LNWorker: Add suggest_rebalance methods for sending and receiving.

These methods return a list of channels that can be rebalanced,
in order to receive or send a given amount.

Also add 'channels' parameter to submarine swaps.
Previously, swaps were not considering which channel to use.

When we do not have liquidity to pay an invoice:
 - add 'rebalance' option in order to pay an invoice
 - use the suggested channel in the 'swap' option

When we do not have the liquidity to receive an invoice:
 - add 'Rebalance' and 'Swap' buttons to the receive tab

electrum/gui/qt/main_window.py

@@ -1185,9 +1185,31 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         self.receive_address_help.setVisible(False)
         self.receive_URI_e = ButtonsTextEdit()
         self.receive_lightning_e = ButtonsTextEdit()
-        self.receive_lightning_help = WWLabel('')
+
+        self.receive_lightning_help_text = WWLabel('')
+        self.receive_rebalance_button = QPushButton('Rebalance')
+        self.receive_rebalance_button.suggestion = None
+        def on_receive_rebalance():
+            if self.receive_rebalance_button.suggestion:
+                chan1, chan2, delta = self.receive_rebalance_button.suggestion
+                self.rebalance_dialog(chan1, chan2, amount_sat=delta)
+        self.receive_rebalance_button.clicked.connect(on_receive_rebalance)
+        self.receive_swap_button = QPushButton('Swap')
+        self.receive_swap_button.suggestion = None
+        def on_receive_swap():
+            if self.receive_swap_button.suggestion:
+                chan, swap_recv_amount_sat = self.receive_swap_button.suggestion
+                self.run_swap_dialog(is_reverse=True, recv_amount_sat=swap_recv_amount_sat, channels=[chan])
+        self.receive_swap_button.clicked.connect(on_receive_swap)
+        buttons = QHBoxLayout()
+        buttons.addWidget(self.receive_rebalance_button)
+        buttons.addWidget(self.receive_swap_button)
+        vbox = QVBoxLayout()
+        vbox.addWidget(self.receive_lightning_help_text)
+        vbox.addLayout(buttons)
+        self.receive_lightning_help = QWidget()
         self.receive_lightning_help.setVisible(False)
-        #self.receive_URI_e.setFocusPolicy(Qt.ClickFocus)
+        self.receive_lightning_help.setLayout(vbox)
 
         fixedSize = 200
         for e in [self.receive_address_e, self.receive_URI_e, self.receive_lightning_e]:
@@ -1273,12 +1295,13 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
 
     def show_receive_request(self, req):
         addr = req.get_address() or ''
+        amount_sat = req.get_amount_sat() or 0
         address_help = '' if addr else _('Amount too small to be received onchain')
         lnaddr = req.lightning_invoice
         bip21_lightning = lnaddr if self.config.get('bip21_lightning', False) else None
         URI = req.get_bip21_URI(lightning=bip21_lightning)
         lightning_online = self.wallet.lnworker and self.wallet.lnworker.num_peers() > 0
-        can_receive_lightning = self.wallet.lnworker and (req.get_amount_sat() or 0) <= self.wallet.lnworker.num_sats_can_receive()
+        can_receive_lightning = self.wallet.lnworker and amount_sat <= self.wallet.lnworker.num_sats_can_receive()
         if lnaddr is None:
             ln_help = _('This request does not have a Lightning invoice.')
             lnaddr = ''
@@ -1286,7 +1309,15 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
             ln_help = _('You must be online to receive Lightning payments.')
             lnaddr = ''
         elif not can_receive_lightning:
+            self.receive_rebalance_button.suggestion = self.wallet.lnworker.suggest_rebalance_to_receive(amount_sat)
+            self.receive_swap_button.suggestion = self.wallet.lnworker.suggest_swap_to_receive(amount_sat)
             ln_help = _('Your Lightning channels do not have the capacity to receive this amount.')
+            can_rebalance = bool(self.receive_rebalance_button.suggestion)
+            can_swap = bool(self.receive_swap_button.suggestion)
+            self.receive_rebalance_button.setEnabled(can_rebalance)
+            self.receive_rebalance_button.setVisible(can_rebalance)
+            self.receive_swap_button.setEnabled(can_swap)
+            self.receive_swap_button.setVisible(can_swap)
             lnaddr = ''
         else:
             ln_help = ''
@@ -1303,7 +1334,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         self.receive_URI_e.setText(URI)
         self.receive_URI_qr.setData(URI)
         self.receive_lightning_e.setText(lnaddr)  # TODO maybe prepend "lightning:" ??
-        self.receive_lightning_help.setText(ln_help)
+        self.receive_lightning_help_text.setText(ln_help)
         self.receive_lightning_qr.setData(lnaddr_qr)
         # macOS hack (similar to #4777)
         self.receive_lightning_e.repaint()
@@ -1691,26 +1722,33 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
             coins = self.get_coins(nonlocal_only=True)
             can_pay_onchain = invoice.get_address() and self.wallet.can_pay_onchain(invoice.get_outputs(), coins=coins)
             can_pay_with_new_channel, channel_funding_sat = self.wallet.can_pay_with_new_channel(amount_sat, coins=coins)
-            can_pay_with_swap, swap_recv_amount_sat = self.wallet.can_pay_with_swap(amount_sat, coins=coins)
+            can_pay_with_swap = self.wallet.lnworker.suggest_swap_to_send(amount_sat, coins=coins)
+            can_rebalance = self.wallet.lnworker.suggest_rebalance_to_send(amount_sat)
             choices = {}
+            if can_rebalance:
+                msg = ''.join([
+                    _('Rebalance channels'), '\n',
+                    _('Funds will be sent between your channels in order to raise your sending capacity.')
+                ])
+                choices[0] = msg
             if can_pay_onchain:
                 msg = ''.join([
                     _('Pay onchain'), '\n',
                     _('Funds will be sent to the invoice fallback address.')
                 ])
-                choices[0] = msg
+                choices[1] = msg
             if can_pay_with_new_channel:
                 msg = ''.join([
                     _('Open a new channel'), '\n',
                     _('You will be able to pay once the channel is open.')
                 ])
-                choices[1] = msg
+                choices[2] = msg
             if can_pay_with_swap:
                 msg = ''.join([
                     _('Rebalance your channels with a submarine swap'), '\n',
                     _('You will be able to pay once the swap is confirmed.')
                 ])
-                choices[2] = msg
+                choices[3] = msg
             if not choices:
                 raise NotEnoughFunds()
             msg = _('You cannot pay that invoice using Lightning.')
@@ -1720,11 +1758,15 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
             if r is not None:
                 self.save_pending_invoice()
                 if r == 0:
-                    self.pay_onchain_dialog(coins, invoice.get_outputs())
+                    chan1, chan2, delta = can_rebalance
+                    self.rebalance_dialog(chan1, chan2, amount_sat=delta)
                 elif r == 1:
-                    self.channels_list.new_channel_dialog(amount_sat=channel_funding_sat)
+                    self.pay_onchain_dialog(coins, invoice.get_outputs())
                 elif r == 2:
-                    self.run_swap_dialog(is_reverse=False, recv_amount_sat=swap_recv_amount_sat)
+                    self.channels_list.new_channel_dialog(amount_sat=channel_funding_sat)
+                elif r == 3:
+                    chan, swap_recv_amount_sat = can_pay_with_swap
+                    self.run_swap_dialog(is_reverse=False, recv_amount_sat=swap_recv_amount_sat, channels=[chan])
             return
 
         # FIXME this is currently lying to user as we truncate to satoshis
@@ -1736,14 +1778,14 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         coro = self.wallet.lnworker.pay_invoice(invoice.lightning_invoice, amount_msat=amount_msat)
         self.run_coroutine_from_thread(coro)
 
-    def run_swap_dialog(self, is_reverse=None, recv_amount_sat=None):
+    def run_swap_dialog(self, is_reverse=None, recv_amount_sat=None, channels=None):
         if not self.network:
             self.window.show_error(_("You are offline."))
             return
         def get_pairs_thread():
             self.network.run_from_another_thread(self.wallet.lnworker.swap_manager.get_pairs())
         BlockingWaitingDialog(self, _('Please wait...'), get_pairs_thread)
-        d = SwapDialog(self, is_reverse=is_reverse, recv_amount_sat=recv_amount_sat)
+        d = SwapDialog(self, is_reverse=is_reverse, recv_amount_sat=recv_amount_sat, channels=channels)
         return d.run()
 
     def on_request_status(self, wallet, key, status):
@@ -2079,7 +2121,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
 
     def query_choice(self, msg, choices):
         # Needed by QtHandler for hardware wallets
-        dialog = WindowModalDialog(self.top_level_window())
+        dialog = WindowModalDialog(self.top_level_window(), title='Question')
         clayout = ChoicesLayout(msg, choices)
         vbox = QVBoxLayout(dialog)
         vbox.addLayout(clayout.layout())

electrum/gui/qt/swap_dialog.py

@@ -28,7 +28,7 @@ class SwapDialog(WindowModalDialog):
 
     tx: Optional[PartialTransaction]
 
-    def __init__(self, window: 'ElectrumWindow', is_reverse=None, recv_amount_sat=None):
+    def __init__(self, window: 'ElectrumWindow', is_reverse=None, recv_amount_sat=None, channels=None):
         WindowModalDialog.__init__(self, window, _('Submarine Swap'))
         self.window = window
         self.config = window.config
@@ -36,6 +36,7 @@ class SwapDialog(WindowModalDialog):
         self.swap_manager = self.lnworker.swap_manager
         self.network = window.network
         self.tx = None  # for the forward-swap only
+        self.channels = channels
         self.is_reverse = is_reverse if is_reverse is not None else True
         vbox = QVBoxLayout(self)
         self.description_label = WWLabel(self.get_description())
@@ -287,6 +288,7 @@ class SwapDialog(WindowModalDialog):
             expected_onchain_amount_sat=onchain_amount,
             password=password,
             tx=tx,
+            channels=self.channels,
         )
         self.window.run_coroutine_from_thread(coro)
 

electrum/lnworker.py

@@ -29,7 +29,7 @@ from . import constants, util
 from . import keystore
 from .util import profiler, chunks, OldTaskGroup
 from .invoices import Invoice, PR_UNPAID, PR_EXPIRED, PR_PAID, PR_INFLIGHT, PR_FAILED, PR_ROUTING, LN_EXPIRY_NEVER
-from .util import NetworkRetryManager, JsonRPCClient
+from .util import NetworkRetryManager, JsonRPCClient, NotEnoughFunds
 from .lnutil import LN_MAX_FUNDING_SAT
 from .keystore import BIP32_KeyStore
 from .bitcoin import COIN
@@ -1998,7 +1998,7 @@ class LNWallet(LNWorker):
         """calculate routing hints (BOLT-11 'r' field)"""
         routing_hints = []
         if channels is None:
-            channels = list(self.get_channels_to_include_in_invoice(amount_msat))
+            channels = list(self.get_channels_for_receiving(amount_msat))
             random.shuffle(channels)  # let's not leak channel order
         scid_to_my_channels = {chan.short_channel_id: chan for chan in channels
                                if chan.short_channel_id is not None}
@@ -2052,29 +2052,50 @@ class LNWallet(LNWorker):
                 chan.balance(LOCAL) if not chan.is_closed() and (chan.is_frozen_for_sending() if frozen else True) else 0
                 for chan in self.channels.values())) / 1000
 
-    def num_sats_can_send(self) -> Decimal:
-        can_send_dict = defaultdict(int)
-        with self.lock:
-            if self.channels:
-                for c in self.channels.values():
-                    if c.is_active() and not c.is_frozen_for_sending():
-                        if not self.channel_db and not self.is_trampoline_peer(c.node_id):
-                            continue
-                        if self.channel_db:
-                            can_send_dict[0] += c.available_to_spend(LOCAL)
-                        else:
-                            can_send_dict[c.node_id] += c.available_to_spend(LOCAL)
-        can_send = max(can_send_dict.values()) if can_send_dict else 0
+    def get_channels_for_sending(self):
+        for c in self.channels.values():
+            if c.is_active() and not c.is_frozen_for_sending():
+                if self.channel_db or self.is_trampoline_peer(c.node_id):
+                    yield c
+
+    def fee_estimate(self, amount_sat):
         # Here we have to guess a fee, because some callers (submarine swaps)
         # use this method to initiate a payment, which would otherwise fail.
         fee_base_msat = TRAMPOLINE_FEES[3]['fee_base_msat']
         fee_proportional_millionths = TRAMPOLINE_FEES[3]['fee_proportional_millionths']
         # inverse of fee_for_edge_msat
-        can_send_minus_fees = (can_send - fee_base_msat) * 1_000_000 // ( 1_000_000 + fee_proportional_millionths)
-        can_send_minus_fees = max(0, can_send_minus_fees)
-        return Decimal(can_send_minus_fees) / 1000
+        amount_msat = amount_sat * 1000
+        amount_minus_fees = (amount_msat - fee_base_msat) * 1_000_000 // ( 1_000_000 + fee_proportional_millionths)
+        return Decimal(amount_msat - amount_minus_fees) / 1000
 
-    def get_channels_to_include_in_invoice(self, amount_msat=None) -> Sequence[Channel]:
+    def num_sats_can_send(self, deltas=None) -> Decimal:
+        """
+        without trampoline, sum of all channel capacity
+        with trampoline, MPP must use a single trampoline
+        """
+        if deltas is None:
+            deltas = {}
+        def send_capacity(chan):
+            if chan in deltas:
+                delta_msat = deltas[chan] * 1000
+                if delta_msat > chan.available_to_spend(REMOTE):
+                    delta_msat = 0
+            else:
+                delta_msat = 0
+            return chan.available_to_spend(LOCAL) + delta_msat
+        can_send_dict = defaultdict(int)
+        with self.lock:
+            for c in self.get_channels_for_sending():
+                if self.channel_db:
+                    can_send_dict[0] += send_capacity(c)
+                else:
+                    can_send_dict[c.node_id] += send_capacity(c)
+        can_send = max(can_send_dict.values()) if can_send_dict else 0
+        can_send_sat = Decimal(can_send)/1000
+        can_send_sat -= self.fee_estimate(can_send_sat)
+        return max(can_send_sat, 0)
+
+    def get_channels_for_receiving(self, amount_msat=None) -> Sequence[Channel]:
         if not amount_msat:  # assume we want to recv a large amt, e.g. finding max.
             amount_msat = float('inf')
         with self.lock:
@@ -2103,16 +2124,26 @@ class LNWallet(LNWorker):
             channels = channels[:10]
             return channels
 
-    def num_sats_can_receive(self) -> Decimal:
+    def num_sats_can_receive(self, deltas=None) -> Decimal:
         """Return a conservative estimate of max sat value we can realistically receive
         in a single payment. (MPP is allowed)
 
         The theoretical max would be `sum(chan.available_to_spend(REMOTE) for chan in self.channels)`,
         but that would require a sender using MPP to magically guess all our channel liquidities.
         """
+        if deltas is None:
+            deltas = {}
+        def recv_capacity(chan):
+            if chan in deltas:
+                delta_msat = deltas[chan] * 1000
+                if delta_msat > chan.available_to_spend(LOCAL):
+                    delta_msat = 0
+            else:
+                delta_msat = 0
+            return chan.available_to_spend(REMOTE) + delta_msat
         with self.lock:
-            recv_channels = self.get_channels_to_include_in_invoice()
-            recv_chan_msats = [chan.available_to_spend(REMOTE) for chan in recv_channels]
+            recv_channels = self.get_channels_for_receiving()
+            recv_chan_msats = [recv_capacity(chan) for chan in recv_channels]
         if not recv_chan_msats:
             return Decimal(0)
         can_receive_msat = max(
@@ -2121,6 +2152,90 @@ class LNWallet(LNWorker):
         )
         return Decimal(can_receive_msat) / 1000
 
+    def _suggest_channels_for_rebalance(self, direction, amount_sat) -> Sequence[Tuple[Channel, int]]:
+        """
+        Suggest a channel and amount to send/receive with that channel, so that we will be able to receive/send amount_sat
+        This is used when suggesting a swap or rebalance in order to receive a payment
+        """
+        with self.lock:
+            func = self.num_sats_can_send if direction == SENT else self.num_sats_can_receive
+            delta = amount_sat - func()
+            assert delta > 0
+            delta += self.fee_estimate(amount_sat)
+            # add safety margin, for example if channel reserves is not met
+            # also covers swap server percentage fee
+            delta += delta // 20
+            suggestions = []
+            channels = self.get_channels_for_sending() if direction == SENT else self.get_channels_for_receiving()
+            for chan in channels:
+                if func(deltas={chan:delta}) >= amount_sat:
+                    suggestions.append((chan, delta))
+        if not suggestions:
+            raise NotEnoughFunds
+        return suggestions
+
+    def _suggest_rebalance(self, direction, amount_sat):
+        """
+        Suggest a rebalance in order to be able to send or receive amount_sat.
+        Returns (from_channel, to_channel, amount to shuffle)
+        """
+        try:
+            suggestions = self._suggest_channels_for_rebalance(direction, amount_sat)
+        except NotEnoughFunds:
+            return False
+        for chan2, delta in suggestions:
+            # margin for fee caused by rebalancing
+            delta += self.fee_estimate(amount_sat)
+            # find other channel or trampoline that can send delta
+            for chan1 in self.channels.values():
+                if chan1.is_frozen_for_sending() or not chan1.is_active():
+                    continue
+                if chan1 == chan2:
+                    continue
+                if not self.channel_db and chan1.node_id == chan2.node_id:
+                    continue
+                if direction == SENT:
+                    if chan1.can_pay(delta*1000):
+                        return (chan1, chan2, delta)
+                else:
+                    if chan1.can_receive(delta*1000):
+                        return (chan2, chan1, delta)
+            else:
+                continue
+        else:
+            return False
+
+    def suggest_rebalance_to_send(self, amount_sat):
+        return self._suggest_rebalance(SENT, amount_sat)
+
+    def suggest_rebalance_to_receive(self, amount_sat):
+        return self._suggest_rebalance(RECEIVED, amount_sat)
+
+    def suggest_swap_to_send(self, amount_sat, coins):
+        # fixme: if swap_amount_sat is lower than the minimum swap amount, we need to propose a higher value
+        assert amount_sat > self.num_sats_can_send()
+        try:
+            suggestions = self._suggest_channels_for_rebalance(SENT, amount_sat)
+        except NotEnoughFunds:
+            return
+        for chan, swap_recv_amount in suggestions:
+            # check that we can send onchain
+            swap_server_mining_fee = 10000 # guessing, because we have not called get_pairs yet
+            swap_funding_sat = swap_recv_amount + swap_server_mining_fee
+            swap_output = PartialTxOutput.from_address_and_value(ln_dummy_address(), int(swap_funding_sat))
+            if not self.wallet.can_pay_onchain([swap_output], coins=coins):
+                continue
+            return (chan, swap_recv_amount)
+
+    def suggest_swap_to_receive(self, amount_sat):
+        assert amount_sat > self.num_sats_can_receive()
+        try:
+            suggestions = self._suggest_channels_for_rebalance(RECEIVED, amount_sat)
+        except NotEnoughFunds:
+            return
+        for chan, swap_recv_amount in suggestions:
+            return (chan, swap_recv_amount)
+
     async def rebalance_channels(self, chan1, chan2, amount_msat):
         lnaddr, invoice = self.create_invoice(
             amount_msat=amount_msat,

electrum/submarine_swaps.py

@@ -262,6 +262,7 @@ class SwapManager(Logger):
             expected_onchain_amount_sat: int,
             password,
             tx: PartialTransaction = None,
+            channels = None,
     ) -> str:
         """send on-chain BTC, receive on Lightning
 
@@ -279,6 +280,7 @@ class SwapManager(Logger):
             message='swap',
             expiry=3600 * 24,
             fallback_address=None,
+            channels=channels,
         )
         payment_hash = lnaddr.paymenthash
         preimage = self.lnworker.get_preimage(payment_hash)
@@ -358,6 +360,7 @@ class SwapManager(Logger):
             *,
             lightning_amount_sat: int,
             expected_onchain_amount_sat: int,
+            channels = None,
     ) -> bool:
         """send on Lightning, receive on-chain
 
@@ -457,7 +460,7 @@ class SwapManager(Logger):
             self.prepayments[prepay_hash] = preimage_hash
             asyncio.ensure_future(self.lnworker.pay_invoice(fee_invoice, attempts=10))
         # initiate payment.
-        success, log = await self.lnworker.pay_invoice(invoice, attempts=10)
+        success, log = await self.lnworker.pay_invoice(invoice, attempts=10, channels=channels)
         return success
 
     def _add_or_reindex_swap(self, swap: SwapData) -> None:

electrum/tests/test_lnpeer.py

@@ -243,7 +243,7 @@ class MockLNWallet(Logger, NetworkRetryManager[LNPeerAddr]):
     get_channel_by_id = LNWallet.get_channel_by_id
     channels_for_peer = LNWallet.channels_for_peer
     calc_routing_hints_for_invoice = LNWallet.calc_routing_hints_for_invoice
-    get_channels_to_include_in_invoice = LNWallet.get_channels_to_include_in_invoice
+    get_channels_for_receiving = LNWallet.get_channels_for_receiving
     handle_error_code_from_failed_htlc = LNWallet.handle_error_code_from_failed_htlc
     is_trampoline_peer = LNWallet.is_trampoline_peer
     wait_for_received_pending_htlcs_to_get_removed = LNWallet.wait_for_received_pending_htlcs_to_get_removed

electrum/wallet.py

@@ -82,7 +82,6 @@ from .logging import get_logger
 from .lnworker import LNWallet
 from .paymentrequest import PaymentRequest
 from .util import read_json_file, write_json_file, UserFacingException
-from .lnutil import ln_dummy_address
 
 if TYPE_CHECKING:
     from .network import Network
@@ -1359,25 +1358,6 @@ class Abstract_Wallet(AddressSynchronizer, ABC):
             return False, None
         return True, channel_funding_sat
 
-    def can_pay_with_swap(self, amount_sat, coins=None):
-        # fixme: if swap_amount_sat is lower than the minimum swap amount, we need to propose a higher value
-        if self.lnworker is None:
-            return False, None
-        num_sats_can_send = int(self.lnworker.num_sats_can_send())
-        if amount_sat <= num_sats_can_send:
-            return True, None
-        lightning_needed = amount_sat - num_sats_can_send
-        lightning_needed += (lightning_needed // 20) # operational safety margin
-        swap_recv_amount = lightning_needed # the server's percentage fee is presumably within the above margin
-        swap_server_mining_fee = 10000      # guessing, because we have not called get_pairs yet
-        swap_funding_sat = swap_recv_amount + swap_server_mining_fee
-        swap_output = PartialTxOutput.from_address_and_value(ln_dummy_address(), swap_funding_sat)
-        can_do_swap_onchain = self.can_pay_onchain([swap_output], coins=coins)
-        can_do_swap_lightning = self.lnworker.num_sats_can_receive() >= swap_recv_amount
-        if can_do_swap_onchain and can_do_swap_lightning:
-            return True, swap_recv_amount
-        return False, None
-
     def make_unsigned_transaction(
             self, *,
             coins: Sequence[PartialTxInput],