Update example for logical to inode v2 ioctl

Now we have the LOGICAL_INO_V2 available, we can use it to overcome the
limitations of the output for this example.

During the first call to the ioctl, we find out if v2 is available. If
so, we keep using it, otherwise we fall back to v1.

We still use the default 64k buffer. But, if v2 is used and we see
missed_bytes, we retry with a buffer that will fit all results.

examples/show_block_group_data_extent_filenames.py

@@ -15,10 +15,40 @@
 
 path_cache = {}
 
+
+def using_v1(fd, vaddr):
+    return btrfs.ioctl.logical_to_ino(fd, vaddr)
+
+
+def using_v2(fd, vaddr):
+    inodes, bytes_missed = btrfs.ioctl.logical_to_ino_v2(fd, vaddr, ignore_offset=True)
+    if bytes_missed > 0:
+        inodes, bytes_missed = \
+            btrfs.ioctl.logical_to_ino_v2(fd, vaddr, bufsize=65536+bytes_missed,
+                                          ignore_offset=True)
+    return inodes, bytes_missed
+
+
+def find_out_about_v1_or_v2(fd, vaddr):
+    global logical_to_ino_fn
+    try:
+        inodes, bytes_missed = using_v2(fd, vaddr)
+        logical_to_ino_fn = using_v2
+        return inodes, bytes_missed
+    except IOError as e:
+        if e.errno == errno.ENOTTY:
+            inodes, bytes_missed = using_v1(fd, vaddr)
+            logical_to_ino_fn = using_v1
+            return inodes, bytes_missed
+        raise
+
+
+logical_to_ino_fn = find_out_about_v1_or_v2
+
 for extent in fs.extents(vaddr, vaddr + block_group.length - 1):
     if isinstance(extent, btrfs.ctree.ExtentItem) and extent.flags & btrfs.ctree.EXTENT_FLAG_DATA:
         print(extent)
-        inodes, bytes_missed = btrfs.ioctl.logical_to_ino(fs.fd, extent.vaddr)
+        inodes, bytes_missed = logical_to_ino_fn(fs.fd, extent.vaddr)
         for inode in inodes:
             if inode.root == btrfs.ctree.FS_TREE_OBJECTID or \
                 (inode.root >= btrfs.ctree.FIRST_FREE_OBJECTID and
@@ -39,5 +69,10 @@
             else:
                 print("    root {} inode {} offset {}".format(
                     inode.root, inode.inum, inode.offset))
+        if len(inodes) == 0 and logical_to_ino_fn == using_v1:
+            print("    [... no result and no kernel support for LOGICAL_INO_V2]")
         if bytes_missed > 0:
-            print("    [...]")
+            if logical_to_ino_fn == using_v1:
+                print("    [... remainder can't be shown; no kernel support for LOGICAL_INO_V2]")
+            else:
+                print("    [... remainder can't be shown; too many items for buffer size]")