Detection of sparse files is broken on btrfs

index d41c0ea..d0a7a55 100644
@@ -18,6 +18,7 @@
 #include <system.h>
 #include <inttostr.h>
 #include <quotearg.h>
+#include <sys/statfs.h>
 #include "common.h"
 
 struct tar_sparse_file;

@@ -261,12 +262,58 @@ sparse_scan_file_raw (struct tar_sparse_file *file)
   return tar_sparse_scan (file, scan_end, NULL);
 }
 
+enum sparse_fs_behavior
+  {
+    sparse_fs_behavior_init = 0,
+    sparse_fs_behavior_fine,
+    sparse_fs_behavior_uncertain
+  };
+
+static enum sparse_fs_behavior
+check_sparse_behavior (int fd)
+{
+  struct statfs buf;
+  if (fstatfs (fd, &buf))
+    return sparse_fs_behavior_fine;
+
+  if (buf.f_type == 0x9123683e)
+    return sparse_fs_behavior_uncertain; /* btrfs */
+
+  return sparse_fs_behavior_fine;
+}
+
+static bool
+wholesparse_detection_prohibited (struct tar_stat_info *st)
+{
+  static dev_t cached_device = 0;
+  static enum sparse_fs_behavior behavior;
+
+  if (behavior == sparse_fs_behavior_init
+      || cached_device != st->stat.st_dev)
+    {
+      cached_device = st->stat.st_dev;
+      behavior = check_sparse_behavior (st->fd);
+    }
+
+  return behavior == sparse_fs_behavior_uncertain;
+}
+
+
 static bool
 sparse_scan_file_wholesparse (struct tar_sparse_file *file)
 {
   struct tar_stat_info *st = file->stat_info;
   struct sp_array sp = {0, 0};
 
+  /* Some file-systems report st_blksize=0 for files which have some
+     inode-inlined data.  This is, per bug-tar@, rather unfortunate
+     behavior, but we need to deal with these filesystems somehow.  So,
+     let's prohibit the "wholesparse" detection method for such filesystems,
+     and let's hope that 'SEEK_HOLE/SEEK_DATA' works (if not, we fallback to
+     slow-but-safe 'raw' method anyway).  */
+  if (wholesparse_detection_prohibited (file->stat_info))
+    return false;
+
   /* Note that this function is called only for truly sparse files of size >= 1
      block size (checked via ST_IS_SPARSE before).  See the thread
      http://www.mail-archive.com/[email protected]/msg04209.html for more info */