source: do not fire a configured event again if nothing changed

When handling a POST request to /source, Subiquity sends a 'source
configured' event. This signals other controllers / models that they
need to restart their tasks that depend on the source being used.

However, if the user of the installer goes back all the way to the
source page and submits it again without changing the settings, there
should be no reason to restart the machinery.

If a call to source ends up doing no modification to the model (i.e.,
not changing the source used or the search_drivers setting), we now
avoid emitting the 'source configured' event ; except if the model has
not been configured yet.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquity/server/controllers/source.py

@@ -13,7 +13,6 @@
 # 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 contextlib
 import logging
 import os
 from typing import Any, Optional
@@ -79,6 +78,7 @@ def __init__(self, app):
         self._handler = None
         self.source_path: Optional[str] = None
         self.ai_source_id: Optional[str] = None
+        self._configured: bool = False
 
     def make_autoinstall(self):
         return {
@@ -148,10 +148,30 @@ def get_handler(
 
     async def configured(self):
         await super().configured()
+        self._configured = True
         self.app.base_model.set_source_variant(self.model.current.variant)
 
     async def POST(self, source_id: str, search_drivers: bool = False) -> None:
-        self.model.search_drivers = search_drivers
-        with contextlib.suppress(KeyError):
-            self.model.current = self.model.get_matching_source(source_id)
-        await self.configured()
+        # Marking the source model configured has an effect on many of the
+        # other controllers. Oftentimes, it would involve cancelling and
+        # restarting various operations.
+        # Let's try not to trigger the event again if we are not changing any
+        # of the settings.
+        changed = False
+        if self.model.search_drivers != search_drivers:
+            changed = True
+            self.model.search_drivers = search_drivers
+
+        try:
+            new_source = self.model.get_matching_source(source_id)
+        except KeyError:
+            # TODO going forward, we should probably stop silently ignoring
+            # unmatched sources.
+            pass
+        else:
+            if self.model.current != new_source:
+                changed = True
+                self.model.current = new_source
+
+        if changed or not self._configured:
+            await self.configured()