[ADD] Update syncthing buffer asynchronously

Update the syncthing buffer asynchronously using threads (when
supported). The URL library plays well with Emacs' cooperative threading
so this change entirely avoids blocking Emacs while retrieving
information from syncthing (without needing to rewrite all the url
retrieval calls to be async).

- Every thread gets its own current buffer, match state, etc. so this
doesn't conflict with other threads.
- Emacs threads are actually cooperative so there are no races here.
This may change in the future but this seems relatively safe for now.

Author: Stebalien

syncthing-draw.el

@@ -776,6 +776,16 @@
 (defun syncthing--draw (server)
   "Setup buffer and draw widgets for SERVER."
   (syncthing-trace)
+  (cond
+   ;; Threading isn't supported, continue.
+   ((not (bound-and-true-p main-thread)))
+   ;; We are the sync thread, continue.
+   ((eq (current-thread) (syncthing-buffer-update-thread syncthing-buffer)))
+   ;; We are the main thread, wait for syncing to complete before drawing.
+   ((eq (current-thread)
+        (when-let* ((thread (syncthing-buffer-update-thread syncthing-buffer)))
+          (thread-join thread))))
+   (t (error "Can only draw in the syncthing buffer from the update thread or the main thread")))
   (when (syncthing-buffer-initialized syncthing-buffer)
     (setf (syncthing-buffer-point syncthing-buffer) (point)))
   (let ((inhibit-read-only t))

syncthing-state.el

@@ -32,7 +32,7 @@
   name collapse-after-start initialized
   fold-folders skip-fold-folders
   fold-devices skip-fold-devices
-  point)
+  point update-thread)
 
 (defvar-local syncthing-watcher nil
   "Buffer-local instance for event poller.")

syncthing-update.el

@@ -15,16 +15,27 @@
 (require 'syncthing-watcher)
 
 
-(defun syncthing--update (&rest _)
-  "Update function for every refresh iteration."
-  (syncthing-trace)
-  (when-let* ((buf (syncthing-buffer-name syncthing-buffer)))
-    (with-current-buffer buf
+(defun syncthing--update-synchronous (buffer)
+  "Synchronously update the specified syncthing BUFFER."
+  (when (buffer-live-p buffer)
+    (with-current-buffer buffer
       (syncthing--ping syncthing-server)
       (syncthing--server-update syncthing-server)
       (syncthing--init-state)
       (syncthing--draw syncthing-server)
-      (setf (syncthing-buffer-collapse-after-start syncthing-buffer) nil))))
+      (setf (syncthing-buffer-collapse-after-start syncthing-buffer) nil
+            (syncthing-buffer-update-thread syncthing-buffer) nil))))
+
+(defun syncthing--update (&rest _)
+  "Update function for every refresh iteration."
+  (syncthing-trace)
+  (when-let* ((buf (current-buffer)))
+    (if (and (bound-and-true-p main-thread)
+             (not (and-let* ((thread (syncthing-buffer-update-thread syncthing-buffer)))
+                    (thread-live-p thread))))
+        (setf (syncthing-buffer-update-thread syncthing-buffer)
+              (make-thread (lambda () (syncthing--update-synchronous buf)) "Syncthing Update"))
+      (syncthing--update-synchronous buf))))
 
 (defun syncthing--calc-speed (server data)
   "Calculate upload and download rate DATA for SERVER."