lnwatcher: keep watching sweep TXOs that are dust due to high fees

- if fee estimates are high atm, some outputs are not worth to sweep
- however, fee estimates might be only-temporarily very high
  - previously in such a case lnwatcher would just discard outputs as dust,
    and mark the channel REDEEMED (and hence never watch it or try again)
  - now, instead, if the outputs would not be dust if fee estimates were lower,
    lnwatcher will keep watching the channel
    - and if estimates go down, lnwatcher will sweep them then
- relatedly, previously txbatcher.is_dust() used allow_fallback_to_static_rates=True,
    and it erroneously almost always fell back to the static rates (150 s/b) during
	startup (race: lnwatcher was faster than the network managed to get estimates)
	- now, instead, txbatcher.is_dust() does not fallback to static rates,
	  and the callers are supposed to handle NoDynamicFeeEstimates.
	  - I think this makes much more sense. The previous meaning of "is_dust"
	    with the fallback was weird. Now it means: "is dust at current feerates".

fixes https://github.com/spesmilo/electrum/issues/9980

electrum/address_synchronizer.py

@@ -1064,7 +1064,7 @@ class AddressSynchronizer(Logger, EventListener):
             return TxMinedDepth.FREE
         tx_mined_depth = self.get_tx_height(txid)
         height, conf = tx_mined_depth.height, tx_mined_depth.conf
-        if conf > 20:
+        if conf > 20:  # FIXME unify with lnutil.REDEEM_AFTER_DOUBLE_SPENT_DELAY ?
             return TxMinedDepth.DEEP
         elif conf > 0:
             return TxMinedDepth.SHALLOW

electrum/lnwatcher.py

@@ -5,11 +5,12 @@
 from typing import TYPE_CHECKING, Optional
 
 from . import util
-from .util import TxMinedInfo, BelowDustLimit
+from .util import TxMinedInfo, BelowDustLimit, NoDynamicFeeEstimates
 from .util import EventListener, event_listener, log_exceptions, ignore_exceptions
 from .transaction import Transaction, TxOutpoint
 from .logging import Logger
 from .address_synchronizer import TX_HEIGHT_LOCAL
+from .lnutil import REDEEM_AFTER_DOUBLE_SPENT_DELAY
 
 
 if TYPE_CHECKING:
@@ -172,32 +173,31 @@ class LNWatcher(Logger, EventListener):
                 # do not keep watching if prevout does not exist
                 self.logger.info(f'prevout does not exist for {name}: {prevout}')
                 continue
-            was_added = self.maybe_redeem(sweep_info)
+            watch_sweep_info = self.maybe_redeem(sweep_info)
             spender_txid = self.adb.get_spender(prevout)  # note: LOCAL spenders don't count
             spender_tx = self.adb.get_transaction(spender_txid) if spender_txid else None
             if spender_tx:
                 # the spender might be the remote, revoked or not
                 htlc_sweepinfo = chan.maybe_sweep_htlcs(closing_tx, spender_tx)
                 for prevout2, htlc_sweep_info in htlc_sweepinfo.items():
-                    htlc_was_added = self.maybe_redeem(htlc_sweep_info)
+                    watch_htlc_sweep_info = self.maybe_redeem(htlc_sweep_info)
                     htlc_tx_spender = self.adb.get_spender(prevout2)
                     self.lnworker.wallet.set_default_label(prevout2, htlc_sweep_info.name)
                     if htlc_tx_spender:
                         keep_watching |= not self.adb.is_deeply_mined(htlc_tx_spender)
                         self.maybe_add_accounting_address(htlc_tx_spender, htlc_sweep_info)
                     else:
-                        keep_watching |= htlc_was_added
+                        keep_watching |= watch_htlc_sweep_info
                 keep_watching |= not self.adb.is_deeply_mined(spender_txid)
                 self.maybe_extract_preimage(chan, spender_tx, prevout)
                 self.maybe_add_accounting_address(spender_txid, sweep_info)
             else:
-                keep_watching |= was_added
+                keep_watching |= watch_sweep_info
             self.maybe_add_pending_forceclose(
                 chan=chan,
                 spender_txid=spender_txid,
                 is_local_ctx=is_local_ctx,
                 sweep_info=sweep_info,
-                sweep_info_txo_is_nondust=was_added,
             )
         return keep_watching
 
@@ -205,10 +205,16 @@ class LNWatcher(Logger, EventListener):
         return self._pending_force_closes
 
     def maybe_redeem(self, sweep_info: 'SweepInfo') -> bool:
-        """ returns False if it was dust """
+        """ returns 'keep_watching' """
         try:
             self.lnworker.wallet.txbatcher.add_sweep_input('lnwatcher', sweep_info)
         except BelowDustLimit:
+            # utxo is considered dust at *current* fee estimates.
+            # but maybe the fees atm are very high? We will retry later.
+            pass
+        except NoDynamicFeeEstimates:
+            pass  # will retry later
+        if sweep_info.is_anchor():
             return False
         return True
 
@@ -259,10 +265,18 @@ class LNWatcher(Logger, EventListener):
         spender_txid: Optional[str],
         is_local_ctx: bool,
         sweep_info: 'SweepInfo',
-        sweep_info_txo_is_nondust: bool,  # i.e. we want to sweep it
+    ) -> None:
-    ):
+        """Adds chan into set of ongoing force-closures if the user should keep the wallet open, waiting for it.
-        """ we are waiting for ctx to be confirmed and there are received htlcs """
+        (we are waiting for ctx to be confirmed and there are received htlcs)
-        if is_local_ctx and sweep_info.name == 'received-htlc' and sweep_info_txo_is_nondust:
+        """
+        if is_local_ctx and sweep_info.name == 'received-htlc':
+            cltv = sweep_info.cltv_abs
+            assert cltv is not None, f"missing cltv for {sweep_info}"
+            if self.adb.get_local_height() > cltv + REDEEM_AFTER_DOUBLE_SPENT_DELAY:
+                # We had plenty of time to sweep. The remote also had time to time out the htlc.
+                # Maybe its value has been ~dust at current and past fee levels (every time we checked).
+                # We should not keep warning the user forever.
+                return
             tx_mined_status = self.adb.get_tx_height(spender_txid)
             if tx_mined_status.height == TX_HEIGHT_LOCAL:
                 self._pending_force_closes.add(chan)

electrum/submarine_swaps.py

@@ -32,7 +32,7 @@ from .transaction import (
 from .util import (
     log_exceptions, ignore_exceptions, BelowDustLimit, OldTaskGroup, ca_path, gen_nostr_ann_pow,
     get_nostr_ann_pow_amount, make_aiohttp_proxy_connector, get_running_loop, get_asyncio_loop, wait_for2,
-    run_sync_function_on_asyncio_thread, trigger_callback
+    run_sync_function_on_asyncio_thread, trigger_callback, NoDynamicFeeEstimates
 )
 from . import lnutil
 from .lnutil import hex_to_bytes, REDEEM_AFTER_DOUBLE_SPENT_DELAY, Keypair
@@ -485,6 +485,9 @@ class SwapManager(Logger):
             except BelowDustLimit:
                 self.logger.info('utxo value below dust threshold')
                 return
+            except NoDynamicFeeEstimates:
+                self.logger.info('got NoDynamicFeeEstimates')
+                return
 
     def get_fee_for_txbatcher(self):
         return self._get_tx_fee(self.config.FEE_POLICY_SWAPS)

electrum/txbatcher.py

@@ -99,6 +99,7 @@ class TxBatcher(Logger):
 
     @locked
     def add_sweep_input(self, key: str, sweep_info: 'SweepInfo') -> None:
+        """Can raise BelowDustLimit or NoDynamicFeeEstimates."""
         if sweep_info.txin and sweep_info.txout:
             # detect legacy htlc using name and csv delay
             if sweep_info.name in ['received-htlc', 'offered-htlc'] and sweep_info.csv_delay == 0:
@@ -263,20 +264,25 @@ class TxBatch(Logger):
         self.batch_payments.append(output)
 
     def is_dust(self, sweep_info: SweepInfo) -> bool:
+        """Can raise NoDynamicFeeEstimates."""
         if sweep_info.is_anchor():
             return False
         if sweep_info.txout is not None:
             return False
-        value = sweep_info.txin._trusted_value_sats
+        value = sweep_info.txin.value_sats()
         witness_size = len(sweep_info.txin.make_witness(71*b'\x00'))
         tx_size_vbytes = 84 + witness_size//4     # assumes no batching, sweep to p2wpkh
         self.logger.info(f'{sweep_info.name} size = {tx_size_vbytes}')
-        fee = self.fee_policy.estimate_fee(tx_size_vbytes, network=self.wallet.network, allow_fallback_to_static_rates=True)
+        fee = self.fee_policy.estimate_fee(tx_size_vbytes, network=self.wallet.network)
         return value - fee <= dust_threshold()
 
     @locked
     def add_sweep_input(self, sweep_info: 'SweepInfo') -> None:
+        """Can raise BelowDustLimit or NoDynamicFeeEstimates."""
         if self.is_dust(sweep_info):
+            # note: this uses the current fee estimates. Just because something is dust
+            #       at the current fee levels, if fees go down, it might still become
+            #       worthwhile to sweep. So callers might want to retry later.
             raise BelowDustLimit
         txin = sweep_info.txin
         if txin.prevout in self._unconfirmed_sweeps: