wallet: don't try to get_input_tx from network when offline

related: https://github.com/spesmilo/electrum/issues/6648#issuecomment-708499893

Trying to fetch the prev tx from the network is a blocking operation with
a 10 sec timeout - we should not hang for 10 seconds if there is no network connection.

electrum/network.py

@@ -328,6 +328,7 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
         self.debug = False
 
         self._set_status('disconnected')
+        self._has_ever_managed_to_connect_to_server = False
 
         # lightning network
         self.channel_db = None  # type: Optional[ChannelDB]
@@ -339,6 +340,10 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
             self.local_watchtower.start_network(self)
             asyncio.ensure_future(self.local_watchtower.start_watching())
 
+    def has_internet_connection(self) -> bool:
+        """Our guess whether the device has Internet-connectivity."""
+        return self._has_ever_managed_to_connect_to_server
+
     def is_lightning_running(self):
         return self.channel_db is not None
 
@@ -768,6 +773,7 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
         if server == self.default_server:
             await self.switch_to_interface(server)
 
+        self._has_ever_managed_to_connect_to_server = True
         self._add_recent_server(server)
         util.trigger_callback('network_updated')
 

electrum/wallet.py

@@ -1613,7 +1613,7 @@ class Abstract_Wallet(AddressSynchronizer, ABC):
         # will likely be.  If co-signing a transaction it may not have
         # all the input txs, in which case we ask the network.
         tx = self.db.get_transaction(tx_hash)
-        if not tx and self.network:
+        if not tx and self.network and self.network.has_internet_connection():
             try:
                 raw_tx = self.network.run_from_another_thread(
                     self.network.get_transaction(tx_hash, timeout=10))