Bug 637544 - Skip fsync() on btrfs

For g_file_set_contents() we fsync() before renaming the file over the
original in order to ensure that we don't end up with an invalid file.
btrfs provides this guarantee for us without the fsync() so skip it
there.

glib/gfileutils.c

@@ -53,6 +53,10 @@
 #include "gstdio.h"
 #include "glibintl.h"
 
+#ifdef __linux__ /* for btrfs check */
+#include <linux/magic.h>
+#include <sys/vfs.h>
+#endif
 
 /**
  * g_mkdir_with_parents:
@@ -963,6 +967,20 @@ write_to_temp_file (const gchar  *contents,
       
       goto out;
     }
+
+#ifdef __linux__
+  {
+    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.
+     */
+
+    if (fstatfs (fd, &buf) == 0 && buf.f_type == BTRFS_SUPER_MAGIC)
+      goto no_fsync;
+  }
+#endif
   
 #ifdef HAVE_FSYNC
   {
@@ -994,6 +1012,7 @@ write_to_temp_file (const gchar  *contents,
       }
   }
 #endif
+ no_fsync:
   
   errno = 0;
   if (fclose (file) == EOF)