler
+	}
+
+	httpsServer := &http.Server{Handler: handler}
+
+	go func() {
+		if serveErr := httpServer.Serve(httpL); serveErr != nil && serveErr != cmux.ErrListenerClosed && !errors.Is(serveErr, http.ErrServerClosed) {
+			logging.LogErrorf("multiplexed HTTP server error: %s", serveErr)
+		}
+	}()
+
+	go func() {
+		if serveErr := httpsServer.Serve(tlsListener); serveErr != nil && serveErr != cmux.ErrListenerClosed && !errors.Is(serveErr, http.ErrServerClosed) {
+			logging.LogErrorf("multiplexed HTTPS server error: %s", serveErr)
+		}
+	}()
+
+	return m.Serve()
+}