GNU bug report logs - #4655
23.1.50; buffer-swap-text oddity

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From: npostavs <at> users.sourceforge.net
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: lekktu <at> gmail.com, monnier <at> iro.umontreal.ca, dgutov <at> yandex.ru, rost <at> math.uni-bielefeld.de, 4655 <at> debbugs.gnu.org, larsi <at> gnus.org
Subject: bug#4655: 23.1.50; buffer-swap-text oddity
Date: Wed, 13 Jul 2016 19:50:49 -0400

[Message part 1 (text/plain, inline)]

Eli Zaretskii <eliz <at> gnu.org> writes:

> Yes, something like "upon return from this form ...".  I think it's a
> good idea.
>
> Otherwise the text is good, IMO.

Okay, here is my final version, I will push tomorrow.

[v3-0001-Document-buffer-swap-text-save-excursion-interact.patch (text/x-diff, inline)]

From f0295dbfceb3264f93d583537ff184b63779adea Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs <at> gmail.com>
Date: Sun, 10 Jul 2016 21:52:23 -0400
Subject: [PATCH v3] Document buffer-swap-text+save-excursion interaction

* doc/lispref/buffers.texi (Swapping Text):
* src/buffer.c (Fbuffer_swap_text): Add warning about interaction of
`buffer-swap-text' and `save-excursion' (Bug #4655).
---
 doc/lispref/buffers.texi | 6 ++++++
 src/buffer.c             | 4 +++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/doc/lispref/buffers.texi b/doc/lispref/buffers.texi
index 1f7f263..740d7cf 100644
--- a/doc/lispref/buffers.texi
+++ b/doc/lispref/buffers.texi
@@ -1211,6 +1211,12 @@ Swapping Text
 overlays, the text properties, the undo list, the value of the
 @code{enable-multibyte-characters} flag (@pxref{Text Representations,
 enable-multibyte-characters}), etc.
+
+@strong{Warning:} If this function is called from within a
+@code{save-excursion} form, the current buffer will be set to
+@var{buffer} upon leaving the form, since the marker used by
+@code{save-excursion} to save the position and buffer will be swapped
+as well.
 @end defun
 
   If you use @code{buffer-swap-text} on a file-visiting buffer, you
diff --git a/src/buffer.c b/src/buffer.c
index e4269c0..89f4479 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2231,7 +2231,9 @@ advance_to_char_boundary (ptrdiff_t byte_pos)
 
 DEFUN ("buffer-swap-text", Fbuffer_swap_text, Sbuffer_swap_text,
        1, 1, 0,
-       doc: /* Swap the text between current buffer and BUFFER.  */)
+       doc: /* Swap the text between current buffer and BUFFER.
+Using this function from `save-excursion' might produce surprising
+results, see Info node `(elisp)Swapping Text'.  */)
   (Lisp_Object buffer)
 {
   struct buffer *other_buffer;
-- 
2.8.0

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