Auto discover the domain at start

`realm discover` without a domain name attempts to find one from DHCP.
This attempts to run it from the controller start method.
Only creates the discover task if there is support
Avoids exception on controller start.

subiquity/server/controllers/ad.py

@@ -13,6 +13,7 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import asyncio
 import logging
 import os
 import re
@@ -54,6 +55,18 @@ class DcPingStrategy:
 
         return AdDomainNameValidation.OK
 
+    async def discover(self) -> str:
+        """ Attempts to discover a domain through the network.
+            Returns the domain or an empty string on error. """
+        cp = await arun_command([self.cmd, self.arg], env={})
+        discovered = ""
+        if cp.returncode == 0:
+            # A typical output looks like:
+            # 'creative.com\n  type: kerberos\n  realm-name: CREATIVE.COM\n...'
+            discovered = cp.stdout.split('\n')[0].strip()
+
+        return discovered
+
 
 class StubDcPingStrategy(DcPingStrategy):
     """ For testing purpose. This class doesn't talk to the network.
@@ -67,6 +80,9 @@ class StubDcPingStrategy(DcPingStrategy):
 
         return AdDomainNameValidation.OK
 
+    async def discover(self) -> str:
+        return "ubuntu.com"
+
     def has_support(self) -> bool:
         return True
 
@@ -84,6 +100,15 @@ class ADController(SubiquityController):
         else:
             self.ping_strgy = DcPingStrategy()
 
+    def start(self):
+        if self.ping_strgy.has_support():
+            asyncio.create_task(self._try_discover_domain())
+
+    async def _try_discover_domain(self):
+        discovered_domain = await self.ping_strgy.discover()
+        if discovered_domain:
+            self.model.set_domain(discovered_domain)
+
     async def GET(self) -> Optional[ADConnectionInfo]:
         """Returns the currently configured AD settings"""
         return self.model.conn_info
@@ -116,8 +141,6 @@ class ADController(SubiquityController):
             to configure AD are present in the live system."""
         return self.ping_strgy.has_support()
 
-    # async def discover_domain_controller(self) -> str:
-
 
 # Helper out-of-class functions grouped.
 class AdValidators:

subiquity/tests/api/test_api.py

@@ -1659,8 +1659,8 @@ class TestActiveDirectory(TestAPI):
         async with start_server('examples/simple.json') as instance:
             endpoint = '/active_directory'
             ad_dict = await instance.get(endpoint)
-            # Starts empty
+            # Starts with the detected domain.
-            self.assertIsNone(ad_dict)
+            self.assertEqual('ubuntu.com', ad_dict['domain_name'])
 
             # Post works by "returning None"
             ad_dict = {