[gnome-continuous-yocto/gnomeostree-3.28-rocko: 6289/8267] filemap: fix skip logic

[Emmanuele Bassi <ebassi src gnome org>]

commit ce7895a61ca9c5b7b85d9cc3a0b05ae66a0d8570
Author: Ed Bartosh <ed bartosh linux intel com>
Date:   Tue Jun 13 14:21:50 2017 +0300

    filemap: fix skip logic
    
    Fixed bug in processing 'skip' parameter:
       don't read input file if end of bmap block is less than skip
    
    Simplified logic of positioning to the start of data inside a
    partially skipped bmap block.
    
    (From OE-Core rev: c19f78a0713c8ac9d28b78f86c6d7b96157788f0)
    
    Signed-off-by: Ed Bartosh <ed bartosh linux intel com>
    Signed-off-by: Richard Purdie <richard purdie linuxfoundation org>

 scripts/lib/wic/filemap.py |   11 +++++++----
 1 files changed, 7 insertions(+), 4 deletions(-)
---
diff --git a/scripts/lib/wic/filemap.py b/scripts/lib/wic/filemap.py
index 1f1aacc..585b7ea 100644
--- a/scripts/lib/wic/filemap.py
+++ b/scripts/lib/wic/filemap.py
@@ -545,11 +545,14 @@ def sparse_copy(src_fname, dst_fname, offset=0, skip=0, api=None):
         start = first * fmap.block_size
         end = (last + 1) * fmap.block_size
 
+        if skip >= end:
+            continue
+
         if start < skip < end:
-            fmap._f_image.seek(skip, os.SEEK_SET)
-        else:
-            fmap._f_image.seek(start, os.SEEK_SET)
-        dst_file.seek(offset + start, os.SEEK_SET)
+            start = skip
+
+        fmap._f_image.seek(start, os.SEEK_SET)
+        dst_file.seek(offset + start - skip, os.SEEK_SET)
 
         chunk_size = 1024 * 1024
         to_read = end - start