test_pay.py: Add test for blockheight disagreement.

tests/test_pay.py

@@ -2744,3 +2744,57 @@ def test_createonion_limits(node_factory):
     with pytest.raises(RpcError, match=r'Payloads exceed maximum onion packet size.'):
         hops[0]['payload'] += '01'
         l1.rpc.createonion(hops=hops, assocdata="BB" * 32)
+
+
+@pytest.mark.xfail(strict=True)
+@unittest.skipIf(not DEVELOPER, "needs use_shadow")
+def test_blockheight_disagreement(node_factory, bitcoind, executor):
+    """
+    While a payment is in-transit from payer to payee, a block
+    might be mined, so that the blockheight the payer used to
+    initiate the payment is no longer the blockheight when the
+    payee receives it.
+    This leads to a failure which *used* to be
+    `final_expiry_too_soon`, a non-permanent failure, but
+    which is *now* `incorrect_or_unknown_payment_details`,
+    a permanent failure.
+    `pay` treats permanent failures as, well, permanent, and
+    gives up on receiving such failure from the payee, but
+    this particular subcase of blockheight disagreement is
+    actually a non-permanent failure (the payer only needs
+    to synchronize to the same blockheight as the payee).
+    """
+    l1, l2 = node_factory.line_graph(2)
+
+    sync_blockheight(bitcoind, [l1, l2])
+
+    # Arrange l1 to stop getting new blocks.
+    def no_more_blocks(req):
+        return {"result": None,
+                "error": {"code": -8, "message": "Block height out of range"}, "id": req['id']}
+    l1.daemon.rpcproxy.mock_rpc('getblockhash', no_more_blocks)
+
+    # Increase blockheight and make sure l2 knows it.
+    # Why 2? Because `pay` uses min_final_cltv_expiry + 1.
+    # But 2 blocks coming in close succession, plus slow
+    # forwarding nodes and block propagation, are still
+    # possible on the mainnet, thus this test.
+    bitcoind.generate_block(2)
+    sync_blockheight(bitcoind, [l2])
+
+    # Have l2 make an invoice.
+    inv = l2.rpc.invoice(1000, 'l', 'd')['bolt11']
+
+    # Have l1 pay l2
+    def pay(l1, inv):
+        l1.rpc.dev_pay(inv, use_shadow=False)
+    fut = executor.submit(pay, l1, inv)
+
+    # Make sure l1 sends out the HTLC.
+    l1.daemon.wait_for_logs([r'NEW:: HTLC LOCAL'])
+
+    # Unblock l1 from new blocks.
+    l1.daemon.rpcproxy.mock_rpc('getblockhash', None)
+
+    # pay command should complete without error
+    fut.result()