Extend button.el to take callback data

* doc/lispref/display.texi (Button Buffer Commands)
(Button Buffer Commands): Document this.
* lisp/button.el (backward-button, forward-button): Accept a
NO-ERROR parameter.
(button-activate): Make it possible to have specific data in the
callback action.

doc/lispref/display.texi

@@ -6473,7 +6473,9 @@ that is the value of that property, passing it the single argument
 @var{button}).  If @var{use-mouse-action} is non-@code{nil}, try to
 invoke the button's @code{mouse-action} property instead of
 @code{action}; if the button has no @code{mouse-action} property, use
-@code{action} as normal.
+@code{action} as normal.  If the @code{button-data} property is
+present in @var{button}, use that as the argument for the
+@code{action} function instead of @var{button}.
 @end defun
 
 @defun button-label button
@@ -6541,14 +6543,16 @@ event's position is used.  If there's no button at @var{pos}, do
 nothing and return @code{nil}, otherwise return @code{t}.
 @end deffn
 
-@deffn Command forward-button n &optional wrap display-message
+@deffn Command forward-button n &optional wrap display-message no-error
 Move to the @var{n}th next button, or @var{n}th previous button if
 @var{n} is negative.  If @var{n} is zero, move to the start of any
 button at point.  If @var{wrap} is non-@code{nil}, moving past either
 end of the buffer continues from the other end.  If
 @var{display-message} is non-@code{nil}, the button's help-echo string
 is displayed.  Any button with a non-@code{nil} @code{skip} property
-is skipped over.  Returns the button found.
+is skipped over.  Returns the button found, and signals an error if no
+buttons can be found.  If @var{no-error} in non-@code{nil}, return nil
+instead of signalling the error.
 @end deffn
 
 @deffn Command backward-button n &optional wrap display-message
@@ -6558,7 +6562,9 @@ button at point.  If @var{wrap} is non-@code{nil}, moving past either
 end of the buffer continues from the other end.  If
 @var{display-message} is non-@code{nil}, the button's help-echo string
 is displayed.  Any button with a non-@code{nil} @code{skip} property
-is skipped over.  Returns the button found.
+is skipped over.  Returns the button found, and signals an error if no
+buttons can be found.  If @var{no-error} in non-@code{nil}, return nil
+instead of signalling the error.
 @end deffn
 
 @defun next-button pos &optional count-current

etc/NEWS

@@ -1968,6 +1968,12 @@ valid event type.
 
 * Lisp Changes in Emacs 27.1
 
++++
+** Buttons (created with 'make-button' and related functions) can
+now use the 'button-data' property.  If present, the data in this
+property will be passed on to the 'action' function instead of the
+button itself in 'button-activate'.
+
 ** 'defcustom' now takes a ':local' keyword that can be either t or
 'permanent', which mean that the variable should be automatically
 buffer-local.  'permanent' also sets the variable's 'permanent-local'

lisp/button.el

@@ -235,15 +235,19 @@ The action can either be a marker or a function.  If it's a
 marker then goto it.  Otherwise if it is a function then it is
 called with BUTTON as only argument.  BUTTON is either an
 overlay, a buffer position, or (for buttons in the mode-line or
-header-line) a string."
+header-line) a string.
+
+If BUTTON has a `button-data' value, call the function with this
+value instad of BUTTON."
   (let ((action (or (and use-mouse-action (button-get button 'mouse-action))
-		    (button-get button 'action))))
+		    (button-get button 'action)))
+        (data (button-get button 'button-data)))
     (if (markerp action)
 	(save-selected-window
 	  (select-window (display-buffer (marker-buffer action)))
 	  (goto-char action)
 	  (recenter 0))
-      (funcall action button))))
+      (funcall action (or data button)))))
 
 (defun button-label (button)
   "Return BUTTON's text label."
@@ -324,6 +328,10 @@ using `make-text-button'.  Note, however, that if there is an existing
 face property at the site of the button, the button face may not be visible.
 You may want to use `make-button' in that case.
 
+If the property `button-data' is present, it will later be used
+as the argument for the `action' callback function instead of the
+default argument, which is the button itself.
+
 BEG can also be a string, in which case it is made into a button.
 
 Also see `insert-text-button'."
@@ -462,13 +470,17 @@ return t."
 	(button-activate button use-mouse-action)
 	t))))
 
-(defun forward-button (n &optional wrap display-message)
+(defun forward-button (n &optional wrap display-message no-error)
   "Move to the Nth next button, or Nth previous button if N is negative.
 If N is 0, move to the start of any button at point.
 If WRAP is non-nil, moving past either end of the buffer continues from the
 other end.
 If DISPLAY-MESSAGE is non-nil, the button's help-echo string is displayed.
 Any button with a non-nil `skip' property is skipped over.
+
+If NO-ERROR, return nil if no further buttons could be found
+instead of erroring out.
+
 Returns the button found."
   (interactive "p\nd\nd")
   (let (button)
@@ -497,22 +509,28 @@ Returns the button found."
 	    (unless (button-get button 'skip)
 	      (setq n (1- n)))))))
     (if (null button)
-	(user-error (if wrap "No buttons!" "No more buttons"))
+        (if no-error
+            nil
+	  (user-error (if wrap "No buttons!" "No more buttons")))
       (let ((msg (and display-message (button-get button 'help-echo))))
 	(when msg
 	  (message "%s" msg)))
       button)))
 
-(defun backward-button (n &optional wrap display-message)
+(defun backward-button (n &optional wrap display-message no-error)
   "Move to the Nth previous button, or Nth next button if N is negative.
 If N is 0, move to the start of any button at point.
 If WRAP is non-nil, moving past either end of the buffer continues from the
 other end.
 If DISPLAY-MESSAGE is non-nil, the button's help-echo string is displayed.
 Any button with a non-nil `skip' property is skipped over.
+
+If NO-ERROR, return nil if no further buttons could be found
+instead of erroring out.
+
 Returns the button found."
   (interactive "p\nd\nd")
-  (forward-button (- n) wrap display-message))
+  (forward-button (- n) wrap display-message no-error))
 
 
 (provide 'button)