adb.get_spender: subscribe to outputs explicitly, instead of as a side effect

This allows to subscribe only if we need it (for channel closing tx and htlcs)

electrum/address_synchronizer.py

@@ -1047,7 +1047,6 @@ class AddressSynchronizer(Logger, EventListener):
     def get_spender(self, outpoint: str) -> Optional[str]:
         """
         returns txid spending outpoint.
-        subscribes to addresses as a side effect.
         """
         prev_txid, index = outpoint.split(':')
         spender_txid = self.db.get_spent_outpoint(prev_txid, int(index))
@@ -1055,15 +1054,15 @@ class AddressSynchronizer(Logger, EventListener):
         tx_mined_status = self.get_tx_height(spender_txid)
         if tx_mined_status.height() in [TX_HEIGHT_LOCAL, TX_HEIGHT_FUTURE]:
             spender_txid = None
-        if not spender_txid:
-            return None
+        return spender_txid
+
+    def subscribe_to_outputs(self, spender_txid: str):
         spender_tx = self.get_transaction(spender_txid)
         for i, o in enumerate(spender_tx.outputs()):
             if o.address is None:
                 continue
             if not self.is_mine(o.address):
                 self.add_address(o.address)
-        return spender_txid
 
     def get_tx_mined_depth(self, txid: str):
         if not txid:

electrum/lnwatcher.py

@@ -111,6 +111,7 @@ class LNWatcher(Logger, EventListener):
         closing_txid = self.adb.get_spender(funding_outpoint)
         closing_height = self.adb.get_tx_height(closing_txid)
         if closing_txid:
+            self.adb.subscribe_to_outputs(closing_txid)
             closing_tx = self.adb.get_transaction(closing_txid)
             if closing_tx:
                 keep_watching = await self.sweep_commitment_transaction(funding_outpoint, closing_tx)
@@ -189,6 +190,8 @@ class LNWatcher(Logger, EventListener):
             if spender_tx:
                 # the spender might be the remote, revoked or not
                 htlc_sweepinfo = chan.maybe_sweep_htlcs(closing_tx, spender_tx)
+                if htlc_sweepinfo:
+                    self.adb.subscribe_to_outputs(spender_txid)
                 for prevout2, htlc_sweep_info in htlc_sweepinfo.items():
                     self.lnworker.wallet.set_default_label(prevout2, htlc_sweep_info.name)
                     if isinstance(htlc_sweep_info, KeepWatchingTXO):  # haven't yet decided if we want to sweep

electrum/plugins/watchtower/watchtower.py

@@ -140,6 +140,7 @@ class WatchTower(Logger, EventListener):
         # inspect_tx_candidate might have added new addresses, in which case we return early
         closing_txid = self.adb.get_spender(funding_outpoint)
         if closing_txid:
+            self.adb.subscribe_to_outputs(closing_txid)
             closing_tx = self.adb.get_transaction(closing_txid)
             if closing_tx:
                 keep_watching = await self.sweep_commitment_transaction(funding_outpoint, closing_tx)