filesystem: revamp udev handling

In LP: #2009141, we are hitting kernel limits and pyudev buffer limits.
We don't care about specific events, so much as getting one event,
waiting for things to calm down, then reprobing.

Turn off the event firehose ASAP, and drain the event queue more
agressively.

subiquity/server/controllers/filesystem.py

@@ -1479,14 +1479,7 @@ def ensure_probing(self):
         else:
             log.debug("Triggered Probert run on udev event")
 
-    def _udev_event(self):
-        cp = run_command(["udevadm", "settle", "-t", "0"])
-        if cp.returncode != 0:
-            log.debug("waiting 0.1 to let udev event queue settle")
-            self.stop_listening_udev()
-            loop = asyncio.get_running_loop()
-            loop.call_later(0.1, self.start_listening_udev)
-            return
+    def drain_udev(self):
         # Drain the udev events in the queue -- if we stopped listening to
         # allow udev to settle, it's good bet there is more than one event to
         # process and we don't want to kick off a full block probe for each
@@ -1495,7 +1488,20 @@ def _udev_event(self):
         while select.select([self._monitor.fileno()], [], [], 0)[0]:
             action, dev = self._monitor.receive_device()
             log.debug("_udev_event %s %s", action, dev)
-        self.ensure_probing()
+
+    def _udev_event(self):
+        self.stop_listening_udev()
+
+        cp = run_command(["udevadm", "settle", "-t", "0"])
+        self.drain_udev()
+
+        if cp.returncode != 0:
+            log.debug("waiting 0.1 to let udev event queue settle")
+            loop = asyncio.get_running_loop()
+            loop.call_later(0.1, self._udev_event)
+            return
+
+        self.start_listening_udev()
 
     def make_autoinstall(self):
         if self.model.dd_target is None: