Subject: 30.0.50; Tabbing through widgets can signal beginning-of-buffer error
From: Stephen Berman <[email-masked]>
To: [email-masked]
Date: Fri, 22 Mar 2024 15:45:16 +0100
List-Id: <debbugs-submit.debbugs.gnu.org>

0. emacs -Q

1. Evaluate the following sexp:

(let ((buf (get-buffer-create "*Widget Test*")))
  (switch-to-buffer buf)
  (dolist (el '("First" "Second" "Third"))
    (widget-create 'push-button el))
  (use-local-map widget-keymap)
  (widget-setup)
  (goto-char (point-min)))

Now the current buffer is *Widget Test* containing three push-button
widgets labeled "First", "Second", and "Third", and point is at the
start of the first widget, at BOB.

2. Hit the TAB key (bound to widget-forward) three times: this moves
point successively from "First" to "Second" to "Third" and then back to
"First" -- but on returning to the initial position after the third TAB,
a beginning-of-buffer error is also signaled.

3. Likewise, hitting S-TAB (bound to widget-backward) three times moves
backwards across the widgets, from "Third" to "Second" to "First", again
signaling a beginning-of-buffer error after the last S-TAB.

These beginning-of-buffer errors are due to widget-move (the workhorse
behind widget-forward and widget-backward) calling backward-char in a
loop without checking for BOB.  The attached patch fixes this.  The
patch also includes additions to widget-test-widget-move (from which
most of the above sexp was taken) that test moving to a widget at BOB.
(If the patch is acceptable, whoever commits it should use the correct
bug# before pushing it, or I can do that myself.)

2024-03-22  Stephen Berman  <[email-masked]>

Prevent error on tabbing to widget at beginning of buffer (bug#xxxxx)

* lisp/wid-edit.el (widget-move): Don't move backward when at
beginning of buffer, and keep point on widget's left side.

* test/lisp/wid-edit-tests.el (widget-test-widget-move): Adds
checks that moving to a widget at beginning of buffer does not
signal a beginning-of-buffer error.

diff --git a/lisp/wid-edit.el b/lisp/wid-edit.el
index 172da3db1e0..948a9ed06a5 100644
--- a/lisp/wid-edit.el
+++ b/lisp/wid-edit.el
@@ -1276,9 +1276,9 @@ widget-move
 	  (unless (eq new old)
 	    (setq arg (1+ arg))))))
     (let ((new (widget-tabable-at)))
-      (while (eq (widget-tabable-at) new)
+      (while (and (eq (widget-tabable-at) new) (not (bobp)))
 	(backward-char)))
-    (forward-char))
+    (unless (bobp) (forward-char)))
   (unless suppress-echo
     (widget-echo-help (point)))
   (run-hooks 'widget-move-hook))
diff --git a/test/lisp/wid-edit-tests.el b/test/lisp/wid-edit-tests.el
index 4b049478b29..3aef683f15e 100644
--- a/test/lisp/wid-edit-tests.el
+++ b/test/lisp/wid-edit-tests.el
@@ -336,7 +336,13 @@ widget-test-widget-move
     (widget-forward 2)
     (forward-char)
     (widget-backward 1)
-    (should (string= "Second" (widget-value (widget-at))))))
+    (should (string= "Second" (widget-value (widget-at))))
+    ;; Check that moving to a widget at beginning of buffer does not
+    ;; signal a beginning-of-buffer error (bug#xxxxx).
+    (widget-backward 1)   ; Should not signal beginning-of-buffer error.
+    (widget-forward 2)
+    (should (string= "Third" (widget-value (widget-at))))
+    (widget-forward 1)))  ; Should not signal beginning-of-buffer error.

 (ert-deftest widget-test-color-match ()
   "Test that the :match function for the color widget works."

In GNU Emacs 30.0.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.18.0) of 2024-03-22 built on strobelfs2
Repository revision: c1530a2e4973005633ebe00d447f1f3aa1200301
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101009
System Description: Linux From Scratch r12.0-112

Configured using:
 'configure -C --with-xwidgets 'CFLAGS=-Og -g3'
 PKG_CONFIG_PATH=/opt/qt5/lib/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER
PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS
TREE_SITTER WEBP X11 XDBE XIM XINPUT2 XPM XWIDGETS GTK3 ZLIB