Fixes Hyper-V issue with VHD file format

The VHD format cookie signature needs to be checked in the footer
instead of the header as fixed format VHDs don't have a copy of the
footer at the beginning of the file.

Fixes bug: #1233853

Change-Id: Ibe44f0d895cde2edbc03dcd6ecebac24b0936660

nova/tests/virt/hyperv/test_vhdutils.py

@@ -88,3 +88,47 @@ def test_get_internal_vhd_size_by_file_size_unsupported(self):
         self.assertRaises(vmutils.HyperVException,
                           vhdutil._get_internal_vhd_size_by_file_size,
                           None, root_vhd_size)
+
+    def test_get_vhd_format_vhdx(self):
+        with mock.patch('nova.virt.hyperv.vhdutils.open',
+                        mock.mock_open(read_data=vhdutils.VHDX_SIGNATURE),
+                        create=True) as mock_open:
+
+            format = self._vhdutils.get_vhd_format(self._FAKE_VHD_PATH)
+
+            self.assertEqual(constants.DISK_FORMAT_VHDX, format)
+
+    def test_get_vhd_format_vhd(self):
+        with mock.patch('nova.virt.hyperv.vhdutils.open',
+                        mock.mock_open(read_data=vhdutils.VHD_SIGNATURE),
+                        create=True) as mock_open:
+            f = mock_open.return_value
+            f.tell.return_value = 1024
+
+            format = self._vhdutils.get_vhd_format(self._FAKE_VHD_PATH)
+
+            self.assertEqual(constants.DISK_FORMAT_VHD, format)
+
+    def test_get_vhd_format_invalid_format(self):
+        with mock.patch('nova.virt.hyperv.vhdutils.open',
+                        mock.mock_open(read_data='invalid'),
+                        create=True) as mock_open:
+            f = mock_open.return_value
+            f.tell.return_value = 1024
+
+            self.assertRaises(vmutils.HyperVException,
+                              self._vhdutils.get_vhd_format,
+                              self._FAKE_VHD_PATH)
+
+    def test_get_vhd_format_zero_length_file(self):
+        with mock.patch('nova.virt.hyperv.vhdutils.open',
+                        mock.mock_open(read_data=''),
+                        create=True) as mock_open:
+            f = mock_open.return_value
+            f.tell.return_value = 0
+
+            self.assertRaises(vmutils.HyperVException,
+                              self._vhdutils.get_vhd_format,
+                              self._FAKE_VHD_PATH)
+
+            f.seek.assert_called_once_with(0, 2)

nova/virt/hyperv/vhdutils.py

@@ -15,6 +15,16 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+"""
+Utility class for VHD related operations.
+
+Official VHD format specs can be retrieved at:
+http://technet.microsoft.com/en-us/library/bb676673.aspx
+See "Download the Specifications Without Registering"
+
+Official VHDX format specs can be retrieved at:
+http://www.microsoft.com/en-us/download/details.aspx?id=34750
+"""
 import struct
 import sys
 
@@ -34,6 +44,9 @@
 VHD_FOOTER_SIZE_DYNAMIC = 512
 VHD_BLK_SIZE_OFFSET = 544
 
+VHD_SIGNATURE = 'conectix'
+VHDX_SIGNATURE = 'vhdxfile'
+
 
 class VHDUtils(object):
 
@@ -179,13 +192,19 @@ def get_vhd_info(self, vhd_path):
 
     def get_vhd_format(self, path):
         with open(path, 'rb') as f:
-            signature = f.read(8)
-        if signature == 'vhdxfile':
-            return constants.DISK_FORMAT_VHDX
-        elif signature == 'conectix':
-            return constants.DISK_FORMAT_VHD
-        else:
-            raise vmutils.HyperVException(_('Unsupported virtual disk format'))
+            # Read header
+            if f.read(8) == VHDX_SIGNATURE:
+                return constants.DISK_FORMAT_VHDX
+
+            # Read footer
+            f.seek(0, 2)
+            file_size = f.tell()
+            if file_size >= 512:
+                f.seek(-512, 2)
+                if f.read(8) == VHD_SIGNATURE:
+                    return constants.DISK_FORMAT_VHD
+
+        raise vmutils.HyperVException(_('Unsupported virtual disk format'))
 
     def get_best_supported_vhd_format(self):
         return constants.DISK_FORMAT_VHD