network: test if interface is alive before iface.taskgroup.spawn

closes https://github.com/spesmilo/electrum/issues/7677

```
 E/n | network | taskgroup died.
 Traceback (most recent call last):
   File "/opt/electrum/electrum/network.py", line 1204, in main
     [await group.spawn(job) for job in self._jobs]
   File "/home/voegtlin/.local/lib/python3.8/site-packages/aiorpcx/curio.py", line 297, in __aexit__
     await self.join()
   File "/opt/electrum/electrum/util.py", line 1255, in join
     task.result()
   File "/opt/electrum/electrum/network.py", line 1277, in _maintain_sessions
     await maintain_main_interface()
   File "/opt/electrum/electrum/network.py", line 1268, in maintain_main_interface
     await self._ensure_there_is_a_main_interface()
   File "/opt/electrum/electrum/network.py", line 1245, in _ensure_there_is_a_main_interface
     await self._switch_to_random_interface()
   File "/opt/electrum/electrum/network.py", line 648, in _switch_to_random_interface
     await self.switch_to_interface(random.choice(servers))
   File "/opt/electrum/electrum/network.py", line 714, in switch_to_interface
     await i.taskgroup.spawn(self._request_server_info(i))
   File "/home/voegtlin/.local/lib/python3.8/site-packages/aiorpcx/curio.py", line 204, in spawn
     self._add_task(task)
   File "/home/voegtlin/.local/lib/python3.8/site-packages/aiorpcx/curio.py", line 150, in _add_task
     raise RuntimeError('task group terminated')
 RuntimeError: task group terminated
```

I believe the "suppress spurious cancellations" block was added as SilentTaskGroup raised
CancelledError instead of RuntimeError for this scenario.

electrum/interface.py

@@ -489,7 +489,7 @@ class Interface(Logger):
                 self.logger.warning(f"disconnecting due to {repr(e)}")
                 self.logger.debug(f"(disconnect) trace for {repr(e)}", exc_info=True)
             finally:
-                self.got_disconnected.set()
+                self.got_disconnected.set()  # set this ASAP, ideally before any awaits
                 await self.network.connection_down(self)
                 # if was not 'ready' yet, schedule waiting coroutines:
                 self.ready.cancel()
@@ -534,6 +534,9 @@ class Interface(Logger):
 
         self.ready.set_result(1)
 
+    def is_connected_and_ready(self) -> bool:
+        return self.ready.done() and not self.got_disconnected.is_set()
+
     async def _save_certificate(self) -> None:
         if not os.path.exists(self.cert_path):
             # we may need to retry this a few times, in case the handshake hasn't completed

electrum/network.py

@@ -437,7 +437,7 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
 
     def is_connected(self):
         interface = self.interface
-        return interface is not None and interface.ready.done()
+        return interface is not None and interface.is_connected_and_ready()
 
     def is_connecting(self):
         return self.connection_status == 'connecting'
@@ -707,8 +707,9 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
 
         i = self.interfaces[server]
         if old_interface != i:
+            if not i.is_connected_and_ready():
+                return
             self.logger.info(f"switching to {server}")
-            assert i.ready.done(), "interface we are switching to is not ready yet"
             blockchain_updated = i.blockchain != self.blockchain()
             self.interface = i
             await i.taskgroup.spawn(self._request_server_info(i))
@@ -1195,7 +1196,7 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
         await self.taskgroup.spawn(self._run_new_interface(self.default_server))
 
         async def main():
-            self.logger.info("starting taskgroup.")
+            self.logger.info(f"starting taskgroup ({hex(id(taskgroup))}).")
             try:
                 # note: if a task finishes with CancelledError, that
                 # will NOT raise, and the group will keep the other tasks running
@@ -1203,9 +1204,9 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
                     await group.spawn(self._maintain_sessions())
                     [await group.spawn(job) for job in self._jobs]
             except Exception as e:
-                self.logger.exception("taskgroup died.")
+                self.logger.exception(f"taskgroup died ({hex(id(taskgroup))}).")
             finally:
-                self.logger.info("taskgroup stopped.")
+                self.logger.info(f"taskgroup stopped ({hex(id(taskgroup))}).")
         asyncio.run_coroutine_threadsafe(main(), self.asyncio_loop)
 
         util.trigger_callback('network_updated')
@@ -1238,13 +1239,13 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
             util.trigger_callback('network_updated')
 
     async def _ensure_there_is_a_main_interface(self):
-        if self.is_connected():
+        if self.interface:
             return
         # if auto_connect is set, try a different server
         if self.auto_connect and not self.is_connecting():
             await self._switch_to_random_interface()
         # if auto_connect is not set, or still no main interface, retry current
-        if not self.is_connected() and not self.is_connecting():
+        if not self.interface and not self.is_connecting():
             if self._can_retry_addr(self.default_server, urgent=True):
                 await self.switch_to_interface(self.default_server)
 
@@ -1271,15 +1272,9 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
                     await self.interface.taskgroup.spawn(self._request_fee_estimates, self.interface)
 
         while True:
-            try:
-                await maybe_start_new_interfaces()
-                await maintain_healthy_spread_of_connected_servers()
-                await maintain_main_interface()
-            except asyncio.CancelledError:
-                # suppress spurious cancellations
-                group = self.taskgroup
-                if not group or group.joined:
-                    raise
+            await maybe_start_new_interfaces()
+            await maintain_healthy_spread_of_connected_servers()
+            await maintain_main_interface()
             await asyncio.sleep(0.1)
 
     @classmethod