bug#51523: 29.0.50; gnus-mime-view-part-externally very slow

[Gregory Heytings <gregory@heytings.org>]

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>
> This reminds me of something I've wanted more than a handful of times -- 
> it'd be nice to have a function/macro like
>
> (when-file-has-changed "/etc/mailcap"
>   (do-the-parsing))
>

See attached patch.

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From 827f6aa9fb616d9467dcccc011a5d3eb4dbac02d Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Mon, 1 Nov 2021 00:53:05 +0000
Subject: [PATCH] Add a macro to execute body only when a file has changed.

* lisp/subr.el (when-file-has-changed): New macro.
(when-file-has-changed--hash-table): Internal variable used by the
new macro.
---
 lisp/subr.el | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/lisp/subr.el b/lisp/subr.el
index f6dbd00532..b29c56f8c6 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -3967,6 +3967,22 @@ function-get
             nil                         ;Re-try `get' on the same `f'.
           (setq f fundef))))
     val))
+
+(defvar when-file-has-changed--hash-table (make-hash-table)
+  "Internal variable used by `when-file-has-changed'.")
+
+(defmacro when-file-has-changed (file body)
+  "Execute BODY only if file has changed.
+The modification time of the FILE is compared to the modification
+time of FILE during a previous invocation of
+`when-file-has-changed'.  If they differ, BODY is executed."
+  `(let* ((attr (file-attributes ,file 'integer))
+	  (mtime (file-attribute-modification-time attr))
+	  (saved-mtime (gethash (intern ,file)
+				when-file-has-changed--hash-table)))
+     (when (not (equal mtime saved-mtime))
+       (puthash (intern ,file) mtime when-file-has-changed--hash-table)
+       ,body)))
 \f
 ;;;; Support for yanking and text properties.
 ;; Why here in subr.el rather than in simple.el?  --Stef
-- 
2.33.0