allow app state transition from NEEDS_CONFIRMATION to WAITING

consider the following scenario, admittedly one that is not possible
today:

 * There is an "install model" that is required for server and not
   desktop (let's say "mirror")
 * The user initially selects a desktop install and moves through the
   screens until they are asked for confirmation.
 * At this point the user moves back through the screens and selects a
   server variant.  Now the application state of NEEDS_CONFIRMATION is
   misleading; the state needs to move back to WAITING until the mirror
   model is configured.

This is all probably excessively general but I feel like the core
control flow of the installer needs to be able to handle this sort of
thing...

subiquity/models/subiquity.py

@@ -121,6 +121,7 @@ class SubiquityModel:
         self.userdata = {}
 
         self._confirmation = asyncio.Event()
+        self._confirmation_task = None
 
         self._configured_names = set()
         self._install_model_names = install_model_names
@@ -136,6 +137,15 @@ class SubiquityModel:
             self._install_model_names.for_variant(variant)
         self._cur_postinstall_model_names = \
             self._postinstall_model_names.for_variant(variant)
+        unconfigured_install_model_names = \
+            self._cur_install_model_names - self._configured_names
+        if unconfigured_install_model_names:
+            if self._install_event.is_set():
+                self._install_event = asyncio.Event()
+            if self._confirmation_task is not None:
+                self._confirmation_task.cancel()
+        else:
+            self._install_event.set()
 
     def configured(self, model_name):
         self._configured_names.add(model_name)
@@ -163,7 +173,17 @@ class SubiquityModel:
         await self._postinstall_event.wait()
 
     async def wait_confirmation(self):
-        await self._confirmation.wait()
+        if self._confirmation_task is None:
+            self._confirmation_task = asyncio.get_event_loop().create_task(
+                self._confirmation.wait())
+        try:
+            await self._confirmation_task
+        except asyncio.CancelledError:
+            return False
+        else:
+            return True
+        finally:
+            self._confirmation_task = None
 
     def is_postinstall_only(self, model_name):
         return model_name in self._cur_postinstall_model_names and \

subiquity/server/controllers/install.py

@@ -199,17 +199,19 @@ class InstallController(SubiquityController):
     async def install(self, *, context):
         context.set('is-install-context', True)
         try:
-            self.app.update_state(ApplicationState.WAITING)
+            while True:
+                self.app.update_state(ApplicationState.WAITING)
 
-            await self.model.wait_install()
+                await self.model.wait_install()
 
-            if not self.app.interactive:
-                if 'autoinstall' in self.app.kernel_cmdline:
-                    self.model.confirm()
+                if not self.app.interactive:
+                    if 'autoinstall' in self.app.kernel_cmdline:
+                        self.model.confirm()
 
-            self.app.update_state(ApplicationState.NEEDS_CONFIRMATION)
+                self.app.update_state(ApplicationState.NEEDS_CONFIRMATION)
 
-            await self.model.wait_confirmation()
+                if await self.model.wait_confirmation():
+                    break
 
             self.app.update_state(ApplicationState.RUNNING)