From 4895ca16f76aa0ec044212a2b96ef8646cf4d0ed Mon Sep 17 00:00:00 2001
From: Lars Ingebrigtsen <larsi@gnus.org>
Date: Thu, 28 Jul 2022 12:23:53 +0200
Subject: [PATCH] Ensure that we don't call print-unreadable-function from "
 prin1"

* src/print.c (PRINTPREPARE): Bind the current buffer so that we
can retrieve it later.
(print_vectorlike): Use it (bug#56773).
(syms_of_print): New internal `print--unreadable-callback-buffer'
variable.
---
 src/print.c             | 19 +++++++++++++++++++
 test/lisp/subr-tests.el | 10 ++++++++++
 test/src/print-tests.el |  1 -
 3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/print.c b/src/print.c
index 384a639b317..48c945d08a0 100644
--- a/src/print.c
+++ b/src/print.c
@@ -105,6 +105,7 @@ bool print_output_debug_flag EXTERNALLY_VISIBLE = 1;
      = !NILP (BVAR (current_buffer, enable_multibyte_characters));	\
    Lisp_Object original = printcharfun;					\
    record_unwind_current_buffer ();					\
+   specbind(Qprint__unreadable_callback_buffer, Fcurrent_buffer ());	\
    if (NILP (printcharfun)) printcharfun = Qt;				\
    if (BUFFERP (printcharfun))						\
      {									\
@@ -1655,6 +1656,17 @@ print_vectorlike (Lisp_Object obj, Lisp_Object printcharfun, bool escapeflag,
 	 infinite recursion in the function called.  */
       Lisp_Object func = Vprint_unreadable_function;
       specbind (Qprint_unreadable_function, Qnil);
+
+      /* If we're being called from `prin1-to-string' or the like,
+	 we're now in the secret " prin1" buffer.  This can lead to
+	 problems if, for instance, the callback function switches a
+	 window to this buffer -- this will make Emacs segfault.  */
+      if (!NILP (Vprint__unreadable_callback_buffer)
+	  && Fbuffer_live_p (Vprint__unreadable_callback_buffer))
+	{
+	  record_unwind_current_buffer ();
+	  set_buffer_internal (XBUFFER (Vprint__unreadable_callback_buffer));
+	}
       Lisp_Object result = CALLN (Ffuncall, func, obj,
 				  escapeflag? Qt: Qnil);
       unbind_to (count, Qnil);
@@ -2913,6 +2925,13 @@ be printed.  */);
   Vprint_unreadable_function = Qnil;
   DEFSYM (Qprint_unreadable_function, "print-unreadable-function");
 
+  DEFVAR_LISP ("print--unreadable-callback-buffer",
+	       Vprint__unreadable_callback_buffer,
+	       doc: /* Dynamically bound to indicate current buffer.  */);
+  Vprint__unreadable_callback_buffer = Qnil;
+  DEFSYM (Qprint__unreadable_callback_buffer,
+	  "print--unreadable-callback-buffer");
+
   defsubr (&Sflush_standard_output);
 
   /* Initialized in print_create_variable_mapping.  */
diff --git a/test/lisp/subr-tests.el b/test/lisp/subr-tests.el
index 20f81d1ddc5..1d85631a4b8 100644
--- a/test/lisp/subr-tests.el
+++ b/test/lisp/subr-tests.el
@@ -1122,5 +1122,15 @@ final or penultimate step during initialization."))
       (should (equal (butlast l n)
                      (subr-tests--butlast-ref l n))))))
 
+(ert-deftest test-print-unreadable-function-buffer ()
+  (with-temp-buffer
+    (let ((current (current-buffer))
+          callback-buffer)
+      (let ((print-unreadable-function
+             (lambda (_object _escape)
+               (setq callback-buffer (current-buffer)))))
+        (prin1-to-string (make-marker)))
+      (should (eq current callback-buffer)))))
+
 (provide 'subr-tests)
 ;;; subr-tests.el ends here
diff --git a/test/src/print-tests.el b/test/src/print-tests.el
index f818b4d4715..91187d9f45c 100644
--- a/test/src/print-tests.el
+++ b/test/src/print-tests.el
@@ -529,6 +529,5 @@ otherwise, use a different charset."
                   (should (equal (% (- (length numbers) loopback-index) loop)
                                  0)))))))))))
 
-
 (provide 'print-tests)
 ;;; print-tests.el ends here
-- 
2.39.2