Calling signmessage returns an NotSupported instead of forwarding call to RPC

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
 
+### Fixed
+
+ - Fixed `signmessage` hanging forever and blocking all subsequent signing operations. The signer refuses `SIGN_MESSAGE` requests by design, so the node now rejects the RPC with an `Unimplemented` ("not supported") error instead of forwarding it to the serial hsmd queue. ([#739](https://github.com/Blockstream/greenlight/issues/739))
+
 ## [0.4.0] - 2026-05-21
 
 ### Fixed

libs/gl-plugin/src/node/wrapper.rs

@@ -498,11 +498,22 @@ impl Node for WrappedNodeServer {
         self.inner.set_channel(r).await
     }
 
+    /// `signmessage` is not part of the API we support: the signer
+    /// refuses `SIGN_MESSAGE` (hsmd type 23) by design, since we use
+    /// that message to attest TLS certificates. Forwarding the call to
+    /// the node would enqueue an hsmd request that never gets a
+    /// response, and since hsmd is a serial queue that blocks all
+    /// subsequent signing operations until the node restarts. We
+    /// therefore reject the call here, before it can reach hsmd.
     async fn sign_message(
         &self,
-        r: Request<pb::SignmessageRequest>,
+        _r: Request<pb::SignmessageRequest>,
     ) -> Result<Response<pb::SignmessageResponse>, Status> {
-        self.inner.sign_message(r).await
+        Err(Status::unimplemented(
+            "signmessage is not supported by Greenlight: the signer \
+             refuses to sign arbitrary messages, so the call can never \
+             complete.",
+        ))
     }
 
     async fn stop(

libs/gl-testing/tests/test_node.py

@@ -52,6 +52,34 @@ def test_node_signer(clients, executor):
     h.shutdown()
 
 
+def test_node_signmessage_rejected(clients):
+    """`signmessage` is rejected outright and does not wedge the node.
+
+    The signer refuses `SIGN_MESSAGE` (hsmd type 23) by design, so
+    forwarding the RPC to the node would leave a request pending in the
+    serial hsmd queue forever, blocking all subsequent signing
+    operations.
+    """
+    c = clients.new()
+    c.register(configure=True)
+    n = c.node()
+    h = c.signer().run_in_thread()
+
+    req = clnpb.SignmessageRequest(message="hello world").SerializeToString()
+    with pytest.raises(ValueError, match="not supported by Greenlight"):
+        n.inner.call("/cln.Node/SignMessage", req)
+
+    # The rejection must not have consumed a slot in the hsmd queue,
+    # so signing operations still work afterwards.
+    inv = n.invoice(
+        label="test",
+        amount_msat=clnpb.AmountOrAny(amount=clnpb.Amount(msat=42000)),
+        description="desc",
+    )
+    assert inv.bolt11
+    h.shutdown()
+
+
 @pytest.mark.skip(reason="routehints seem to be missing in regtest")
 def test_node_network(node_factory, clients, bitcoind):
     """Setup a small network and check that we can send/receive payments.