lightningd: always broadcast the latest close tx

Before this patch we would only update `channel->last_tx` with the newly
proposed closure tx from the peer if the fee of the new one was lower.

In negotiations where we are at the higher end and the peer starts
lower, all peer's subsequent proposals will be higher than his initial
proposal and in this case we would never update `channel->last_tx`
and would wrongly broadcast his initial proposal at the end of the
negotiation.

Fixes https://github.com/ElementsProject/lightning/issues/3549

Changelog-Fixed: Always broadcast the latest close transaction at the end of the close fee negotiation, instead of sometimes broadcasting the peer's initial closing proposal.

tests/test_closing.py

@@ -393,6 +393,87 @@ def test_deprecated_closing_compat(node_factory, bitcoind, chainparams):
         l1.rpc.call('check', ['close', nodeid, "Given enough eyeballs, all bugs are shallow."])
 
 
+def closing_fee(node_factory, bitcoind, chainparams, opts):
+    rate = opts['funder_feerate_per_kw']
+    funder = node_factory.get_node(feerates=(rate, rate, rate))
+
+    rate = opts['fundee_feerate_per_kw']
+    fundee = node_factory.get_node(feerates=(rate, rate, rate))
+
+    funder_id = funder.info['id']
+    fundee_id = fundee.info['id']
+
+    fund_amount = 10**6
+
+    funder.rpc.connect(fundee_id, 'localhost', fundee.port)
+    funder.fund_channel(fundee, fund_amount)
+
+    assert bitcoind.rpc.getmempoolinfo()['size'] == 0
+
+    if opts['close_initiated_by'] == 'funder':
+        funder.rpc.close(peer_id=fundee_id)
+    else:
+        assert opts['close_initiated_by'] == 'fundee'
+        fundee.rpc.close(peer_id=funder_id)
+
+    # Get the closing transaction from the bitcoind mempool and get its fee
+
+    mempool = None
+    mempool_tx_ids = None
+
+    def get_mempool_when_size_1():
+        nonlocal mempool, mempool_tx_ids
+        mempool = bitcoind.rpc.getrawmempool(True)
+        mempool_tx_ids = list(mempool.keys())
+        return len(mempool_tx_ids) == 1
+
+    wait_for(get_mempool_when_size_1)
+
+    close_tx_id = mempool_tx_ids[0]
+    fee_mempool = round(mempool[close_tx_id]['fee'] * 10**8)
+
+    # Get the proclaimed closing fee from the two nodes' statuses
+
+    status_agreed_regex = re.compile("agreed on a closing fee of ([0-9]+) satoshi")
+
+    # [fee_from_funder_status, fee_from_fundee_status]
+    fees_from_status = [None, None]
+
+    def get_fee_from_status(node, peer_id, i):
+        nonlocal fees_from_status
+        status = only_one(only_one(node.rpc.listpeers(peer_id)['peers'][0]['channels'])['status'])
+        m = status_agreed_regex.search(status)
+        if not m:
+            return False
+        fees_from_status[i] = int(m.group(1))
+        return True
+
+    wait_for(lambda: get_fee_from_status(funder, fundee_id, 0))
+    wait_for(lambda: get_fee_from_status(fundee, funder_id, 1))
+
+    assert fee_mempool == fees_from_status[0]
+    assert fee_mempool == fees_from_status[1]
+    assert fee_mempool == opts['expected_close_fee']
+
+
+def test_closing_fee(node_factory, bitcoind, chainparams):
+    """Test that the closing negotiation strategy works"""
+    # feerate 27625 -> closing fee negotiation starts at 20000
+    # feerate 29006 -> closing fee negotiation starts at 21000
+
+    opts = {
+        'funder_feerate_per_kw': 29006,
+        'fundee_feerate_per_kw': 27625,
+        'close_initiated_by': 'funder',
+        'expected_close_fee': 20333
+    }
+
+    closing_fee(node_factory, bitcoind, chainparams, opts)
+
+    opts['close_initiated_by'] = 'fundee'
+    closing_fee(node_factory, bitcoind, chainparams, opts)
+
+
 @unittest.skipIf(not DEVELOPER, "needs DEVELOPER=1")
 def test_penalty_inhtlc(node_factory, bitcoind, executor, chainparams):
     """Test penalty transaction with an incoming HTLC"""