bug#20848: 24.5; window-state-put fails when strongly dedicated windows are involved

[Vasilij Schneidermann <v.schneidermann@gmail.com>]

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I've written a package for managing window configurations and switched
from using `current-window-configuration' and `set-window-configuration'
for their storage to `window-state-get' and `window-state-put' as
recommended by the Emacs Lisp manual to have serializable state and the
possibility of transforming it in a more elegant manner.

Today an user reported to me that the package doesn't work when strongly
dedicated buffers are involved.  I've refined their reproduction recipe
to the following piece of code which can be evaluated from `emacs -Q` in
the scratch buffer:

(progn
  (switch-to-buffer "*Messages*")
  (let ((old-window-config (window-state-get)))
    (set-window-dedicated-p nil t)
    (switch-to-buffer-other-window "*scratch*")
    (window-state-put old-window-config (frame-root-window))))

To get out of this, it's sufficient to use M-: (set-window-dedicated-p
nil nil) afterwards.

Compare with the following version that doesn't error out:

(progn
  (switch-to-buffer "*Messages*")
  (let ((old-window-config (current-window-configuration)))
    (set-window-dedicated-p nil t)
    (switch-to-buffer-other-window "*scratch*")
    (set-window-configuration old-window-config)))

Is this a bug? If not, is there a way of working around this in a sane
fashion (like, not removing the dedication status of the window)?



In GNU Emacs 24.5.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.16.2)
 of 2015-04-20 on bitzer.hoetzel.info
Windowing system distributor `The X.Org Foundation', version 11.0.11701000
System Description: Arch Linux

Configured using:
 `configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 --param=ssp-buffer-size=4' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro'

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
None found.

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Memory information:
((conses 16 70995 6884)
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 (strings 32 9046 4656)
 (string-bytes 1 248445)
 (vectors 16 8907)
 (vector-slots 8 383084 18361)
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 (intervals 56 177 6)
 (buffers 960 11)
 (heap 1024 48846 939))

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