lnaddr: make min_cltv logic less error-prone

round-tripping the value behaved unexpectedly before...

electrum/lnaddr.py

@@ -272,7 +272,6 @@ class LnAddr(object):
         self.pubkey = None
         self.net = constants.net if net is None else net  # type: Type[AbstractNet]
         self._amount = amount  # type: Optional[Decimal]  # in bitcoins
-        self._min_final_cltv_expiry = 18
 
     @property
     def amount(self) -> Optional[Decimal]:
@@ -326,7 +325,10 @@ class LnAddr(object):
         )
 
     def get_min_final_cltv_expiry(self) -> int:
-        return self._min_final_cltv_expiry
+        cltv = self.get_tag('c')
+        if cltv is None:
+            return 18
+        return int(cltv)
 
     def get_tag(self, tag):
         for k, v in self.tags:
@@ -482,7 +484,7 @@ def lndecode(invoice: str, *, verbose=False, net=None) -> LnAddr:
             addr.pubkey = pubkeybytes
 
         elif tag == 'c':
-            addr._min_final_cltv_expiry = tagdata.uint
+            addr.tags.append(('c', tagdata.uint))
 
         elif tag == '9':
             features = tagdata.uint

electrum/tests/test_bolt11.py

@@ -146,6 +146,7 @@ class TestBolt11(ElectrumTestCase):
     def test_min_final_cltv_expiry_roundtrip(self):
         for cltv in (1, 15, 16, 31, 32, 33, 150, 511, 512, 513, 1023, 1024, 1025):
             lnaddr = LnAddr(paymenthash=RHASH, amount=Decimal('0.001'), tags=[('d', '1 cup coffee'), ('x', 60), ('c', cltv)])
+            self.assertEqual(cltv, lnaddr.get_min_final_cltv_expiry())
             invoice = lnencode(lnaddr, PRIVKEY)
             self.assertEqual(cltv, lndecode(invoice).get_min_final_cltv_expiry())