Allow preserving symlinks with file-precious-flag set

* doc/lispref/files.texi (Saving Buffers): Document it.
* lisp/files.el (file-preserve-symlinks-on-save): New user option
(bug#18125).
(basic-save-buffer-2): Use it.

doc/lispref/files.texi

@@ -503,6 +503,13 @@ all hard links between the file you save and other file names.
 
 Some modes give this variable a non-@code{nil} buffer-local value
 in particular buffers.
+
+@vindex file-preserve-symlinks-on-save
+If this option is non-@code{nil} and you're visiting files via a
+symbolic link, Emacs break the symbolic link and write the buffer to a
+file with the same name as the symbolic link.  To instead write to the
+file the symbolic link points to (and thereby preserving the link),
+set @code{file-preserve-symlinks-on-save} to @code{t}.
 @end defopt
 
 @defopt require-final-newline

etc/NEWS

@@ -2460,6 +2460,11 @@ images are marked.
 
 ** Miscellaneous
 
++++
+*** New user option 'file-preserve-symlinks-on-save'.
+This controls what Emacs does when saving buffers visited via a
+symbolic link, and 'file-precious-flag' is non-nil.
+
 +++
 *** New user option 'copy-directory-create-symlink'.
 If non-nil, will make `copy-directory' (when used on a symbolic

lisp/files.el

@@ -5446,6 +5446,14 @@ symbolic link and copy the contents instead."
   :version "28.1"
   :group 'files)
 
+(defcustom file-preserve-symlinks-on-save nil
+  "If non-nil, saving a buffer visited via a symlink won't overwrite the symlink.
+This is only relevant if `file-precious-flag' is non-nil -- if
+this is nil, Emacs will preserve the symlinks anyway."
+  :type 'boolean
+  :version "28.1"
+  :group 'files)
+
 (defvar-local save-buffer-coding-system nil
   "If non-nil, use this coding system for saving the buffer.
 More precisely, use this coding system in place of the
@@ -5648,7 +5656,14 @@ Before and after saving the buffer, this function runs
 				     buffer-file-name)))
 	    ;; We succeeded in writing the temp file,
 	    ;; so rename it.
-	    (rename-file tempname buffer-file-name t))
+	    (rename-file tempname
+                         (if (and file-preserve-symlinks-on-save
+                                  (file-symlink-p buffer-file-name))
+                             ;; Write to the file that the symlink
+                             ;; points to.
+                             (file-chase-links buffer-file-name)
+                           buffer-file-name)
+                         t))
 	;; If file not writable, see if we can make it writable
 	;; temporarily while we write it.
 	;; But no need to do so if we have just backed it up