gfileutils: Remove outdated BTRFS fsync optimization from set_contents

This code was skipping fsync on BTRFS because of an old guarantee about
the overwrite-by-rename behavior that no longer holds true. This has
been confirmed by the BTRFS developers to no longer be guaranteed since
Kernel 3.17 (August 2014), but it was guaranteed when this optimization
was first introduced in 2010.

This could result in empty files after crashes in applications using
g_file_set_contents(). Most prominently this might have been the cause
of dconf settings getting lost on BTRFS after crashes due to the
frequency with which such writes can happen in dconf.

See: https://gitlab.gnome.org/GNOME/dconf/-/issues/73

glib/gfileutils.c

@@ -56,11 +56,6 @@
 #include "gstdioprivate.h"
 #include "glibintl.h"
 
-#ifdef HAVE_LINUX_MAGIC_H /* for btrfs check */
-#include <linux/magic.h>
-#include <sys/vfs.h>
-#endif
-
 
 /**
  * SECTION:fileutils
@@ -1091,23 +1086,6 @@ fd_should_be_fsynced (int                    fd,
 #ifdef HAVE_FSYNC
   struct stat statbuf;
 
-#ifdef BTRFS_SUPER_MAGIC
-  {
-    struct statfs buf;
-
-    /* On Linux, on btrfs, skip the fsync since rename-over-existing is
-     * guaranteed to be atomic and this is the only case in which we
-     * would fsync() anyway.
-     *
-     * See https://btrfs.wiki.kernel.org/index.php/FAQ#What_are_the_crash_guarantees_of_overwrite-by-rename.3F
-     */
-
-    if ((flags & G_FILE_SET_CONTENTS_CONSISTENT) &&
-        fstatfs (fd, &buf) == 0 && buf.f_type == BTRFS_SUPER_MAGIC)
-      return FALSE;
-  }
-#endif  /* BTRFS_SUPER_MAGIC */
-
   /* If the final destination exists and is > 0 bytes, we want to sync the
    * newly written file to ensure the data is on disk when we rename over
    * the destination. Otherwise if we get a system crash we can lose both

meson.build

@@ -282,7 +282,6 @@ headers = [
   'grp.h',
   'inttypes.h',
   'limits.h',
-  'linux/magic.h',
   'locale.h',
   'mach/mach_time.h',
   'memory.h',