Subject: Re: bug#68006: 30.0.50; Image-mode speed

From: Manuel Giraud <[email-masked]>
Date: Sat, 30 Dec 2023 13:37:50 +0100

Eli Zaretskii <[email-masked]> writes:

>> From: Manuel Giraud <[email-masked]>
>> Cc: [email-masked]
>> Date: Fri, 29 Dec 2023 12:11:44 +0100
>> 
>> What about this new patch?  It works for me as intended but might need
>> better names and better docs.  Thanks.
>> 
>> +(defcustom image-mode-eager-cache-flush t
>> +  "Non-nil means flush image from cache eagerly.
>> +When set to nil, be aware that the image cache could grow fast
>> +but an image previously displayed will show faster."
>
> This doc string should explain the effects better, in terms of
> user-facing behavior, not in terms of technical aspects of the code.

And here is a new version of the patch.  Maybe, we need a NEWS entry for
this too.
>From 49f1af6333b90a700c52fd4845583c0506d14795 Mon Sep 17 00:00:00 2001
From: Manuel Giraud <[email-masked]>
Date: Fri, 29 Dec 2023 11:48:21 +0100
Subject: [PATCH] Add some user control in image-mode caching

Bug#68006

* lisp/image-mode.el (image-mode-eager-cache-flush): New custom.
(image-toggle-display-image): Add a contents hash in image spec
when flushing cache lazily.

* lisp/image/image-dired-util.el (image-dired-thumb-name):
* lisp/image.el (image-contents-sha1): Move
'image-dired-contents-sha1' to 'image-contents-sha1' in toplevel
image library.
---
 lisp/image-mode.el             | 20 ++++++++++++++++++--
 lisp/image.el                  | 11 ++++++++++-
 lisp/image/image-dired-util.el |  8 +-------
 3 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/lisp/image-mode.el b/lisp/image-mode.el
index d5ca6348c92..68be3faaae7 100644
--- a/lisp/image-mode.el
+++ b/lisp/image-mode.el
@@ -93,6 +93,16 @@ image-auto-resize-on-window-resize
   :version "27.1"
   :group 'image)
 
+(defcustom image-mode-eager-cache-flush t
+  "Non-nil means an image is flushed from the cache before being
+read again.  When set to nil, an image spec is not removed from
+the cache so it will be displayed faster the second time.
+
+See `image-cache-eviction-delay'."
+  :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 +964,14 @@ 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)
+    (if image-mode-eager-cache-flush
+        ;; Discard any stale image data before looking it up again.
+        (image-flush image)
+      ;; Add a content based hash into image spec to be sure that the
+      ;; cache is updated should the on disk image change.
+      (when (and filename (file-exists-p filename))
+        (setq image (append image (list :hash (image-contents-sha1 filename))))))
+
     (setq image (append image (image-transform-properties image)))
     (setq props
 	  `(display ,image
diff --git a/lisp/image.el b/lisp/image.el
index e20fbcf4c98..0f1c74d9250 100644
--- a/lisp/image.el
+++ b/lisp/image.el
@@ -401,7 +401,16 @@ image-type-from-file-name
 	    ;; If nothing seems to be supported, return first type that matched.
 	    (or first (setq first type))))))))
 
- ;;;###autoload
+;;;###autoload
+(defun image-contents-sha1 (filename)
+  "Compute the SHA-1 of the first 4KiB of FILENAME's contents.
+The first 4KiB does not represent the whole file but was chosen
+because it is fast enough for interactive usage."
+  (with-temp-buffer
+    (insert-file-contents-literally filename nil 0 4096)
+    (sha1 (current-buffer))))
+
+;;;###autoload
 (defun image-supported-file-p (file)
   "Say whether Emacs has native support for displaying TYPE.
 The value is a symbol specifying the image type, or nil if type
diff --git a/lisp/image/image-dired-util.el b/lisp/image/image-dired-util.el
index e17cc6c919f..e5f526fec57 100644
--- a/lisp/image/image-dired-util.el
+++ b/lisp/image/image-dired-util.el
@@ -59,12 +59,6 @@ image-dired-dir
       (message "Thumbnail directory created: %s" image-dired-dir))
     image-dired-dir))
 
-(defun image-dired-contents-sha1 (filename)
-  "Compute the SHA-1 of the first 4KiB of FILENAME's contents."
-  (with-temp-buffer
-    (insert-file-contents-literally filename nil 0 4096)
-    (sha1 (current-buffer))))
-
 (defun image-dired-thumb-name (file)
   "Return absolute file name for thumbnail FILE.
 Depending on the value of `image-dired-thumbnail-storage' and
@@ -99,7 +93,7 @@ image-dired-thumb-name
            (concat (md5 (concat "file://" file)) ".png")
            (expand-file-name thumbdir (xdg-cache-home))))
       (let ((name (if (eq 'sha1-contents image-dired-thumb-naming)
-                      (image-dired-contents-sha1 file)
+                      (image-contents-sha1 file)
                     ;; Defaults to SHA-1 of file name
                     (sha1 file))))
         (cond ((or (eq 'image-dired image-dired-thumbnail-storage)
-- 
2.43.0

-- 
Manuel Giraud