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

[Gregory Heytings <gregory@heytings.org>]

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[Stefan just told me that this email got corrupted when I sent it, here it 
is again.]

I attach a better patch, it uses a file-has-change-p function instead of a 
when-file-has-changed macro.

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From 4106b342fe40d4b585f43c19ba45ccc6f3f6c1e1 Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Mon, 1 Nov 2021 09:25:23 +0000
Subject: [PATCH] Read mailcaps again only when necessary.

* lisp/net/mailcap.el (mailcap-parse-mailcaps): Read mailcaps again
only when at least one of the mailcap files has changed.  Fixes
bug#51523.

* lisp/files.el (file-has-changed-p): New function.
(file-has-changed-p--hash-table): Internal variable used by the
new function.
---
 lisp/files.el       | 16 ++++++++++++++++
 lisp/net/mailcap.el | 25 +++++++++++++------------
 2 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/lisp/files.el b/lisp/files.el
index 1e65d0ce83..5e7be3844e 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -6181,6 +6181,22 @@ file-in-directory-p
 	  (unless mismatch
 	    (file-equal-p root dir)))))))
 
+(defvar file-has-changed-p--hash-table (make-hash-table)
+  "Internal variable used by `file-has-changed-p'.")
+
+(defun file-has-changed-p (file)
+  "Return non-nil if FILE has changed.
+The modification time of FILE is compared to the modification
+time of FILE during a previous invocation of `file-has-changed-p'.
+Therefore the first invocation of `file-has-changed-p' always
+returns non-nil."
+  (let* ((attr (file-attributes file 'integer))
+	  (mtime (file-attribute-modification-time attr))
+	  (saved-mtime (gethash (intern file)
+				file-has-changed-p--hash-table)))
+     (when (not (equal mtime saved-mtime))
+       (puthash (intern file) mtime file-has-changed-p--hash-table))))
+
 (defun copy-directory (directory newname &optional keep-time parents copy-contents)
   "Copy DIRECTORY to NEWNAME.  Both args must be strings.
 This function always sets the file modes of the output files to match
diff --git a/lisp/net/mailcap.el b/lisp/net/mailcap.el
index 83d0eeef9f..4dedd38c22 100644
--- a/lisp/net/mailcap.el
+++ b/lisp/net/mailcap.el
@@ -447,18 +447,19 @@ mailcap-parse-mailcaps
               ("/etc/mailcap" system)
               ("/usr/etc/mailcap" system)
 	      ("/usr/local/etc/mailcap" system)))))
-    ;; The ~/.mailcap entries will end up first in the resulting data.
-    (dolist (spec (reverse
-                   (if (stringp path)
-                       (split-string path path-separator t)
-                     path)))
-      (let ((source (and (consp spec) (cadr spec)))
-            (file-name (if (stringp spec)
-                           spec
-                         (car spec))))
-        (when (and (file-readable-p file-name)
-                   (file-regular-p file-name))
-          (mailcap-parse-mailcap file-name source))))
+    (when (seq-some (lambda (f) (file-has-changed-p (car f))) path)
+      ;; The ~/.mailcap entries will end up first in the resulting data.
+      (dolist (spec (reverse
+		     (if (stringp path)
+			 (split-string path path-separator t)
+		       path)))
+	(let ((source (and (consp spec) (cadr spec)))
+	      (file-name (if (stringp spec)
+			     spec
+			   (car spec))))
+	  (when (and (file-readable-p file-name)
+		     (file-regular-p file-name))
+	    (mailcap-parse-mailcap file-name source)))))
     (setq mailcap-parsed-p t)))
 
 (defun mailcap-parse-mailcap (fname &optional source)
-- 
2.33.0