Merge pull request #29 from rackerlabs/pluggable_hardware_managers

RFR: Pluggable Hardware Managers

setup.cfg

@@ -21,3 +21,6 @@ console_scripts =
 teeth_agent.modes =
     standby = teeth_agent.standby:StandbyMode
     decom = teeth_agent.decom:DecomMode
+
+teeth_agent.hardware_managers =
+    generic = teeth_agent.hardware:GenericHardwareManager

teeth_agent/agent.py

@@ -118,7 +118,7 @@ class TeethAgent(object):
         self.api = api.TeethAgentAPIServer(self)
         self.command_results = collections.OrderedDict()
         self.heartbeater = TeethAgentHeartbeater(self)
-        self.hardware = hardware.HardwareInspector()
+        self.hardware = hardware.load_hardware_manager()
         self.command_lock = threading.Lock()
         self.log = structlog.get_logger()
         self.started_at = None
@@ -235,7 +235,7 @@ def build_agent(api_url,
     if not listen_host:
         listen_host = advertise_host
 
-    mac_addr = hardware.HardwareInspector().get_primary_mac_address()
+    mac_addr = hardware.load_hardware_manager().get_primary_mac_address()
     api_client = overlord_agent_api.APIClient(api_url)
 
     log.info('fetching agent configuration from API',

teeth_agent/hardware.py

@@ -14,7 +14,65 @@ See the License for the specific language governing permissions and
 limitations under the License.
 """
 
+import abc
+
+import stevedore
+import structlog
+
+_global_manager = None
+
+
+class HardwareSupport(object):
+    """These are just guidelines to suggest values that might be returned by
+    calls to `evaluate_hardware_support`. No HardwareManager in mainline
+    teeth-agent will ever offer a value greater than `MAINLINE`. Service
+    Providers should feel free to return values greater than SERVICE_PROVIDER
+    to distinguish between additional levels of support.
+    """
+    NONE = 0
+    GENERIC = 1
+    MAINLINE = 2
+    SERVICE_PROVIDER = 3
+
+
+class HardwareManager(object):
+    @abc.abstractmethod
+    def evaluate_hardware_support(cls):
+        pass
+
+    @abc.abstractmethod
+    def get_primary_mac_address(self):
+        pass
+
+
+class GenericHardwareManager(HardwareManager):
+    def evaluate_hardware_support(cls):
+        return HardwareSupport.GENERIC
 
-class HardwareInspector(object):
     def get_primary_mac_address(self):
         return open('/sys/class/net/eth0/address', 'r').read().strip('\n')
+
+
+def _compare_extensions(ext1, ext2):
+    mgr1 = ext1.obj
+    mgr2 = ext2.obj
+    return mgr1.evaluate_hardware_support() - mgr2.evaluate_hardware_support()
+
+
+def load_hardware_manager():
+    log = structlog.get_logger()
+    extension_manager = stevedore.ExtensionManager(
+        namespace='teeth_agent.hardware_managers',
+        invoke_on_load=True)
+
+    # There will always be at least one extension available (the
+    # GenericHardwareManager).
+    preferred_extension = sorted(extension_manager, _compare_extensions)[0]
+    preferred_manager = preferred_extension.obj
+
+    if preferred_manager.evaluate_hardware_support() <= 0:
+        raise RuntimeError('No suitable HardwareManager could be found')
+
+    log.info('selected hardware manager',
+             manager_name=preferred_extension.entry_point_target)
+    return preferred_manager

teeth_agent/tests/hardware.py

@@ -20,9 +20,9 @@ import unittest
 from teeth_agent import hardware
 
 
-class TestBaseTeethAgent(unittest.TestCase):
+class TestGenericHardwareManager(unittest.TestCase):
     def setUp(self):
-        self.hardware = hardware.HardwareInspector()
+        self.hardware = hardware.GenericHardwareManager()
 
     @mock.patch('__builtin__.open')
     def test_decom_mode(self, mocked_open):