bug#81562: [PATCH] Restore current_buffer when window_text_pixel_size fails

From 208c486db2d54016ebea6f8caaa34ecc84ee4f0e Mon Sep 17 00:00:00 2001
From: SkyCanvas <[email protected]>
Date: Sun, 16 Aug 2026 22:03:05 +0800
Subject: [PATCH] Validate arguments of window-text-pixel-size up front

In the b != current_buffer branch, if window_text_pixel_size encounters
an error (such as receiving an argument of the wrong type), the
current_buffer pointer remains at b and is not correctly switched back.

FROM, TO, X-LIMIT, Y-LIMIT and MODE-LINES are now checked with
CHECK_* macros before the buffer switch.

* src/xdisp.c (Fwindow_text_pixel_size): Validate arguments before
calling window_text_pixel_size.

* test/src/xdisp-tests.el
(xdisp-tests--window-text-pixel-size-args-before-switch)
(xdisp-tests--window-text-pixel-size-invalid-args): New tests.
---
 src/xdisp.c             | 30 +++++++++++++++++++++++++++
 test/src/xdisp-tests.el | 46 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+)

diff --git a/src/xdisp.c b/src/xdisp.c
index beb8d980ee3..12f8627c650 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -12128,6 +12128,36 @@ DEFUN ("window-text-pixel-size", Fwindow_text_pixel_size, Swindow_text_pixel_siz
   struct buffer *old_b = NULL;
   Lisp_Object value;
 
+  if (!NILP (from) && !EQ (from, Qt))
+    {
+      if (CONSP (from))
+	{
+	  CHECK_TYPE (INTEGERP (XCAR (from)) || MARKERP (XCAR (from)),
+		      Qinteger_or_marker_p, XCAR (from));
+	  CHECK_FIXNUM (XCDR (from));
+	}
+      else
+	CHECK_TYPE (INTEGERP (from) || MARKERP (from),
+		    Qinteger_or_marker_p, from);
+    }
+
+  if (!NILP (to) && !EQ (to, Qt))
+    CHECK_TYPE (INTEGERP (to) || MARKERP (to), Qinteger_or_marker_p, to);
+
+  if (!NILP (x_limit) && !EQ (x_limit, Qt))
+    CHECK_FIXNAT (x_limit);
+
+  if (!NILP (y_limit))
+    CHECK_FIXNAT (y_limit);
+
+  if (!NILP (mode_lines) && !EQ (mode_lines, Qt))
+    {
+      CHECK_SYMBOL (mode_lines);
+      if (!EQ (mode_lines, Qmode_line) && !EQ (mode_lines, Qtab_line)
+	  && !EQ (mode_lines, Qheader_line))
+        error ("MODE-LINES must be mode-line, tab-line or header-line");
+    }
+
   if (b != current_buffer)
     {
       old_b = current_buffer;
diff --git a/test/src/xdisp-tests.el b/test/src/xdisp-tests.el
index 8eb57253972..3722dadb1cc 100644
--- a/test/src/xdisp-tests.el
+++ b/test/src/xdisp-tests.el
@@ -213,4 +213,50 @@ xdisp-test-format-mode-line
       (should (equal m1 m2))
       (should (equal s1 s2)))))
 
+(ert-deftest xdisp-tests--window-text-pixel-size-args-before-switch ()
+  "Invalid arguments must be rejected before switching to WINDOW's buffer."
+  (let ((buf-a (generate-new-buffer "buf-a"))
+        (buf-b (generate-new-buffer "buf-b")))
+    (unwind-protect
+        (progn
+          (with-current-buffer buf-b
+            (insert "test content for window-text-pixel-size"))
+          (set-window-buffer (selected-window) buf-b)
+          (set-buffer buf-a)
+          (should-error
+           (window-text-pixel-size (selected-window)
+                                   (cons 1 "not-a-fixnum")
+                                   nil nil nil nil nil)
+           :type 'wrong-type-argument)
+          (should (eq (current-buffer) buf-a)))
+      (ignore-errors (kill-buffer buf-a))
+      (ignore-errors (kill-buffer buf-b)))))
+
+(ert-deftest xdisp-tests--window-text-pixel-size-invalid-args ()
+  ;; FROM must be nil, t, a position (integer or marker), or a cons
+  ;; (POSITION . VERTICAL-OFFSET) where VERTICAL-OFFSET is a fixnum.
+  (should-error (window-text-pixel-size nil "foo")
+                :type 'wrong-type-argument)
+  (should-error (window-text-pixel-size nil '("foo" . 1))
+                :type 'wrong-type-argument)
+  (should-error (window-text-pixel-size nil '(1 . "foo"))
+                :type 'wrong-type-argument)
+  ;; TO must be nil, t, or a position.
+  (should-error (window-text-pixel-size nil nil "foo")
+                :type 'wrong-type-argument)
+  ;; X-LIMIT must be nil, t, or a nonnegative fixnum.
+  (should-error (window-text-pixel-size nil nil nil "foo")
+                :type 'wrong-type-argument)
+  (should-error (window-text-pixel-size nil nil nil -1)
+                :type 'wrong-type-argument)
+  ;; Y-LIMIT must be nil or a nonnegative fixnum; unlike X-LIMIT,
+  ;; t is not valid.
+  (should-error (window-text-pixel-size nil nil nil nil t)
+                :type 'wrong-type-argument)
+  ;; MODE-LINES must be nil, t, or one of the three line symbols.
+  (should-error (window-text-pixel-size nil nil nil nil nil 'mod-line))
+  ;; Valid arguments must still be accepted.
+  (should (consp (window-text-pixel-size nil '(1 . 0))))
+  (should (consp (window-text-pixel-size nil nil nil t nil 'mode-line))))
+
 ;;; xdisp-tests.el ends here
-- 
2.55.0