Improve syncing VC buffers before generating diffs

* lisp/vc/vc.el (vc-maybe-buffer-sync): Delete.  Correct
handling of indirect buffers is now implicitly achieved by
vc-buffer-sync-fileset.
(vc-buffer-sync-fileset): Make NOT-ESSENTIAL argument optional,
new MISSING-IN-DIRS optional argument.  Rewrite to handle
directories named in the fileset, not only files.
(vc-ediff): Replace call to vc-maybe-buffer-sync with a call to
vc-buffer-sync-fileset.
(vc-root-diff): Similarly replace call to vc-maybe-buffer-sync.
This means the user is prompted to save additional buffers, that
they likely want to save before generating the diffs.
* test/lisp/vc/vc-misc-tests.el: New file.

lisp/vc/vc.el

@@ -2281,10 +2281,6 @@ state of each file in the fileset."
        t (list backend (list rootdir)) rev1 rev2
        (called-interactively-p 'interactive)))))
 
-(defun vc-maybe-buffer-sync (not-essential)
-  (with-current-buffer (or (buffer-base-buffer) (current-buffer))
-    (when buffer-file-name (vc-buffer-sync not-essential))))
-
 ;;;###autoload
 (defun vc-diff (&optional historic not-essential fileset)
   "Display diffs between file revisions.
@@ -2303,11 +2299,40 @@ Optional argument FILESET, if non-nil, overrides the fileset."
       (vc-diff-internal t fileset nil nil
 			(called-interactively-p 'interactive)))))
 
-(defun vc-buffer-sync-fileset (fileset not-essential)
-  (dolist (filename (cadr fileset))
-    (when-let* ((buffer (find-buffer-visiting filename)))
-      (with-current-buffer buffer
-	(vc-buffer-sync not-essential)))))
+(defun vc-buffer-sync-fileset (fileset &optional not-essential missing-in-dirs)
+  "Call `vc-buffer-sync' for most buffers visiting files in FILESET.
+NOT-ESSENTIAL means it is okay to continue if the user says not to save.
+
+For files named explicitly in FILESET, this function always syncs their
+buffers.  By contrast, for directories named in FILESET, its behavior
+depends on MISSING-IN-DIRS.  For each directory named in FILESET, it
+considers buffers visiting any file contained within that directory or
+its subdirectories.  If MISSING-IN-DIRS is nil, it syncs only those
+buffers whose files exist on disk.  Otherwise it syncs all of them."
+  ;; This treatment of directories named in FILESET is wanted for, at
+  ;; least, users with `vc-find-revision-no-save' set to non-nil: not
+  ;; treating directories this way would imply calling `vc-buffer-sync'
+  ;; on all buffers generated by \\`C-x v ~' during \\`C-x v D'.
+  (let (dirs buffers)
+    (dolist (name (cadr fileset))
+      (if (file-directory-p name)
+          (push name dirs)
+        (when-let* ((buf (find-buffer-visiting name)))
+          (push buf buffers))))
+    (when dirs
+      (setq buffers
+            (cl-nunion buffers
+                       (match-buffers
+                        (lambda (buf)
+                          (and-let*
+                              ((file (buffer-local-value 'buffer-file-name buf))
+                               ((or missing-in-dirs (file-exists-p file)))
+                               ((cl-some (lambda (dir)
+                                           (file-in-directory-p file dir))
+                                         dirs)))))))))
+    (dolist (buf buffers)
+      (with-current-buffer buf
+        (vc-buffer-sync not-essential)))))
 
 ;;;###autoload
 (defun vc-diff-mergebase (_files rev1 rev2)
@@ -2384,8 +2409,9 @@ saving the buffer."
   (interactive (list current-prefix-arg t))
   (if historic
       (call-interactively 'vc-version-ediff)
-    (vc-maybe-buffer-sync not-essential)
-    (vc-version-ediff (cadr (vc-deduce-fileset t)) nil nil)))
+    (let ((fileset (vc-deduce-fileset)))
+      (vc-buffer-sync-fileset fileset not-essential)
+      (vc-version-ediff (cadr fileset) nil nil))))
 
 ;;;###autoload
 (defun vc-root-diff (historic &optional not-essential)
@@ -2401,7 +2427,6 @@ saving the buffer."
   (if historic
       ;; We want the diff for the VC root dir.
       (call-interactively 'vc-root-version-diff)
-    (vc-maybe-buffer-sync not-essential)
     (let ((backend (vc-deduce-backend))
 	  (default-directory default-directory)
 	  rootdir)
@@ -2416,10 +2441,11 @@ saving the buffer."
       ;; relative to it.  Bind default-directory to the root directory
       ;; here, this way the *vc-diff* buffer is setup correctly, so
       ;; relative file names work.
-      (let ((default-directory rootdir))
-        (vc-diff-internal
-         t (list backend (list rootdir)) nil nil
-         (called-interactively-p 'interactive))))))
+      (let ((default-directory rootdir)
+            (fileset `(,backend (,rootdir))))
+        (vc-buffer-sync-fileset fileset not-essential)
+        (vc-diff-internal t fileset nil nil
+                          (called-interactively-p 'interactive))))))
 
 ;;;###autoload
 (defun vc-root-dir ()

test/lisp/vc/vc-misc-tests.el

@@ -0,0 +1,67 @@
+;;; vc-misc-tests.el --- backend-agnostic VC tests  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2025 Free Software Foundation, Inc.
+
+;; Author: Sean Whitton <spwhitton@spwhitton.name>
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'ert-x)
+(require 'vc)
+
+(ert-deftest vc-test-buffer-sync-fileset ()
+  "Test `vc-buffer-sync-fileset'."
+  (cl-flet ((test-it (&rest args)
+              (let (buffers)
+                (cl-letf (((symbol-function 'vc-buffer-sync)
+                           (lambda (&rest _)
+                             (push (current-buffer) buffers))))
+                  (apply #'vc-buffer-sync-fileset args)
+                  (sort buffers)))))
+    (ert-with-temp-directory temp
+      (let* ((default-directory temp)
+             (present (find-file-noselect "present"))
+             (missing (find-file-noselect "missing"))
+             (only-present (list present))
+             (only-missing (list missing))
+             (missing+present (list missing present)))
+        (with-current-buffer present (basic-save-buffer))
+        (with-temp-file "unvisited")
+        ;; Regular behavior for files.
+        (should (equal (test-it `(Git ("missing")))
+                       only-missing))
+        (should (equal (test-it `(Git ("present" "missing")))
+                       missing+present))
+        ;; Regular behavior for directories.
+        (should (equal (test-it `(Git (,temp)))
+                       only-present))
+        ;; Two ways to override regular behavior for directories.
+        (should (equal (test-it `(Git (,temp)) nil t)
+                       missing+present))
+        (should (equal (test-it `(Git (,temp "missing")))
+                       missing+present))
+        ;; Doesn't sync PRESENT twice.
+        (should (equal (test-it `(Git ("present" ,temp)))
+                       only-present))
+        (should (equal (test-it `(Git ("missing" ,temp "present")))
+                       missing+present))))))
+
+(provide 'vc-misc-tests)
+;;; vc-misc-tests.el ends here