pyln: add RPCException for finer method failure control.

contrib/pyln-client/README.md

@@ -78,6 +78,9 @@ def hello(plugin, name="world"):
     It gets reported as the description when registering the function
     as a method with `lightningd`.
 
+    If this returns (a dict), that's the JSON "result" returned.  If
+    it raises an exception, that causes a JSON "error" return (raising
+    pyln.client.RpcException allows finer control over the return).
     """
     greeting = plugin.get_option('greeting')
     s = '{} {}'.format(greeting, name)

contrib/pyln-client/pyln/client/__init__.py

@@ -1,5 +1,5 @@
 from .lightning import LightningRpc, RpcError, Millisatoshi
-from .plugin import Plugin, monkey_patch
+from .plugin import Plugin, monkey_patch, RpcException
 
 
 __version__ = "0.8.0"
@@ -9,6 +9,7 @@
     "LightningRpc",
     "Plugin",
     "RpcError",
+    "RpcException",
     "Millisatoshi",
     "__version__",
     "monkey_patch"

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

@@ -61,6 +61,14 @@ def __init__(self, name: str, func: Callable[..., JSONType],
         self.after: List[str] = []
 
 
+class RpcException(Exception):
+    # -32600 == "Invalid Request"
+    def __init__(self, message: str, code: int = -32600):
+        self.code = code
+        self.message = message
+        super().__init__("RpcException: {}".format(message))
+
+
 class Request(dict):
     """A request object that wraps params and allows async return
     """
@@ -102,21 +110,27 @@ def set_result(self, result: Any) -> None:
         self.state = RequestState.FINISHED
         self.termination_tb = "".join(traceback.extract_stack().format()[:-1])
 
-    def set_exception(self, exc: Exception) -> None:
+    def set_exception(self, exc: Union[Exception, RpcException]) -> None:
         if self.state != RequestState.PENDING:
             assert(self.termination_tb is not None)
             raise ValueError(
                 "Cannot set the exception of a request that is not pending, "
                 "current state is {state}. Request previously terminated at\n"
                 "{tb}".format(state=self.state, tb=self.termination_tb))
         self.exc = exc
+        if isinstance(exc, RpcException):
+            code = exc.code
+            message = exc.message
+        else:
+            code = -32600  # "Invalid Request"
+            message = ("Error while processing {method}: {exc}"
+                       .format(method=self.method, exc=str(exc)))
         self._write_result({
             'jsonrpc': '2.0',
             'id': self.id,
             "error": {
-                "code": -32600,  # "Invalid Request"
-                "message": "Error while processing {method}: {exc}"
-                           .format(method=self.method, exc=str(exc)),
+                "code": code,
+                "message": message,
                 # 'data' field "may be omitted."
                 "traceback": traceback.format_exc(),
             },

contrib/pyln-client/tests/test_plugin.py

@@ -1,5 +1,5 @@
 from pyln.client import Plugin
-from pyln.client.plugin import Request, Millisatoshi
+from pyln.client.plugin import Request, Millisatoshi, RpcException
 import itertools
 import pytest  # type: ignore
 
@@ -172,6 +172,39 @@ def test1(name):
     assert call_list == []
 
 
+def test_method_exceptions():
+    """A bunch of tests that should fail calling the methods."""
+    p = Plugin(autopatch=False)
+
+    def fake_write_result(resultdict):
+        global result_dict
+        result_dict = resultdict
+
+    @p.method("test_raise")
+    def test_raise():
+        raise RpcException("testing RpcException", code=-1000)
+
+    req = Request(p, 1, "test_raise", {})
+    req._write_result = fake_write_result
+    p._dispatch_request(req)
+    assert result_dict['jsonrpc'] == '2.0'
+    assert result_dict['id'] == 1
+    assert result_dict['error']['code'] == -1000
+    assert result_dict['error']['message'] == "testing RpcException"
+
+    @p.method("test_raise2")
+    def test_raise2():
+        raise Exception("normal exception")
+
+    req = Request(p, 1, "test_raise2", {})
+    req._write_result = fake_write_result
+    p._dispatch_request(req)
+    assert result_dict['jsonrpc'] == '2.0'
+    assert result_dict['id'] == 1
+    assert result_dict['error']['code'] == -32600
+    assert result_dict['error']['message'] == "Error while processing test_raise2: normal exception"
+
+
 def test_positional_inject():
     p = Plugin()
     rdict = Request(