Subject: bug#45256: Viewing images over network using TRAMP (errors and unexpected prompts)

From: Juri Linkov <[email-masked]>
Date: Thu, 17 Dec 2020 23:59:55 +0200

>> The simplest solution is just to increase the number of seconds
>> in the user option 'image-auto-resize-on-window-resize'
>> proportionally to network latency.
>
> That's a global variable, right? So you would also delay resizing of
> images located locally.

Right, but disabling image-auto-resize-on-window-resize to nil in init file
will help as a temporary measure until this problem is properly fixed
for slow connections.

>> Locking could be implemented as well.  How would be better to do this?
>> Maybe by using a buffer-local variable?
>
> Yes. Something like
>
> (defvar image-fit-to-window-lock nil
>   "Lock for `image-fit-to-window' timer."
>
> (defun image-fit-to-window (window)
>   "..."
>   (unless image-fit-to-window-lock
>     (unwind-protect
> 	(progn
> 	  (setq-local image-fit-to-window-lock t)
> 	  ...)
>       (setq image-fit-to-window-lock nil))))
>
>
> There's also another thread. When image-fit-to-window is called from
> Emacs the first time, there could also be a running Tramp operation,
> which would be disturbed. See the recent discussion about "Tramp and
> timers" in the emacs-devel ML. Tramp would detect this situation, and
> fire the (new) error remote-file-error. This must also be handled, like
>
> (defun image-fit-to-window (window)
>   "..."
>   (ignore-error 'remote-file-error
>     ...))

Additionally to these changes, I also added canceling the previous timer
before starting a new timer to avoid several simultaneously started timers.
This improved responsiveness in non-remote case.  In remote case it helps
a little too, but still needs the lock for extremely slow connections.
Here is a complete patch (BTW, I'm not sure if the check for file-remote-p
can be removed now):

diff --git a/lisp/image-mode.el b/lisp/image-mode.el
index 032ebf3873..a5b610745c 100644
--- a/lisp/image-mode.el
+++ b/lisp/image-mode.el
@@ -795,7 +795,7 @@ image-toggle-display-image
   (let* ((filename (buffer-file-name))
 	 (data-p (not (and filename
 			   (file-readable-p filename)
-			   (not (file-remote-p filename))
+			   (not (file-remote-p filename)) ; TODO: remove?
 			   (not (buffer-modified-p))
 			   (not (and (boundp 'archive-superior-buffer)
 				     archive-superior-buffer))
@@ -942,6 +942,9 @@ image-after-revert-hook
           (get-buffer-window-list (current-buffer) 'nomini 'visible))
     (image-toggle-display-image)))
 
+(defvar image-auto-resize-timer nil
+  "Timer for `image-auto-resize-on-window-resize' option.")
+
 (defun image--window-state-change (window)
   ;; Wait for a bit of idle-time before actually performing the change,
   ;; so as to batch together sequences of closely consecutive size changes.
@@ -950,8 +953,15 @@ image--window-state-change
   ;; consecutive calls happen without any redisplay between them,
   ;; the costly operation of image resizing should happen only once.
   (when (numberp image-auto-resize-on-window-resize)
-    (run-with-idle-timer image-auto-resize-on-window-resize nil
-                         #'image-fit-to-window window)))
+    (when image-auto-resize-timer
+      (cancel-timer image-auto-resize-timer)
+      (setq image-auto-resize-timer nil))
+    (setq image-auto-resize-timer
+          (run-with-idle-timer image-auto-resize-on-window-resize nil
+                               #'image-fit-to-window window))))
+
+(defvar image-fit-to-window-lock nil
+  "Lock for `image-fit-to-window' timer function.")
 
 (defun image-fit-to-window (window)
   "Adjust size of image to display it exactly in WINDOW boundaries."
@@ -968,7 +978,13 @@ image-fit-to-window
               (when (and image-width image-height
                          (or (not (= image-width  window-width))
                              (not (= image-height window-height))))
-                (image-toggle-display-image)))))))))
+                (unless image-fit-to-window-lock
+                  (unwind-protect
+                      (progn
+                        (setq-local image-fit-to-window-lock t)
+                        (ignore-error 'remote-file-error
+                          (image-toggle-display-image)))
+                    (setq image-fit-to-window-lock nil)))))))))))
 
 
 ;;; Animated images