swapserver: support prepayment of fees

electrum/plugins/swapserver/server.py

@@ -103,7 +103,7 @@ class SwapServer(Logger, EventListener):
             their_pubkey=bytes.fromhex(request['claimPublicKey'])
             assert len(payment_hash) == 32
             assert len(their_pubkey) == 33
-            swap, payment_hash, invoice = sm.add_server_swap(
+            swap, payment_hash, invoice, prepay_invoice = sm.add_server_swap(
                 lightning_amount_sat=lightning_amount_sat,
                 payment_hash=payment_hash,
                 their_pubkey=their_pubkey
@@ -111,7 +111,7 @@ class SwapServer(Logger, EventListener):
             response = {
                 'id': payment_hash.hex(),
                 'invoice': invoice,
-                'minerFeeInvoice': None,
+                'minerFeeInvoice': prepay_invoice,
                 'lockupAddress': swap.lockup_address,
                 'redeemScript': swap.redeem_script.hex(),
                 'timeoutBlockHeight': swap.locktime,
@@ -121,7 +121,7 @@ class SwapServer(Logger, EventListener):
             their_invoice=request['invoice']
             their_pubkey=bytes.fromhex(request['refundPublicKey'])
             assert len(their_pubkey) == 33
-            swap, payment_hash, invoice = sm.add_server_swap(
+            swap, payment_hash, invoice, prepay_invoice = sm.add_server_swap(
                 invoice=their_invoice,
                 their_pubkey=their_pubkey
             )

electrum/submarine_swaps.py

@@ -317,19 +317,32 @@ class SwapManager(Logger):
                 {1:ripemd(payment_hash), 4:our_pubkey, 6:locktime, 9:their_pubkey}
             )
             self.wallet.save_invoice(Invoice.from_bech32(invoice))
+            prepay_invoice = None
         else:
             onchain_amount_sat = self._get_recv_amount(lightning_amount_sat, is_reverse=True)
+            prepay_amount_sat = self.get_claim_fee() * 2
+            main_amount_sat = lightning_amount_sat - prepay_amount_sat
             lnaddr, invoice = self.lnworker.get_bolt11_invoice(
                 payment_hash=payment_hash,
-                amount_msat=lightning_amount_sat * 1000,
+                amount_msat=main_amount_sat * 1000,
                 message='Submarine swap',
                 expiry=3600 * 24,
                 fallback_address=None,
                 channels=None,
             )
             # add payment info to lnworker
-            self.lnworker.add_payment_info_for_hold_invoice(payment_hash, lightning_amount_sat)
+            self.lnworker.add_payment_info_for_hold_invoice(payment_hash, main_amount_sat)
             self.lnworker.register_callback_for_hold_invoice(payment_hash, self.hold_invoice_callback, 60*60*24)
+            prepay_hash = self.lnworker.create_payment_info(amount_msat=prepay_amount_sat*1000)
+            _, prepay_invoice = self.lnworker.get_bolt11_invoice(
+                payment_hash=prepay_hash,
+                amount_msat=prepay_amount_sat * 1000,
+                message='prepay',
+                expiry=3600 * 24,
+                fallback_address=None,
+                channels=None,
+            )
+            self.lnworker.bundle_payments([payment_hash, prepay_hash])
             redeem_script = construct_script(
                 WITNESS_TEMPLATE_REVERSE_SWAP,
                 {1:32, 5:ripemd(payment_hash), 7:their_pubkey, 10:locktime, 13:our_pubkey}
@@ -354,7 +367,7 @@ class SwapManager(Logger):
         swap._payment_hash = payment_hash
         self._add_or_reindex_swap(swap)
         self.add_lnwatcher_callback(swap)
-        return swap, payment_hash, invoice
+        return swap, payment_hash, invoice, prepay_invoice
 
     async def normal_swap(
             self,