pyln-client: send proper JSONRPC2 errors on failed call

Changelog-Added: pyln now sends proper error on bad calls to plugin methods

Co-Authored-By: Sergi Delgado Segura <sergi.delgado.s@gmail.com>

contrib/plugins/helloworld.py

@@ -19,6 +19,12 @@ def hello(plugin, name="world"):
     return s
 
 
+@plugin.method("bye")
+def bye(plugin, name, **kwargs):
+    """This methods requires {name} to be set by the caller !"""
+    return "Bye {}".format(name)
+
+
 @plugin.init()
 def init(options, configuration, plugin, **kwargs):
     plugin.log("Plugin helloworld.py initialized")

contrib/pyln-client/pyln/client/plugin.py

@@ -83,9 +83,12 @@ class Request(dict):
         self._write_result({
             'jsonrpc': '2.0',
             'id': self.id,
-            "error": "Error while processing {method}: {exc}".format(
-                method=self.method, exc=repr(exc)
-            ),
+            "error": {
+                "code": -32600,  # "Invalid Request"
+                "message": "Error while processing {method}: {exc}"
+                           .format(method=self.method, exc=str(exc)),
+                # 'data' field "may be omitted."
+            },
         })
 
     def _write_result(self, result):

tests/test_plugin.py

@@ -180,6 +180,11 @@ def test_rpc_passthrough(node_factory):
     with pytest.raises(RpcError):
         n.rpc.fail()
 
+    # Try to call a method without enough arguments
+    with pytest.raises(RpcError, match="processing bye: missing a required"
+                                       " argument"):
+        n.rpc.bye()
+
 
 def test_plugin_dir(node_factory):
     """--plugin-dir works"""