synchronizer: allow server not finding txn sometimes

User has wallet file with history that includes some txid; corresponding
raw tx is not in the "transactions" dict in the file however.
When the synchronizer starts up, it requests this "missing" txn from
the server... but what if the server does not know about it?
Maybe it was reorged and is not in the new best chain,
and not even in mempool. This was not handled previously.

fix #5122

electrum/network.py

@@ -780,7 +780,7 @@ class Network(PrintError):
             try:
                 return await func(self, *args, **kwargs)
             except aiorpcx.jsonrpc.CodeMessageError as e:
-                raise UntrustedServerReturnedError(original_exception=e)
+                raise UntrustedServerReturnedError(original_exception=e) from e
         return wrapper
 
     @best_effort_reliable

electrum/synchronizer.py

@@ -32,6 +32,7 @@ from aiorpcx import TaskGroup, run_in_thread
 from .transaction import Transaction
 from .util import bh2u, make_aiohttp_session, NetworkJobOnDefaultServer
 from .bitcoin import address_to_scripthash, is_address
+from .network import UntrustedServerReturnedError
 
 if TYPE_CHECKING:
     from .network import Network
@@ -165,7 +166,7 @@ class Synchronizer(SynchronizerBase):
         # Remove request; this allows up_to_date to be True
         self.requested_histories.pop(addr)
 
-    async def _request_missing_txs(self, hist):
+    async def _request_missing_txs(self, hist, *, allow_server_not_finding_tx=False):
         # "hist" is a list of [tx_hash, tx_height] lists
         transaction_hashes = []
         for tx_hash, tx_height in hist:
@@ -179,10 +180,18 @@ class Synchronizer(SynchronizerBase):
         if not transaction_hashes: return
         async with TaskGroup() as group:
             for tx_hash in transaction_hashes:
-                await group.spawn(self._get_transaction, tx_hash)
+                await group.spawn(self._get_transaction(tx_hash, allow_server_not_finding_tx=allow_server_not_finding_tx))
 
-    async def _get_transaction(self, tx_hash):
+    async def _get_transaction(self, tx_hash, *, allow_server_not_finding_tx=False):
+        try:
             result = await self.network.get_transaction(tx_hash)
+        except UntrustedServerReturnedError as e:
+            # most likely, "No such mempool or blockchain transaction"
+            if allow_server_not_finding_tx:
+                self.requested_tx.pop(tx_hash)
+                return
+            else:
+                raise
         tx = Transaction(result)
         try:
             tx.deserialize()
@@ -207,7 +216,7 @@ class Synchronizer(SynchronizerBase):
             # Old electrum servers returned ['*'] when all history for the address
             # was pruned. This no longer happens but may remain in old wallets.
             if history == ['*']: continue
-            await self._request_missing_txs(history)
+            await self._request_missing_txs(history, allow_server_not_finding_tx=True)
         # add addresses to bootstrap
         for addr in self.wallet.get_addresses():
             await self._add_address(addr)