>From 4f941c1fe7bc91b121f305b3a3239cf9a3944a6b Mon Sep 17 00:00:00 2001
From: Manuel Giraud <manuel@ledu-giraud.fr>
Date: Tue, 26 Dec 2023 15:35:17 +0100
Subject: [PATCH] Add some user control on image caching

* lisp/image-mode.el (image-eager-flush): New custom to control
image cache flush.
(image-toggle-display-image): Flush image upon on file disk
change.
* src/image.c (Fimage_timestamp): New function to access image
display timestamp.
---
 lisp/image-mode.el | 16 ++++++++++++++--
 src/image.c        | 25 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/lisp/image-mode.el b/lisp/image-mode.el
index d5ca6348c92..ccea96ecc26 100644
--- a/lisp/image-mode.el
+++ b/lisp/image-mode.el
@@ -93,6 +93,12 @@ image-auto-resize-on-window-resize
   :version "27.1"
   :group 'image)
 
+(defcustom image-eager-flush t
+  "Non-nil means flush image from cache eagerly."
+  :type 'boolean
+  :version "30.1"
+  :group 'image)
+
 (defvar-local image-transform-resize nil
   "The image resize operation.
 Non-nil to resize the image upon first display.
@@ -954,8 +960,14 @@ image-toggle-display-image
                   (plist-put (cdr image) :width
                              (plist-get (cdr image) :max-width)))))
 
-    ;; Discard any stale image data before looking it up again.
-    (image-flush image)
+    ;; Discard image data if its file has changed on disk.
+    (when (or image-eager-flush
+              (and filename
+                   (time-less-p
+                    (image-timestamp image)
+                    (file-attribute-modification-time
+                     (file-attributes filename)))))
+      (image-flush image))
     (setq image (append image (image-transform-properties image)))
     (setq props
 	  `(display ,image
diff --git a/src/image.c b/src/image.c
index 651ec0b34e5..3c2697d1e6f 100644
--- a/src/image.c
+++ b/src/image.c
@@ -1650,6 +1650,30 @@ DEFUN ("image-metadata", Fimage_metadata, Simage_metadata, 1, 2, 0,
   return ext;
 }
 
+DEFUN ("image-timestamp", Fimage_timestamp, Simage_timestamp, 1, 2, 0,
+       doc: /* Return the time in seconds at which the image SPEC was
+last displayed.
+
+FRAME is the frame on which the image was displayed.  FRAME nil or
+omitted means use the selected frame.  */)
+  (Lisp_Object spec, Lisp_Object frame)
+{
+  Lisp_Object timestamp;
+
+  timestamp = Qnil;
+  if (valid_image_p (spec))
+    {
+      struct frame *f = decode_window_system_frame (frame);
+      ptrdiff_t id = lookup_image (f, spec, -1);
+      struct image *img = IMAGE_FROM_ID (f, id);
+
+      if (img)
+	timestamp = make_lisp_time (img->timestamp);
+    }
+
+  return timestamp;
+}
+
 
 /***********************************************************************
 		 Image type independent image structures
@@ -12908,6 +12932,7 @@ syms_of_image (void)
   defsubr (&Simage_size);
   defsubr (&Simage_mask_p);
   defsubr (&Simage_metadata);
+  defsubr (&Simage_timestamp);
   defsubr (&Simage_cache_size);
   defsubr (&Simagep);
 
-- 
2.43.0