Add method to generate device names universally

When attaching block devices, we find the next device name to use by
looking at the used device letters in the block device mappings and
selecting the next available letter, as long as the letter is within
'a' - 'z' or 'aa' - 'zz'. This allows us to represent 702 devices (and
would interestingly hit an IndexError if all 'a' - 'z' and 'aa' - 'zz'
letters were in use already.

This adds a method for generating device names universally, not limited
to any particular maximum, based on a device index. For example:

  vda(index=0), sdb(index=1), vdaa(index=26), sdabc(index=730), ...

This method and its helper method will be used in a subsequent patch.

Part of blueprint conf-max-attach-volumes

Change-Id: I2699f600a98222a4dc58903ec97ca70e57701c93

nova/block_device.py

@@ -509,6 +509,32 @@ def get_device_letter(device_name):
     return _nums.sub('', letter) if device_name else device_name
 
 
+def generate_device_letter(index):
+    """Returns device letter by index (starts by zero)
+       i.e.
+       index   = 0, 1,..., 18277
+       results = a, b,..., zzz
+    """
+    base = ord('z') - ord('a') + 1
+    unit_dev_name = ""
+    while index >= 0:
+        letter = chr(ord('a') + (index % base))
+        unit_dev_name = letter + unit_dev_name
+        index = int(index / base) - 1
+
+    return unit_dev_name
+
+
+def generate_device_name(prefix, index):
+    """Returns device unit name by index (starts by zero)
+       i.e.
+       prefix  = vd
+       index   = 0, 1,..., 18277
+       results = vda, vdb,..., vdzzz
+    """
+    return prefix + generate_device_letter(index)
+
+
 def instance_block_mapping(instance, bdms):
     root_device_name = instance['root_device_name']
     # NOTE(clayg): remove this when xenapi is setting default_root_device

nova/tests/unit/test_block_device.py

@@ -149,6 +149,20 @@ class BlockDeviceTestCase(test.NoDBTestCase):
         self.assertEqual('c', block_device.get_device_letter('hdc'))
         self.assertIsNone(block_device.get_device_letter(None))
 
+    def test_generate_device_name(self):
+        expected = (
+                ('vda', ("vd", 0)),
+                ('vdaa', ("vd", 26)),
+                ('vdabc', ("vd", 730)),
+                ('vdidpok', ("vd", 4194304)),
+                ('sdc', ("sd", 2)),
+                ('sdaa', ("sd", 26)),
+                ('sdiw', ("sd", 256)),
+                ('hdzz', ("hd", 701))
+                )
+        for res, args in expected:
+            self.assertEqual(res, block_device.generate_device_name(*args))
+
     def test_volume_in_mapping(self):
         swap = {'device_name': '/dev/sdb',
                 'swap_size': 1}