[PATCH v8 03/10] qcow2_format.py: change Qcow2BitmapExt initialization method

[Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>]

There are two ways to initialize a class derived from Qcow2Struct:
1. Pass a block of binary data to the constructor.
2. Pass the file descriptor to allow reading the file from constructor.
Let's change the Qcow2BitmapExt initialization method from 1 to 2 to
support a scattered reading in the initialization chain.
The implementation comes with the patch that follows.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
---
 tests/qemu-iotests/qcow2_format.py | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/tests/qemu-iotests/qcow2_format.py b/tests/qemu-iotests/qcow2_format.py
index 2f3681b..1435e34 100644
--- a/tests/qemu-iotests/qcow2_format.py
+++ b/tests/qemu-iotests/qcow2_format.py
@@ -63,7 +63,8 @@ class Qcow2StructMeta(type):
 
 class Qcow2Struct(metaclass=Qcow2StructMeta):
 
-    """Qcow2Struct: base class for qcow2 data structures
+    """
+    Qcow2Struct: base class for qcow2 data structures
 
     Successors should define fields class variable, which is: list of tuples,
     each of three elements:
@@ -113,6 +114,9 @@ class Qcow2BitmapExt(Qcow2Struct):
         ('u64', '{:#x}', 'bitmap_directory_offset')
     )
 
+    def __init__(self, fd):
+        super().__init__(fd=fd)
+
 
 QCOW2_EXT_MAGIC_BITMAPS = 0x23852875
 
@@ -173,7 +177,13 @@ class QcowHeaderExtension(Qcow2Struct):
         self.data_str = data_str
 
         if self.magic == QCOW2_EXT_MAGIC_BITMAPS:
-            self.obj = Qcow2BitmapExt(data=self.data)
+            assert fd is not None
+            position = fd.tell()
+            # Step back to reread data
+            padded = (self.length + 7) & ~7
+            fd.seek(-padded, 1)
+            self.obj = Qcow2BitmapExt(fd=fd)
+            fd.seek(position)
         else:
             self.obj = None
 
-- 
1.8.3.1