nnmail-cache-close should not use nofollow

nnmail-cache-close did not work when ~/.nnmail-cache was a symlink
to some other directory.  Reported by Michael Albinus (Bug#39793).
* lisp/gnus/nnbabyl.el (nnbabyl-create-mbox):
* lisp/gnus/nndiary.el (nndiary-generate-nov-file):
* lisp/gnus/nnfolder.el (nnfolder-possibly-change-group):
* lisp/gnus/nnmbox.el (nnmbox-create-mbox):
* lisp/gnus/nnml.el (nnml-generate-nov-file):
Pass ‘excl’ to nnmail-write-region when creating a file, for safety.
* lisp/gnus/nnmail.el (nnmail-write-region):
Add optional MUSTBENEW arg, to stay consistent with write-region.
Past ‘nofollow’ to set-file-modes only when MUSTBENEW is ‘excl’.

Author: eggert

lisp/gnus/nnbabyl.el

@@ -544,7 +544,7 @@
       (setq buffer-file-name nnbabyl-mbox-file)
       (insert "BABYL OPTIONS:\n\n\^_")
       (nnmail-write-region
-       (point-min) (point-max) nnbabyl-mbox-file t 'nomesg))))
+       (point-min) (point-max) nnbabyl-mbox-file t 'nomesg nil 'excl))))
 
 (defun nnbabyl-read-mbox ()
   (nnmail-activate 'nnbabyl)

lisp/gnus/nndiary.el

@@ -1115,7 +1115,7 @@ all.  This may very well take some time.")
 	  (widen))
 	(setq files (cdr files)))
       (with-current-buffer nov-buffer
-	(nnmail-write-region 1 (point-max) nov nil 'nomesg)
+	(nnmail-write-region 1 (point-max) nov nil 'nomesg nil 'excl)
 	(kill-buffer (current-buffer))))))
 
 (defun nndiary-nov-delete-article (group article)

lisp/gnus/nnfolder.el

@@ -735,7 +735,7 @@ deleted.  Point is left where the deleted region was."
 		       (or nnfolder-file-coding-system-for-write
 			   nnfolder-file-coding-system-for-write)))
 		  (nnmail-write-region (point-min) (point-min)
-				       file t 'nomesg)))
+				       file t 'nomesg nil 'excl)))
 	      (when (setq nnfolder-current-buffer (nnfolder-read-folder group))
 		(set-buffer nnfolder-current-buffer)
 		(push (list group nnfolder-current-buffer)

lisp/gnus/nnmail.el

@@ -1953,12 +1953,14 @@ If TIME is nil, then return the cutoff time for oldness instead."
       (unless (re-search-forward "^Message-ID[ \t]*:" nil t)
 	(insert "Message-ID: " (nnmail-message-id) "\n")))))
 
-(defun nnmail-write-region (start end filename &optional append visit lockname)
+(defun nnmail-write-region (start end filename
+				  &optional append visit lockname mustbenew)
   "Do a `write-region', and then set the file modes."
   (let ((coding-system-for-write nnmail-file-coding-system)
 	(file-name-coding-system nnmail-pathname-coding-system))
-    (write-region start end filename append visit lockname)
-    (set-file-modes filename nnmail-default-file-modes 'nofollow)))
+    (write-region start end filename append visit lockname mustbenew)
+    (set-file-modes filename nnmail-default-file-modes
+		    (when (eq mustbenew 'excl) 'nofollow))))
 
 ;;;
 ;;; Status functions

lisp/gnus/nnmbox.el

@@ -613,7 +613,7 @@
 	  (dir (file-name-directory nnmbox-mbox-file)))
       (and dir (gnus-make-directory dir))
       (nnmail-write-region (point-min) (point-min)
-			   nnmbox-mbox-file t 'nomesg))))
+			   nnmbox-mbox-file t 'nomesg nil 'excl))))
 
 (defun nnmbox-read-mbox ()
   (nnmail-activate 'nnmbox)

lisp/gnus/nnml.el

@@ -901,7 +901,7 @@ Unless no-active is non-nil, update the active file too."
 		(nnheader-insert-nov headers)))
 	    (widen))))
       (with-current-buffer nov-buffer
-	(nnmail-write-region (point-min) (point-max) nov nil 'nomesg)
+	(nnmail-write-region (point-min) (point-max) nov nil 'nomesg nil 'excl)
 	(kill-buffer (current-buffer))))))
 
 (defun nnml-nov-delete-article (group article)