Subject: Re: bug#4851: 23.1.50; narrowing, indirect buffers and set-buffer

From: Stefan Monnier <[email-masked]>
Date: Tue, 03 Nov 2009 16:34:20 -0500

> 1. emacs -Q
> 2. C-x b bla RET
> 3. M-: (insert "This is a test.")
> 4. M-: (narrow-to-region 5 10) 
> => buffer "bla" now displays " is a"
> 5. M-: (make-indirect-buffer (current-buffer) "blip")
> 6. M-: (save-restriction
>         (widen)
>         (set-buffer (get-buffer-create "bloop")))
> => buffer "bla" now displays "This is a test."

Thanks! I finally tracked this bug down.  Sadly, I think this points out
a more general problem, so while the patch below fixes it, we probably
have several other related bugs.

        Stefan

--- src/editfns.c	19 Oct 2009 04:27:14 -0000	1.473
+++ src/editfns.c	3 Nov 2009 21:32:41 -0000
@@ -3275,12 +3275,26 @@
 save_restriction_restore (data)
      Lisp_Object data;
 {
+  struct buffer *cur = NULL;
+  struct buffer *buf = (CONSP (data)
+			? XMARKER (XCAR (data))->buffer
+			: XBUFFER (data));
+
+  if (buf && buf != current_buffer && !NILP (buf->pt_marker))
+    { /* If `buf' uses markers to keep track of PT, BEGV, and ZV (as
+	 is the case if it is or has an indirect buffer), then make
+	 sure it is current before we update BEGV, so
+	 set_buffer_internal takes care of managing those markers.  */
+      cur = current_buffer;
+      set_buffer_internal (buf);
+    }
+
   if (CONSP (data))
     /* A pair of marks bounding a saved restriction.  */
     {
       struct Lisp_Marker *beg = XMARKER (XCAR (data));
       struct Lisp_Marker *end = XMARKER (XCDR (data));
-      struct buffer *buf = beg->buffer; /* END should have the same buffer. */
+      eassert (buf == end->buffer);
 
       if (buf /* Verify marker still points to a buffer.  */
 	  && (beg->charpos != BUF_BEGV (buf) || end->charpos != BUF_ZV (buf)))
@@ -3305,8 +3319,6 @@
   else
     /* A buffer, which means that there was no old restriction.  */
     {
-      struct buffer *buf = XBUFFER (data);
-
       if (buf /* Verify marker still points to a buffer.  */
 	  && (BUF_BEGV (buf) != BUF_BEG (buf) || BUF_ZV (buf) != BUF_Z (buf)))
 	/* The buffer has been narrowed, get rid of the narrowing.  */
@@ -3318,6 +3330,9 @@
 	}
     }
 
+  if (cur)
+    set_buffer_internal (cur);
+
   return Qnil;
 }