plugin: Add opening_fee_msat to LspInvoiceResponse

Include the LSP's JIT channel opening fee in the lsp_invoice response
so clients can display the negotiated fees when presenting an invoice.

The fee is computed from the selected LSP's LSPS2 fee parameters using
max(min_fee_msat, ceil(amount * proportional / 1_000_000)). When the
node has sufficient incoming capacity and no JIT channel is needed, the
fee is reported as 0.

Also surfaces the fee through gl-sdk's ReceiveResponse for UniFFI
bindings consumers.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: cdecker

libs/gl-client-py/glclient/greenlight.proto

@@ -187,8 +187,8 @@ message NodeConfig {
 
 
 // The `GlConfig` is used to pass greenlight-specific startup parameters
-// to the node. The `gl-plugin` will look for a serialized config object in 
-// the node's datastore to load these values from. Please refer to the 
+// to the node. The `gl-plugin` will look for a serialized config object in
+// the node's datastore to load these values from. Please refer to the
 // individual fields to learn what they do.
 message GlConfig {
 	string close_to_addr = 1;
@@ -240,7 +240,7 @@ message LspInvoiceRequest {
   // Optional: for discounts/API keys
   string token = 2; // len=0 => None
   // Pass-through of cln invoice rpc params
-  uint64 amount_msat = 3;  // 0 => Any
+  uint64 amount_msat = 3; // 0 => Any
   string description = 4;
   string label = 5;
 }
@@ -250,6 +250,10 @@ message LspInvoiceResponse {
   uint32 expires_at = 3;
   bytes payment_hash = 4;
   bytes payment_secret = 5;
+  // The fee charged by the LSP for opening a JIT channel, in
+  // millisatoshi. This is 0 if the node already had sufficient
+  // incoming capacity and no JIT channel was needed.
+  uint64 opening_fee_msat = 6;
 }
 
 // Request for streaming node events. Currently empty but defined as

libs/gl-client-py/glclient/greenlight_pb2.py

@@ -236,3 +236,8 @@ def test_lsps_plugin_calls(clients, bitcoind, node_factory, lsps_server):
     assert len(inv.route_hints.route_hints) == 1
     rh = inv.route_hints.route_hints[0]
     assert rh.pubkey == bytes.fromhex(lsp_id)
+
+    # The response should include the LSP opening fee computed from
+    # the LSPS2 fee parameters (min_fee_msat=1000, proportional=1000).
+    # For 31337 msat: max(1000, ceil(31337 * 1000 / 1_000_000)) = 1000
+    assert res.opening_fee_msat == 1000

libs/gl-client-py/glclient/greenlight_pb2.pyi

@@ -247,6 +247,7 @@ impl Node for PluginNodeServer {
                     expires_at: res.expires_at as u32,
                     payment_hash: <cln_rpc::primitives::Sha256 as Borrow<[u8]>>::borrow(&res.payment_hash).to_vec(),
                     payment_secret: res.payment_secret.to_vec(),
+                    opening_fee_msat: 0,
                 }));
             }
 
@@ -279,8 +280,18 @@ impl Node for PluginNodeServer {
         let lsp = &lsps[0];
         log::info!("Selecting {:?} for invoice negotiation", lsp);
 
+        // Compute the expected opening fee from the LSP's fee parameters.
+        let opening_fee_msat = lsp.params.first().map_or(0, |p| {
+            let min_fee: u64 = p.min_fee_msat.parse().unwrap_or(0);
+            let proportional_fee = req
+                .amount_msat
+                .saturating_mul(p.proportional)
+                .div_ceil(1_000_000);
+            std::cmp::max(min_fee, proportional_fee)
+        });
+
         // Use the new RPC method name for versions > v25.05gl1
-        let res = if *version > *"v25.05gl1" {
+        let mut res = if *version > *"v25.05gl1" {
             let mut invreq: crate::requests::LspInvoiceRequestV2 = req.into();
             invreq.lsp_id = lsp.node_id.to_owned();
             rpc.call_typed(&invreq)
@@ -294,6 +305,7 @@ impl Node for PluginNodeServer {
                 .map_err(|e| Status::new(Code::Internal, e.to_string()))?
         };
 
+        res.opening_fee_msat = opening_fee_msat;
         Ok(Response::new(res.into()))
     }