From f8b969b1aced58e74e942a09335ba3f3c752eba1 Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Mon, 1 Nov 2021 21:10:49 +0000
Subject: [PATCH] Improve file-has-changed-p.

* lisp/files (file-has-changed-p): Add a second argument, and use it.
Update the docstring.

* doc/lispref/files.texi: Update the documentation.

* lisp/net/mailcap.el: Add a second argument to the call to
file-has-changed-p.
---
 doc/lispref/files.texi |  8 +++++---
 lisp/files.el          | 28 ++++++++++++++++------------
 lisp/net/mailcap.el    |  4 +++-
 3 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index 250f7a3f9f..ce967ee381 100644
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -1314,13 +1314,15 @@ File Attributes
 @end example
 @end defun
 
-@defun file-has-changed-p filename
+@defun file-has-changed-p filename tag
 This convenience function is useful when, for instance, parsing files
 run-time, and you typically want to re-read a file when it has
 changed.  This function returns non-@code{nil} the first time it's
 called on @var{filename} in an Emacs session, but will return
-@code{nil} on subsequent calls in that session (unless the file
-changes its modification time).
+@code{nil} on subsequent calls in that session (unless the file size
+or modification time has changed in the meantime).  With an optional
+argument @var{tag}, the size and modification time comparisons are
+limited to calls with the same tag.
 @end defun
 
 @defun file-attributes filename &optional id-format
diff --git a/lisp/files.el b/lisp/files.el
index 5e7be3844e..d7dfa9399e 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -6181,21 +6181,25 @@ file-in-directory-p
 	  (unless mismatch
 	    (file-equal-p root dir)))))))
 
-(defvar file-has-changed-p--hash-table (make-hash-table)
+(defvar file-has-changed-p--hash-table (make-hash-table :test #'equal)
   "Internal variable used by `file-has-changed-p'.")
 
-(defun file-has-changed-p (file)
+(defun file-has-changed-p (file &optional tag)
   "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))))
+The size and modification time of FILE is compared to the size
+and 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.
+The optional argument TAG can be used to limit the comparison to
+invocations with identical tags; it can for example be the symbol
+of the calling function."
+  (let* ((fileattr (file-attributes file 'integer))
+	 (attr (cons (file-attribute-size fileattr)
+		     (file-attribute-modification-time fileattr)))
+	 (sym (concat (symbol-name tag) "@" file))
+	 (cachedattr (gethash sym file-has-changed-p--hash-table)))
+     (when (not (equal attr cachedattr))
+       (puthash sym attr 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.
diff --git a/lisp/net/mailcap.el b/lisp/net/mailcap.el
index 4dedd38c22..e40cf2a336 100644
--- a/lisp/net/mailcap.el
+++ b/lisp/net/mailcap.el
@@ -447,7 +447,9 @@ mailcap-parse-mailcaps
               ("/etc/mailcap" system)
               ("/usr/etc/mailcap" system)
 	      ("/usr/local/etc/mailcap" system)))))
-    (when (seq-some (lambda (f) (file-has-changed-p (car f))) path)
+    (when (seq-some (lambda (f)
+                      (file-has-changed-p (car f) 'mail-parse-mailcaps))
+                    path)
       ;; The ~/.mailcap entries will end up first in the resulting data.
       (dolist (spec (reverse
 		     (if (stringp path)
-- 
2.33.0