fix(cgpt.py): Add support for aligning partitions

When using anything other than classic spinning disks with 512 sectors
it is generally best to maintain some alignment with the underlying
physical sector or erase block size. The default alignment most
partitioning tools use these days is 1MB (2048 sectors). Also sometimes
qemu-img requires disk sizes to be aligned to 64KB.

build_library/cgpt.py

@@ -36,7 +36,7 @@ def LoadPartitionConfig(filename):
   valid_keys = set(('_comment', 'metadata', 'layouts'))
   valid_layout_keys = set((
       '_comment', 'type', 'num', 'label', 'blocks', 'block_size', 'fs_blocks',
-      'fs_block_size', 'features', 'uuid'))
+      'fs_block_size', 'features', 'uuid', 'alignment'))
 
   if not os.path.exists(filename):
     raise ConfigNotFound('Partition config %s was not found!' % filename)
@@ -45,9 +45,14 @@ def LoadPartitionConfig(filename):
 
   try:
     metadata = config['metadata']
-    for key in ('block_size', 'fs_block_size'):
+    for key in ('alignment', 'block_size', 'fs_block_size'):
       metadata[key] = int(metadata[key])
 
+    # Sometimes qemu-img expects disks sizes aligned to 64k
+    align_bytes = metadata['alignment'] * metadata['block_size']
+    if align_bytes < 65536 or align_bytes % 65536 != 0:
+      raise InvalidLayout('Invalid alignment, 64KB or better required')
+
     unknown_keys = set(config.keys()) - valid_keys
     if unknown_keys:
       raise InvalidLayout('Unknown items: %r' % unknown_keys)
@@ -67,6 +72,7 @@ def LoadPartitionConfig(filename):
             if not s in part:
               raise InvalidLayout('Layout "%s" missing "%s"' % (layout_name, s))
 
+        part['alignment'] = int(part.get('alignment', metadata['alignment']))
         part['blocks'] = int(part['blocks'])
         part['bytes'] = part['blocks'] * metadata['block_size']
 
@@ -220,19 +226,30 @@ def WritePartitionTable(options, image_type, layout_filename, disk_filename):
   def Cgpt(*args):
     subprocess.check_call(['cgpt'] + [str(a) for a in args])
 
+  def Align(count, alignment):
+    offset = count % alignment
+    if offset:
+      count += alignment - offset
+    return count
+
   config = LoadPartitionConfig(layout_filename)
+  metadata = config['metadata']
   partitions = GetPartitionTable(options, config, image_type)
   disk_block_count = GPT_RESERVED_SECTORS
 
   for partition in partitions:
+    disk_block_count = Align(disk_block_count, partition['alignment'])
     disk_block_count += partition['blocks']
 
   disk_block_count += GPT_RESERVED_SECTORS
+  # Sometimes qemu-img expects disks sizes aligned to 64k
+  disk_block_count = Align(disk_block_count, config['metadata']['alignment'])
 
   Cgpt('create', '-c', '-s', disk_block_count, disk_filename)
 
   sector = GPT_RESERVED_SECTORS
   for partition in partitions:
+    sector = Align(sector, partition['alignment'])
     if partition['type'] != 'blank':
       Cgpt('add', '-i', partition['num'],
                   '-b', sector,

build_library/legacy_disk_layout.json

@@ -1,6 +1,7 @@
 {
   "_comment": "See http://www.chromium.org/chromium-os/building-chromium-os/disk-layout-format",
   "metadata":{
+    "alignment": 2048,
     "block_size": 512,
     "fs_block_size": 4096
   },