fix(disk_util): Move existing image reading code to new function.

This will allow the code to be shared, making it possible to safely
resize existing images with disk_util.

build_library/disk_util

@@ -178,6 +178,35 @@ def GetPartitionTableFromConfig(options):
   return partitions
 
 
+def GetPartitionTableFromImage(options):
+  """Loads very basic partition table info from an existing image.
+
+  Currently only includes blocks and first_block values.
+
+  Args:
+    options: Flags passed to the script
+  Returns:
+    A list defining all existing partitions.
+  """
+  partitions = {}
+  cgpt_show = subprocess.check_output(
+          ['cgpt', 'show', '-q', options.disk_image])
+  for line in cgpt_show.split('\n'):
+    if not line.strip():
+      continue
+    fields = line.split(None, 3)
+    if len(fields) != 4 or not all(f.isdigit() for f in fields[:3]):
+      raise Exception('Invalid output from cgpt show -q: %r' % line)
+
+    part_num = fields[2]
+    partitions[part_num] = {
+            'num': int(part_num),
+            'first_block': int(fields[0]),
+            'blocks': int(fields[1])}
+
+  return partitions
+
+
 def WritePartitionTable(options, config=None, partitions=None):
   """Writes the given partition table to a disk image or device.
 
@@ -347,20 +376,12 @@ def Mount(options):
   """
 
   config, partitions = LoadPartitionConfig(options)
+  image_parts = GetPartitionTableFromImage(options)
   mounts = {}
 
-  cgpt_show = subprocess.check_output(
-          ['cgpt', 'show', '-q', options.disk_image])
-  for line in cgpt_show.split('\n'):
-    if not line.strip():
-      continue
-    fields = line.split(None, 3)
-    if len(fields) != 4 or not all(f.isdigit() for f in fields[:3]):
-      raise Exception('Invalid output from cgpt show -q: %r' % line)
-
-    first_block = int(fields[0])
-    blocks = int(fields[1])
-    part_num = fields[2]
+  for part_num, image_part in image_parts.iteritems():
+    first_block = image_part['first_block']
+    blocks = image_part['blocks']
     part = partitions.get(part_num, {})
     path = part.get('mount', None)
     if not path or not path.startswith('/'):