interface: refuse to overwrite blockchain of main interface

in case of conflicting forks

electrum/interface.py

@@ -503,11 +503,14 @@ class Interface(PrintError):
             # is assumed to be expensive; especially as forks below the max
             # checkpoint are ignored.
             self.print_error("new fork at bad height {}. conflict!!".format(bad))
+            assert self.blockchain != branch
             ismocking = type(branch) is dict
             if ismocking:
                 self.print_error("TODO replace blockchain")
                 return 'fork_conflict', height
             self.print_error('forkpoint conflicts with existing fork', branch.path())
+            self._raise_if_fork_conflicts_with_default_server(branch)
+            self._disconnect_from_interfaces_on_conflicting_blockchain(branch)
             branch.write(b'', 0)
             branch.save_header(bad_header)
             self.blockchain = branch
@@ -524,6 +527,21 @@ class Interface(PrintError):
             assert b.forkpoint == bad
             return 'fork_noconflict', height
 
+    def _raise_if_fork_conflicts_with_default_server(self, chain_to_delete: Blockchain) -> None:
+        main_interface = self.network.interface
+        if not main_interface: return
+        if main_interface == self: return
+        chain_of_default_server = main_interface.blockchain
+        if not chain_of_default_server: return
+        if chain_to_delete == chain_of_default_server:
+            raise GracefulDisconnect('refusing to overwrite blockchain of default server')
+
+    def _disconnect_from_interfaces_on_conflicting_blockchain(self, chain: Blockchain) -> None:
+        ifaces = self.network.disconnect_from_interfaces_on_given_blockchain(chain)
+        if not ifaces: return
+        servers = [interface.server for interface in ifaces]
+        self.print_error("forcing disconnect of other interfaces: {}".format(servers))
+
     async def _search_headers_backwards(self, height, header):
         async def iterate():
             nonlocal height, header

electrum/network.py

@@ -32,7 +32,7 @@ import json
 import sys
 import ipaddress
 import asyncio
-from typing import NamedTuple, Optional
+from typing import NamedTuple, Optional, Sequence
 
 import dns
 import dns.resolver
@@ -43,6 +43,7 @@ from .util import PrintError, print_error, aiosafe, bfh
 from .bitcoin import COIN
 from . import constants
 from . import blockchain
+from .blockchain import Blockchain
 from .interface import Interface, serialize_server, deserialize_server
 from .version import PROTOCOL_VERSION
 from .simple_config import SimpleConfig
@@ -708,14 +709,22 @@ class Network(PrintError):
 
     @with_interface_lock
     def get_blockchains(self):
-        out = {}
+        out = {}  # blockchain_id -> list(interfaces)
         with blockchain.blockchains_lock: blockchain_items = list(blockchain.blockchains.items())
-        for k, b in blockchain_items:
+        for chain_id, bc in blockchain_items:
-            r = list(filter(lambda i: i.blockchain==b, list(self.interfaces.values())))
+            r = list(filter(lambda i: i.blockchain==bc, list(self.interfaces.values())))
             if r:
-                out[k] = r
+                out[chain_id] = r
         return out
 
+    @with_interface_lock
+    def disconnect_from_interfaces_on_given_blockchain(self, chain: Blockchain) -> Sequence[Interface]:
+        chain_id = chain.forkpoint
+        ifaces = self.get_blockchains().get(chain_id) or []
+        for interface in ifaces:
+            self.connection_down(interface.server)
+        return ifaces
+
     def follow_chain(self, index):
         bc = blockchain.blockchains.get(index)
         if bc: