pytest: add test (failing) which allows talking about closed channels.

In particular, if one side sees the final CLOSING_SIGNED and the other
doesn't, we won't talk when it reconnects.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_closing.py

@@ -2786,6 +2786,32 @@ def test_htlc_rexmit_while_closing(node_factory, executor):
     fut2.result(TIMEOUT)
 
 
+@pytest.mark.xfail(strict=True)
+@pytest.mark.openchannel('v1')
+@pytest.mark.developer("needs dev_disconnect")
+def test_you_forgot_closed_channel(node_factory, executor):
+    """Ideally you'd keep talking to us about closed channels"""
+    disconnects = ['@WIRE_CLOSING_SIGNED']
+
+    l1, l2 = node_factory.line_graph(2, opts=[{'may_reconnect': True,
+                                               'dev-no-reconnect': None},
+                                              {'may_reconnect': True,
+                                               'dev-no-reconnect': None,
+                                               'disconnect': disconnects}])
+
+    l1.pay(l2, 200000)
+
+    fut = executor.submit(l1.rpc.close, l2.info['id'])
+
+    # l2 considers the closing done, l1 does not
+    wait_for(lambda: only_one(only_one(l2.rpc.listpeers()['peers'])['channels'])['state'] == 'CLOSINGD_COMPLETE')
+    assert only_one(only_one(l1.rpc.listpeers()['peers'])['channels'])['state'] == 'CLOSINGD_SIGEXCHANGE'
+
+    # l1 reconnects, it should succeed.
+    l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
+    fut.result(TIMEOUT)
+
+
 @unittest.skipIf(TEST_NETWORK == 'liquid-regtest', "Uses regtest addresses")
 @pytest.mark.developer("too slow without fast polling for blocks")
 def test_segwit_anyshutdown(node_factory, bitcoind, executor):