pyln-client: Add support to monkey patch the JSONEncoder

Several times we had issues with plugins not being able to re-encode an RPC
result because they forgot to use the custom encoder class. This allows us to
patch the JSONEncoder when we start the RPC or the plugin and automagically
support classes that provide a `to_json` method.

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

@@ -5,6 +5,21 @@ import logging
 import os
 import socket
 import warnings
+from json import JSONEncoder
+
+
+def _patched_default(self, obj):
+    return getattr(obj.__class__, "to_json", _patched_default.default)(obj)
+
+
+def monkey_patch_json(patch=True):
+    is_patched = JSONEncoder.default == _patched_default
+
+    if patch and not is_patched:
+        _patched_default.default = JSONEncoder.default  # Save unmodified
+        JSONEncoder.default = _patched_default  # Replace it.
+    elif not patch and is_patched:
+        JSONEncoder.default = _patched_default.default
 
 
 class RpcError(ValueError):
@@ -327,7 +342,10 @@ class LightningRpc(UnixDomainSocketRpc):
             return json.JSONEncoder.default(self, o)
 
     class LightningJSONDecoder(json.JSONDecoder):
-        def __init__(self, *, object_hook=None, parse_float=None, parse_int=None, parse_constant=None, strict=True, object_pairs_hook=None):
+        def __init__(self, *, object_hook=None, parse_float=None,
+                     parse_int=None, parse_constant=None,
+                     strict=True, object_pairs_hook=None,
+                     patch_json=True):
             self.object_hook_next = object_hook
             super().__init__(object_hook=self.millisatoshi_hook, parse_float=parse_float, parse_int=parse_int, parse_constant=parse_constant, strict=strict, object_pairs_hook=object_pairs_hook)
 
@@ -357,8 +375,18 @@ class LightningRpc(UnixDomainSocketRpc):
                 obj = self.object_hook_next(obj)
             return obj
 
-    def __init__(self, socket_path, executor=None, logger=logging):
-        super().__init__(socket_path, executor, logger, self.LightningJSONEncoder, self.LightningJSONDecoder())
+    def __init__(self, socket_path, executor=None, logger=logging,
+                 patch_json=True):
+        super().__init__(
+            socket_path,
+            executor,
+            logger,
+            self.LightningJSONEncoder,
+            self.LightningJSONDecoder()
+        )
+
+        if patch_json:
+            monkey_patch_json(patch=True)
 
     def autocleaninvoice(self, cycle_seconds=None, expired_by=None):
         """