Make block device size equal to virtual size

When snapshotting make sure that the size of the underlying block
device is equal to the input media's virtual size. If the input media
is not raw, the QEMU image driver may grow the underlying image. The
block device should be big enough to allow this kind of operations.

image_creator/disk.py

@@ -17,10 +17,8 @@
 
 """Module hosting the Disk class."""
 
-from image_creator.util import get_command
-from image_creator.util import try_fail_repeat
-from image_creator.util import free_space
-from image_creator.util import FatalError
+from image_creator.util import get_command, try_fail_repeat, free_space, \
+    FatalError, image_info
 from image_creator.bundle_volume import BundleVolume
 from image_creator.image import Image
 
@@ -160,22 +158,43 @@ class Disk(object):
 
         mode = os.stat(self.file).st_mode
         device = self.file if stat.S_ISBLK(mode) else self._losetup(self.file)
-        size = blockdev('--getsz', device)
+        size = int(blockdev('--getsz', device))
+        virtual_size = (image_info(device)['virtual-size'] + 511) // 512
+
+        # If the virtual_size is bigger than the block device size, we need to
+        # create a bigger block device padded with zeros, otherwise QEMU will
+        # freeze if it tries to enlarge the underlying file.
+        if virtual_size > size:
+            zeros = virtual_size - size
+            tablefd, table = tempfile.mkstemp()
+            try:
+                try:
+                    os.write(tablefd, '0 %d linear %s 0\n' % (size, device))
+                    os.write(tablefd, '%d %d zero\n' % (size, zeros))
+                finally:
+                    os.close(tablefd)
+                padded = 'snf-image-creator-padded-%s' % uuid.uuid4().hex
+                dmsetup('create', padded, table)
+                self._add_cleanup(try_fail_repeat, dmsetup, 'remove', padded)
+            finally:
+                os.unlink(table)
+            device = "/dev/mapper/%s" % padded
+            size = virtual_size
+
         self.out.output("Snapshotting media source ...", False)
         cowfd, cow = tempfile.mkstemp(dir=self.tmp)
         os.close(cowfd)
         self._add_cleanup(os.unlink, cow)
         # Create cow sparse file
-        dd('if=/dev/null', 'of=%s' % cow, 'bs=512', 'seek=%d' % int(size))
+        dd('if=/dev/null', 'of=%s' % cow, 'bs=512', 'seek=%d' % size)
         cowdev = self._losetup(cow)
 
-        snapshot = uuid.uuid4().hex
+        snapshot = 'snf-image-creator-snapshot-%s' % uuid.uuid4().hex
         tablefd, table = tempfile.mkstemp()
         try:
             try:
                 os.write(tablefd,
-                         "0 %d snapshot %s %s n 8" %
-                         (int(size), device, cowdev))
+                         "0 %d snapshot %s %s n 8\n" % (size, device, cowdev))
             finally:
                 os.close(tablefd)
 

image_creator/util.py

@@ -24,6 +24,8 @@ import hashlib
 import time
 import os
 import re
+import json
+from sh import qemu_img
 
 
 class FatalError(Exception):
@@ -31,6 +33,11 @@ class FatalError(Exception):
     pass
 
 
+def image_info(image):
+    info = qemu_img('info', '--output', 'json', image)
+    return json.loads(str(info))
+
+
 def get_command(command):
     """Return a file system binary command"""
     def find_sbin_command(command, exception):