pytest: Highlight the re-entrancy issue for cln-plugin events

This was pointed out by Daywalker [1]: we are synchronously processing
events from `lightningd` which means that if processing one of the
hooks or requests was slow or delayed, we would not get notifications,
hooks, or RPC requests, massively impacting the flexbility.

This highlights the issue with a failing test (it times out), and in
the next commit we will isolate event processing into their own task,
so to free the event loop from having to wait for an eventual
response.

[1] https://community.corelightning.org/c/developers/hold-invoice-plugin#comment_wrapper_16754493

tests/test_cln_rs.py

@@ -249,6 +249,46 @@ def test_grpc_wrong_auth(node_factory):
     stub.Getinfo(nodepb.GetinfoRequest())
 
 
+@pytest.mark.xfail(
+    reason="Times out because we can't call the RPC method while currently holding on to HTLCs",
+    strict=True,
+)
+def test_cln_plugin_reentrant(node_factory, executor):
+    """Ensure that we continue processing events while already handling.
+
+    We should be continuing to handle incoming events even though a
+    prior event has not completed. This is important for things like
+    the `htlc_accepted` hook which needs to hold on to multiple
+    incoming HTLCs.
+
+    Scenario: l1 uses an `htlc_accepted` to hold on to incoming HTLCs,
+    and we release them using an RPC method.
+
+    """
+    bin_path = Path.cwd() / "target" / RUST_PROFILE / "examples" / "cln-plugin-reentrant"
+    l1 = node_factory.get_node(options={"plugin": str(bin_path)})
+    l2 = node_factory.get_node()
+    l2.connect(l1)
+    l2.fundchannel(l1)
+
+    # Now create two invoices, and pay them both. Neither should
+    # succeed, but we should queue them on the plugin.
+    i1 = l1.rpc.invoice(label='lbl1', msatoshi='42sat', description='desc')['bolt11']
+    i2 = l1.rpc.invoice(label='lbl2', msatoshi='31337sat', description='desc')['bolt11']
+
+    f1 = executor.submit(l2.rpc.pay, i1)
+    f2 = executor.submit(l2.rpc.pay, i2)
+
+    import time
+    time.sleep(3)
+
+    print("Releasing HTLCs after holding them")
+    l1.rpc.call('release')
+
+    assert f1.result()
+    assert f2.result()
+
+
 def test_grpc_keysend_routehint(bitcoind, node_factory):
     """The routehints are a bit special, test that conversions work.