cli: add command to export preimage

..also export preimage in check_hold_invoice return value if available.

I intentionally did not return the preimage in the returned dict of
wallet.export_requests as this seems risky to do considering some users
of the cli might forward the response to a payer and the payserver
exposes it too.

Closes https://github.com/spesmilo/electrum/issues/10176

electrum/commands.py

@@ -1514,10 +1514,22 @@ class Commands(Logger):
             plist = wallet.lnworker.get_payments(status='settled')[bfh(payment_hash)]
             _dir, amount_msat, _fee, _ts = wallet.lnworker.get_payment_value(info, plist)
             result["received_amount_sat"] = amount_msat // 1000
+            result['preimage'] = wallet.lnworker.get_preimage_hex(payment_hash)
         if info is not None:
             result["invoice_amount_sat"] = (info.amount_msat or 0) // 1000
         return result
 
+    @command('wl')
+    async def export_lightning_preimage(self, payment_hash: str, wallet: 'Abstract_Wallet' = None) -> Optional[str]:
+        """
+        Returns the stored preimage of the given payment_hash if it is known.
+
+        arg:str:payment_hash: Hash of the preimage
+        """
+        preimage = wallet.lnworker.get_preimage_hex(payment_hash)
+        assert preimage is None or crypto.sha256(bytes.fromhex(preimage)).hex() == payment_hash
+        return preimage
+
     @command('w')
     async def addtransaction(self, tx, wallet: Abstract_Wallet = None):
         """

electrum/wallet.py

@@ -2930,8 +2930,11 @@ class Abstract_Wallet(ABC, Logger, EventListener):
         if x.is_lightning():
             d['rhash'] = x.rhash
             d['lightning_invoice'] = self.get_bolt11_invoice(x)
-            if self.lnworker and status == PR_UNPAID:
-                d['can_receive'] = self.lnworker.can_receive_invoice(x)
+            if self.lnworker:
+                if status == PR_UNPAID:
+                    d['can_receive'] = self.lnworker.can_receive_invoice(x)
+                elif status == PR_PAID and (preimage := self.lnworker.get_preimage(x.payment_hash)):
+                    d['preimage'] = preimage.hex()
         if address := x.get_address():
             d['address'] = address
             d['URI'] = self.get_request_URI(x)

tests/test_commands.py

@@ -572,6 +572,7 @@ class TestCommandsTestnet(ElectrumTestCase):
             assert settled_status['status'] == 'settled'
             assert settled_status['received_amount_sat'] == 10000
             assert settled_status['invoice_amount_sat'] == 10000
+            assert settled_status['preimage'] == preimage.hex()
 
         with self.assertRaises(AssertionError):
             # cancelling a settled invoice should raise
@@ -723,3 +724,19 @@ class TestCommandsTestnet(ElectrumTestCase):
             }
         }
         self.assertEqual(result, expected_result)
+
+    @mock.patch.object(wallet.Abstract_Wallet, 'save_db')
+    async def test_export_lightning_preimage(self, *mock_args):
+        w = restore_wallet_from_text__for_unittest(
+            'disagree rug lemon bean unaware square alone beach tennis exhibit fix mimic',
+            path='if_this_exists_mocking_failed_648151893',
+            config=self.config)['wallet']
+        cmds = Commands(config=self.config)
+
+        preimage = os.urandom(32)
+        payment_hash = sha256(preimage)
+        w.lnworker.save_preimage(payment_hash, preimage)
+
+        assert await cmds.export_lightning_preimage(payment_hash=payment_hash.hex(), wallet=w) == preimage.hex()
+        assert await cmds.export_lightning_preimage(payment_hash=os.urandom(32).hex(), wallet=w) is None
+