bug#11245: 24.0.95; set-file-selinux-context fails when the filesystem doesn't support contexts

[goeran]

After upgrading to emacs to the one in Fedora 17 test release, numbered
24.0.95, I started to get messages about emacs failing to write backup
files, and it writing a %backup%~ file instead.  Checking in the file
system, emacs had successfully made a backup file.  After a bit of
investigation, I believe it has to do with emacs trying to set SELinux
context on a file system which doesn't support it.  In my case, my home
directory is mounted via NFS.

So to reproduce, I do the following, starting in a directory in an NFS
file system:

On the command line:
    freddi$ touch apa
    freddi$ ll -Z apa
    -rw-rw-r--. göran göran system_u:object_r:nfs_t:SystemLow apa
    freddi$ emacs -Q

Then, in emacs:
    M-x s e t - v a r <tab> <return> b a c k u <tab> b
    y <tab> <return> t <return> C-x C-f a p a <return>
    a p a C-x C-s

Now I get an error message:
    Cannot write backup file; backing up in ~/.emacs.d/%backup%~

But there is an (empty) file "apa~", so the backup DID succeed.

My understanding is that the root cause is the function
set-file-selinux-context in fileio.c failing with a file error when the
lsetfilecon() call fails.  But it is not really an error if it fails
with errno set to ENOTSUP.  A system which does support SELinux may very
well have some parts of the file tree where you can't modify the
context.  Such as an NFS home directory.

It seems to me that the case where lsetfilecon() fails with ENOTSUP
should be handled in the same way as if is_selinux_enabled() returned
false in the first place, i.e. just return without any error.  Do I miss
something?



In GNU Emacs 24.0.95.1 (x86_64-redhat-linux-gnu, GTK+ Version 2.24.10)
 of 2012-04-06 on x86-13.phx2.fedoraproject.org
Windowing system distributor `Fedora Project', version 11.0.11002000
Configured using:
 `configure '--build=x86_64-redhat-linux-gnu'
 '--host=x86_64-redhat-linux-gnu' '--program-prefix='
 '--disable-dependency-tracking' '--prefix=/usr' '--exec-prefix=/usr'
 '--bindir=/usr/bin' '--sbindir=/usr/sbin' '--sysconfdir=/etc'
 '--datadir=/usr/share' '--includedir=/usr/include'
 '--libdir=/usr/lib64' '--libexecdir=/usr/libexec'
 '--localstatedir=/var' '--sharedstatedir=/var/lib'
 '--mandir=/usr/share/man' '--infodir=/usr/share/info' '--with-dbus'
 '--with-gif' '--with-jpeg' '--with-png' '--with-rsvg' '--with-tiff'
 '--with-xft' '--with-xpm' '--with-x-toolkit=gtk' '--with-gpm=no'
 '--with-wide-int' 'build_alias=x86_64-redhat-linux-gnu'
 'host_alias=x86_64-redhat-linux-gnu' 'CFLAGS=-DMAIL_USE_LOCKF -O2 -g
 -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector
 --param=ssp-buffer-size=4 -m64 -mtune=generic' 'LDFLAGS=-Wl,-z,relro ''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: sv_SE.utf8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
  tooltip-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: (only . t)

Recent input:
M-x s e t - v a r <tab> <return> b a c k u <tab> b 
<tab> <return> t <return> C-x C-f a p a <return> a 
p a C-x C-s M-x r e p o r t - e m a <tab> <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Saving file /home/göran/apa...
Cannot write backup file; backing up in ~/.emacs.d/%backup%~
Wrote /home/göran/apa

Load-path shadows:
None found.

Features:
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mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail regexp-opt rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils cus-edit easymenu cus-start cus-load
wid-edit help-fns time-date tooltip ediff-hook vc-hooks lisp-float-type
mwheel x-win x-dnd tool-bar dnd fontset image fringe lisp-mode register
page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock
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button faces cus-face files text-properties overlay sha1 md5 base64
format env code-pages mule custom widget hashtable-print-readable
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system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty emacs)