pyln: Add notifications support to LightningRpc

Changelog-Added: pyln-client: Added support for command notifications to LightningRpc via the `notify` context-manager.

contrib/pyln-client/pyln/client/lightning.py

@@ -1,12 +1,13 @@
-from decimal import Decimal
-from math import floor, log10
-from typing import Optional, Union
 import json
 import logging
 import os
 import socket
 import warnings
+from contextlib import contextmanager
+from decimal import Decimal
 from json import JSONEncoder
+from math import floor, log10
+from typing import Optional, Union
 
 
 def _patched_default(self, obj):
@@ -283,8 +284,9 @@ class UnixDomainSocketRpc(object):
         self.decoder = decoder
         self.executor = executor
         self.logger = logger
+        self._notify = None
 
-        self.next_id = 0
+        self.next_id = 1
 
     def _writeobj(self, sock, obj):
         s = json.dumps(obj, ensure_ascii=False, cls=self.encoder_cls)
@@ -334,18 +336,44 @@ class UnixDomainSocketRpc(object):
         # FIXME: we open a new socket for every readobj call...
         sock = UnixSocket(self.socket_path)
         this_id = self.next_id
+        self.next_id += 0
+        buf = b''
+
+        if self._notify is not None:
+            # Opt into the notifications support
             self._writeobj(sock, {
+                "jsonrpc": "2.0",
+                "method": "notifications",
+                "id": 0,
+                "params": {
+                    "enable": True
+                },
+            })
+            _, buf = self._readobj(sock, buf)
+
+        request = {
             "jsonrpc": "2.0",
             "method": method,
             "params": payload,
             "id": this_id,
-        })
+        }
-        self.next_id += 1
+
-        buf = b''
+        self._writeobj(sock, request)
         while True:
             resp, buf = self._readobj(sock, buf)
-            # FIXME: We should offer a callback for notifications.
+            id = resp.get("id", None)
-            if 'method' not in resp or 'id' in resp:
+            meth = resp.get("method", None)
+
+            if meth == 'message' and self._notify is not None:
+                n = resp['params']
+                self._notify(
+                    message=n.get('message', None),
+                    progress=n.get('progress', None),
+                    request=request
+                )
+                continue
+
+            if meth is None or id is None:
                 break
 
         self.logger.debug("Received response for %s call: %r", method, resp)
@@ -361,6 +389,31 @@ class UnixDomainSocketRpc(object):
             raise ValueError("Malformed response, \"result\" missing.")
         return resp["result"]
 
+    @contextmanager
+    def notify(self, fn):
+        """Register a notification callback to use for a set of RPC calls.
+
+        This is a context manager and should be used like this:
+
+        ```python
+        def fn(message, progress, request, **kwargs):
+            print(message)
+
+        with rpc.notify(fn):
+            rpc.somemethod()
+        ```
+
+        The `fn` function will be called once for each notification
+        the is sent by `somemethod`. This is a context manager,
+        meaning that multiple commands can share the same context, and
+        the same notification function.
+
+        """
+        old = self._notify
+        self._notify = fn
+        yield
+        self._notify = old
+
 
 class LightningRpc(UnixDomainSocketRpc):
     """