synchronizer: better handle history-status mismatch

When receiving the history of an address, the client behaved unexpectedly
if either of two checks failed.
The client checked that the txids in the history are unique, and that the
history matches the previously announced status. If either failed, it
would just log a line and do nothing. Importantly, the synchronizer could
even consider itself is_up_to_date, i.e. the GUI could show the wallet is
synced.

This is now changed such that:
- if the txid uniqueness test fails, we simply disconnect
- if the history is not consistent with previously announced status,
  we wait a bit, make sure is_up_to_date is False in the meantime,
  and then potentially disconnect
See rationale for these in the comments.

related: https://github.com/spesmilo/electrum/issues/7058#issuecomment-783613084

electrum/interface.py

@@ -953,6 +953,12 @@ class Interface(Logger):
                 if height < prev_height:
                     raise RequestCorrupted(f'heights of confirmed txs must be in increasing order')
                 prev_height = height
+        hashes = set(map(lambda item: item['tx_hash'], res))
+        if len(hashes) != len(res):
+            # Either server is sending garbage... or maybe if server is race-prone
+            # a recently mined tx could be included in both last block and mempool?
+            # Still, it's simplest to just disregard the response.
+            raise RequestCorrupted(f"server history has non-unique txids for sh={sh}")
         return res
 
     async def listunspent_for_scripthash(self, sh: str) -> List[dict]:

electrum/synchronizer.py

@@ -24,7 +24,7 @@
 # SOFTWARE.
 import asyncio
 import hashlib
-from typing import Dict, List, TYPE_CHECKING, Tuple
+from typing import Dict, List, TYPE_CHECKING, Tuple, Set
 from collections import defaultdict
 import logging
 
@@ -35,7 +35,7 @@ from .transaction import Transaction, PartialTransaction
 from .util import bh2u, make_aiohttp_session, NetworkJobOnDefaultServer, random_shuffled_copy
 from .bitcoin import address_to_scripthash, is_address
 from .logging import Logger
-from .interface import GracefulDisconnect
+from .interface import GracefulDisconnect, NetworkTimeout
 
 if TYPE_CHECKING:
     from .network import Network
@@ -153,6 +153,7 @@ class Synchronizer(SynchronizerBase):
         super()._reset()
         self.requested_tx = {}
         self.requested_histories = set()
+        self._stale_histories = dict()  # type: Dict[str, asyncio.Task]
 
     def diagnostic_name(self):
         return self.wallet.diagnostic_name()
@@ -160,34 +161,43 @@ class Synchronizer(SynchronizerBase):
     def is_up_to_date(self):
         return (not self.requested_addrs
                 and not self.requested_histories
-                and not self.requested_tx)
+                and not self.requested_tx
+                and not self._stale_histories)
 
     async def _on_address_status(self, addr, status):
         history = self.wallet.db.get_addr_history(addr)
         if history_status(history) == status:
             return
+        # No point in requesting history twice for the same announced status.
+        # However if we got announced a new status, we should request history again:
         if (addr, status) in self.requested_histories:
             return
         # request address history
         self.requested_histories.add((addr, status))
+        self._stale_histories.pop(addr, asyncio.Future()).cancel()
         h = address_to_scripthash(addr)
         self._requests_sent += 1
         async with self._network_request_semaphore:
             result = await self.interface.get_history_for_scripthash(h)
         self._requests_answered += 1
         self.logger.info(f"receiving history {addr} {len(result)}")
-        hashes = set(map(lambda item: item['tx_hash'], result))
         hist = list(map(lambda item: (item['tx_hash'], item['height']), result))
         # tx_fees
         tx_fees = [(item['tx_hash'], item.get('fee')) for item in result]
         tx_fees = dict(filter(lambda x:x[1] is not None, tx_fees))
-        # Check that txids are unique
-        if len(hashes) != len(result):
-            self.logger.info(f"error: server history has non-unique txids: {addr}")
         # Check that the status corresponds to what was announced
-        elif history_status(hist) != status:
-            self.logger.info(f"error: status mismatch: {addr}")
+        if history_status(hist) != status:
+            # could happen naturally if history changed between getting status and history (race)
+            self.logger.info(f"error: status mismatch: {addr}. we'll wait a bit for status update.")
+            # The server is supposed to send a new status notification, which will trigger a new
+            # get_history. We shall wait a bit for this to happen, otherwise we disconnect.
+            async def disconnect_if_still_stale():
+                timeout = self.network.get_network_timeout_seconds(NetworkTimeout.Generic)
+                await asyncio.sleep(timeout)
+                raise SynchronizerFailure(f"timeout reached waiting for addr {addr}: history still stale")
+            self._stale_histories[addr] = await self.taskgroup.spawn(disconnect_if_still_stale)
         else:
+            self._stale_histories.pop(addr, asyncio.Future()).cancel()
             # Store received history
             self.wallet.receive_history_callback(addr, hist, tx_fees)
             # Request transactions we don't have