Allow write-contents-functions to short-circuit buffer save

Bug#28412

* lisp/files.el (basic-save-buffer): Re-arrange function so that
  write-contents-functions are run earlier. If they return non-nil,
  consider the buffer saved without requiring the buffer to be
  visiting a file.
  (save-some-buffers): This function should consider any buffer with a
  buffer-local value for write-contents-functions eligible for
  saving.
* test/lisp/files-tests.el (files-test-no-file-write-contents): New
  test.
* doc/lispref/files.texi (Saving Buffers): Mention in docs.
* etc/NEWS: And in NEWS.

4 files changed, 122 insertions, 67 deletions

diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index 901382fe9bd..6be998f0b2e 100644
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -457,15 +457,23 @@ Even though this is not a normal hook, you can use @code{add-hook} and
 @defvar write-contents-functions
 This works just like @code{write-file-functions}, but it is intended
 for hooks that pertain to the buffer's contents, not to the particular
-visited file or its location.  Such hooks are usually set up by major
-modes, as buffer-local bindings for this variable.  This variable
-automatically becomes buffer-local whenever it is set; switching to a
-new major mode always resets this variable, but calling
-@code{set-visited-file-name} does not.
+visited file or its location, and can be used to create arbitrary save
+processes for buffers that aren't visiting files at all.  Such hooks
+are usually set up by major modes, as buffer-local bindings for this
+variable.  This variable automatically becomes buffer-local whenever
+it is set; switching to a new major mode always resets this variable,
+but calling @code{set-visited-file-name} does not.
 
 If any of the functions in this hook returns non-@code{nil}, the file
 is considered already written and the rest are not called and neither
 are the functions in @code{write-file-functions}.
+
+When using this hook to save buffers that are not visiting files (for
+instance, special-mode buffers), keep in mind that, if the function
+fails to save correctly and returns a @code{nil} value,
+@code{save-buffer} will go on to prompt the user for a file to save
+the buffer in.  If this is undesirable, consider having the function
+fail by raising an error.
 @end defvar
 
 @defopt before-save-hook
diff --git a/etc/NEWS b/etc/NEWS
index 03ef05b2a30..94673697090 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -108,6 +108,14 @@ The effect is similar to that of "toolBar" resource on the tool bar.
 
 * Changes in Emacs 26.1
 
++++
+** Functions in 'write-contents-functions' can fully short-circuit the
+'save-buffer' process.  Previously, saving a buffer that was not
+visiting a file would always prompt for a file name.  Now it only does
+so if 'write-contents-functions' is nil (or all its functions return
+nil).  A non-nil buffer-local value for this variable is sufficient
+for 'save-some-buffers' to consider the buffer for saving.
+
 ---
 ** New variable 'executable-prefix-env' for inserting magic signatures.
 This variable affects the format of the interpreter magic number
diff --git a/lisp/files.el b/lisp/files.el
index de9fab8d32e..72ace246445 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -517,10 +517,12 @@ updates before the buffer is saved, use `before-save-hook'.")
     'write-contents-functions "22.1")
 (defvar write-contents-functions nil
   "List of functions to be called before writing out a buffer to a file.
-Only used by `save-buffer'.
-If one of them returns non-nil, the file is considered already written
-and the rest are not called and neither are the functions in
-`write-file-functions'.
+
+Only used by `save-buffer'.  If one of them returns non-nil, the
+file is considered already written and the rest are not called
+and neither are the functions in `write-file-functions'.  This
+hook can thus be used to create save behavior for buffers that
+are not visiting a file at all.
 
 This variable is meant to be used for hooks that pertain to the
 buffer's contents, not to the particular visited file; thus,
@@ -4875,9 +4877,12 @@ in such cases.")
 
 (defun basic-save-buffer (&optional called-interactively)
   "Save the current buffer in its visited file, if it has been modified.
-The hooks `write-contents-functions' and `write-file-functions' get a chance
-to do the job of saving; if they do not, then the buffer is saved in
-the visited file in the usual way.
+
+The hooks `write-contents-functions', `local-write-file-hooks'
+and `write-file-functions' get a chance to do the job of saving;
+if they do not, then the buffer is saved in the visited file in
+the usual way.
+
 Before and after saving the buffer, this function runs
 `before-save-hook' and `after-save-hook', respectively."
   (interactive '(called-interactively))
@@ -4886,29 +4891,14 @@ Before and after saving the buffer, this function runs
     (if (buffer-base-buffer)
         (set-buffer (buffer-base-buffer)))
     (if (or (buffer-modified-p)
-            ;; handle the case when no modification has been made but
-            ;; the file disappeared since visited
+            ;; Handle the case when no modification has been made but
+            ;; the file disappeared since visited.
             (and buffer-file-name
                  (not (file-exists-p buffer-file-name))))
         (let ((recent-save (recent-auto-save-p))
               setmodes)
-          ;; If buffer has no file name, ask user for one.
-          (or buffer-file-name
-              (let ((filename
-                     (expand-file-name
-                      (read-file-name "File to save in: "
-                                      nil (expand-file-name (buffer-name))))))
-                (if (file-exists-p filename)
-                    (if (file-directory-p filename)
-                        ;; Signal an error if the user specified the name of an
-                        ;; existing directory.
-                        (error "%s is a directory" filename)
-                      (unless (y-or-n-p (format-message
-                                         "File `%s' exists; overwrite? "
-                                         filename))
-                        (error "Canceled"))))
-                (set-visited-file-name filename)))
-          (or (verify-visited-file-modtime (current-buffer))
+          (or (null buffer-file-name)
+              (verify-visited-file-modtime (current-buffer))
               (not (file-exists-p buffer-file-name))
               (yes-or-no-p
                (format
@@ -4920,6 +4910,7 @@ Before and after saving the buffer, this function runs
             (save-excursion
               (and (> (point-max) (point-min))
                    (not find-file-literally)
+                   (null buffer-read-only)
                    (/= (char-after (1- (point-max))) ?\n)
                    (not (and (eq selective-display t)
                              (= (char-after (1- (point-max))) ?\r)))
@@ -4932,46 +4923,65 @@ Before and after saving the buffer, this function runs
                    (save-excursion
                      (goto-char (point-max))
                      (insert ?\n))))
-            ;; Support VC version backups.
-            (vc-before-save)
             ;; Don't let errors prevent saving the buffer.
             (with-demoted-errors (run-hooks 'before-save-hook))
-            (or (run-hook-with-args-until-success 'write-contents-functions)
-                (run-hook-with-args-until-success 'local-write-file-hooks)
-                (run-hook-with-args-until-success 'write-file-functions)
-                ;; If a hook returned t, file is already "written".
-                ;; Otherwise, write it the usual way now.
-                (let ((dir (file-name-directory
-                            (expand-file-name buffer-file-name))))
-                  (unless (file-exists-p dir)
-                    (if (y-or-n-p
-                         (format-message
-                          "Directory `%s' does not exist; create? " dir))
-                        (make-directory dir t)
-                      (error "Canceled")))
-                  (setq setmodes (basic-save-buffer-1))))
+            ;; Give `write-contents-functions' a chance to
+            ;; short-circuit the whole process.
+            (unless (run-hook-with-args-until-success 'write-contents-functions)
+              ;; If buffer has no file name, ask user for one.
+              (or buffer-file-name
+                  (let ((filename
+                         (expand-file-name
+                          (read-file-name "File to save in: "
+                                          nil (expand-file-name (buffer-name))))))
+                    (if (file-exists-p filename)
+                        (if (file-directory-p filename)
+                            ;; Signal an error if the user specified the name of an
+                            ;; existing directory.
+                            (error "%s is a directory" filename)
+                          (unless (y-or-n-p (format-message
+                                             "File `%s' exists; overwrite? "
+                                             filename))
+                            (error "Canceled"))))
+                    (set-visited-file-name filename)))
+              ;; Support VC version backups.
+              (vc-before-save)
+              (or (run-hook-with-args-until-success 'local-write-file-hooks)
+                  (run-hook-with-args-until-success 'write-file-functions)
+                  ;; If a hook returned t, file is already "written".
+                  ;; Otherwise, write it the usual way now.
+                  (let ((dir (file-name-directory
+                              (expand-file-name buffer-file-name))))
+                    (unless (file-exists-p dir)
+                      (if (y-or-n-p
+                           (format-message
+                            "Directory `%s' does not exist; create? " dir))
+                          (make-directory dir t)
+                        (error "Canceled")))
+                    (setq setmodes (basic-save-buffer-1)))))
             ;; Now we have saved the current buffer.  Let's make sure
             ;; that buffer-file-coding-system is fixed to what
             ;; actually used for saving by binding it locally.
-            (if save-buffer-coding-system
-                (setq save-buffer-coding-system last-coding-system-used)
-              (setq buffer-file-coding-system last-coding-system-used))
-            (setq buffer-file-number
-                  (nthcdr 10 (file-attributes buffer-file-name)))
-            (if setmodes
-                (condition-case ()
-                    (progn
-                      (unless
-                          (with-demoted-errors
-                            (set-file-modes buffer-file-name (car setmodes)))
-                        (set-file-extended-attributes buffer-file-name
-                                                      (nth 1 setmodes))))
-                  (error nil))))
-          ;; If the auto-save file was recent before this command,
-          ;; delete it now.
-          (delete-auto-save-file-if-necessary recent-save)
-          ;; Support VC `implicit' locking.
-          (vc-after-save)
+            (when buffer-file-name
+              (if save-buffer-coding-system
+                  (setq save-buffer-coding-system last-coding-system-used)
+                (setq buffer-file-coding-system last-coding-system-used))
+              (setq buffer-file-number
+                    (nthcdr 10 (file-attributes buffer-file-name)))
+              (if setmodes
+                  (condition-case ()
+                      (progn
+                        (unless
+                            (with-demoted-errors
+                                (set-file-modes buffer-file-name (car setmodes)))
+                          (set-file-extended-attributes buffer-file-name
+                                                        (nth 1 setmodes))))
+                    (error nil)))
+              ;; Support VC `implicit' locking.
+              (vc-after-save))
+            ;; If the auto-save file was recent before this command,
+            ;; delete it now.
+            (delete-auto-save-file-if-necessary recent-save))
           (run-hooks 'after-save-hook))
       (or noninteractive
           (not called-interactively)
@@ -5183,7 +5193,9 @@ change the additional actions you can take on files."
                      (and pred
                           (progn
                             (set-buffer buffer)
-                            (and buffer-offer-save (> (buffer-size) 0)))))
+                            (and buffer-offer-save (> (buffer-size) 0))))
+                     (buffer-local-value
+                      'write-contents-functions buffer))
                     (or (not (functionp pred))
                         (with-current-buffer buffer (funcall pred)))
                     (if arg
diff --git a/test/lisp/files-tests.el b/test/lisp/files-tests.el
index b52965a02b4..c6806cdb58e 100644
--- a/test/lisp/files-tests.el
+++ b/test/lisp/files-tests.el
@@ -365,6 +365,33 @@ be invoked with the right arguments."
     (should-error (make-directory a/b))
     (should-not (make-directory a/b t))))
 
+(ert-deftest files-test-no-file-write-contents ()
+  "Test that `write-contents-functions' permits saving a file.
+Usually `basic-save-buffer' will prompt for a file name if the
+current buffer has none.  It should first call the functions in
+`write-contents-functions', and if one of them returns non-nil,
+consider the buffer saved, without prompting for a file
+name (Bug#28412)."
+  (let ((read-file-name-function
+         (lambda (&rest _ignore)
+           (error "Prompting for file name"))))
+    ;; With contents function, and no file.
+    (with-temp-buffer
+      (setq write-contents-functions (lambda () t))
+      (set-buffer-modified-p t)
+      (should (null (save-buffer))))
+    ;; With no contents function and no file.  This should reach the
+    ;; `read-file-name' prompt.
+    (with-temp-buffer
+      (set-buffer-modified-p t)
+      (should-error (save-buffer) :type 'error))
+    ;; Then a buffer visiting a file: should save normally.
+    (files-tests--with-temp-file temp-file-name
+      (with-current-buffer (find-file-noselect temp-file-name)
+        (setq write-contents-functions nil)
+        (insert "p")
+        (should (null (save-buffer)))
+        (should (eq (buffer-size) 1))))))
 
 (provide 'files-tests)
 ;;; files-tests.el ends here