Fix rgrep in dired using directory for search file pattern

* lisp/progmodes/grep.el (grep-read-files): Allow major modes to
define file name to use for default search pattern.
Add non-directory file at point as default search pattern candidate.

* lisp/dired.el (dired-grep-read-files): Use non-directory file at
point for grep file name pattern.  (Bug#34621)

Copyright-paperwork-exempt: yes

lisp/dired.el

@@ -774,6 +774,15 @@ as an argument to `dired-goto-file'."
 	  (file-name-as-directory (abbreviate-file-name filename))
 	(abbreviate-file-name filename)))))
 
+(defun dired-grep-read-files ()
+  "Use file at point as the file for grep's default file-name pattern suggestion.
+If a directory or nothing is found at point, return nil."
+  (let ((file-name (dired-file-name-at-point)))
+    (if (and file-name
+	     (not (file-directory-p file-name)))
+	file-name)))
+(put 'dired-mode 'grep-read-files 'dired-grep-read-files)
+
 ;;;###autoload (define-key ctl-x-map "d" 'dired)
 ;;;###autoload
 (defun dired (dirname &optional switches)

lisp/progmodes/grep.el

@@ -959,8 +959,16 @@ substitution string.  Note dynamic scoping of variables.")
 The pattern can include shell wildcards.  As whitespace triggers
 completion when entering a pattern, including it requires
 quoting, e.g. `\\[quoted-insert]<space>'."
-  (let* ((bn (or (buffer-file-name)
-		 (replace-regexp-in-string "<[0-9]+>\\'" "" (buffer-name))))
+  (let* ((grep-read-files-function (get major-mode 'grep-read-files))
+	 (file-name-at-point
+	   (run-hook-with-args-until-success 'file-name-at-point-functions))
+	 (bn (if grep-read-files-function
+		 (funcall grep-read-files-function)
+	       (or (if (and (stringp file-name-at-point)
+			    (not (file-directory-p file-name-at-point)))
+		       file-name-at-point)
+		   (buffer-file-name)
+		   (replace-regexp-in-string "<[0-9]+>\\'" "" (buffer-name)))))
 	 (fn (and bn
 		  (stringp bn)
 		  (file-name-nondirectory bn)))