interface: check genesis hash on connection

Prevents the client from accidentally connecting to a server on a
different network.
I noticed its possible to connect to mainnet servers on a signet
instance causing the recent peers to get populated with mainnet peers
rendering the wallet instance barely usable. Doing this check should
prevent this and similar issues.

electrum/interface.py

@@ -964,6 +964,14 @@ class Interface(Logger):
             if not self.network.check_interface_against_healthy_spread_of_connected_servers(self):
                 raise GracefulDisconnect(f'too many connected servers already '
                                          f'in bucket {self.bucket_based_on_ipaddress()}')
+
+            try:
+                features = await session.send_request('server.features')
+                server_genesis_hash = assert_dict_contains_field(features, field_name='genesis_hash')
+            except (aiorpcx.jsonrpc.RPCError, RequestCorrupted) as e:
+                raise GracefulDisconnect(e)
+            if server_genesis_hash != constants.net.GENESIS:
+                raise GracefulDisconnect(f'server on different chain: {server_genesis_hash=}. ours: {constants.net.GENESIS}')
             self.logger.info(f"connection established. version: {ver}")
 
             try:

tests/test_interface.py

@@ -11,6 +11,7 @@ from electrum.util import OldTaskGroup, bfh
 from electrum.logging import Logger
 from electrum.simple_config import SimpleConfig
 from electrum.transaction import Transaction
+from electrum import constants
 
 from . import ElectrumTestCase
 
@@ -130,6 +131,7 @@ class ServerSession(aiorpcx.RPCSession, Logger):
     async def handle_request(self, request):
         handlers = {
             'server.version': self._handle_server_version,
+            'server.features': self._handle_server_features,
             'blockchain.estimatefee': self._handle_estimatefee,
             'blockchain.headers.subscribe': self._handle_headers_subscribe,
             'blockchain.block.header': self._handle_block_header,
@@ -146,6 +148,17 @@ class ServerSession(aiorpcx.RPCSession, Logger):
     async def _handle_server_version(self, client_name='', protocol_version=None):
         return ['best_server_impl/0.1', '1.4']
 
+    async def _handle_server_features(self) -> dict:
+        return {
+            'genesis_hash': constants.net.GENESIS,
+            'hosts': {"14.3.140.101": {"tcp_port": 51001, "ssl_port": 51002}},
+            'protocol_max': '1.7.0',
+            'protocol_min': '1.4.3',
+            'pruning': None,
+            'server_version': 'ElectrumX 1.19.0',
+            'hash_function': 'sha256',
+        }
+
     async def _handle_estimatefee(self, number, mode=None):
         return 1000