Add JSON encoder to IPC for set types

Closes qtile#3360. Fixes bug in which groups couldn't be listed over IPC when using JSON
encoding. Adds tests to demonstrate how the serialization works and what
kind of errors to expect when encountering non-serializable types.

libqtile/ipc.py

@@ -129,13 +129,20 @@ def unpack(data: bytes, *, is_json: bool | None = None) -> tuple[Any, bool]:
     def pack(msg: Any, *, is_json: bool = False) -> bytes:
         """Pack the object into a message to pass"""
         if is_json:
-            json_obj = json.dumps(msg)
+            json_obj = json.dumps(msg, default=_IPC._json_encoder)
             return json_obj.encode()
 
         msg_bytes = marshal.dumps(msg)
         size = struct.pack(HDRFORMAT, len(msg_bytes))
         return size + msg_bytes
 
+    @staticmethod
+    def _json_encoder(field: Any) -> Any:
+        """Convert non-serializable types to ones understood by stdlib json module"""
+        if isinstance(field, set):
+            return list(field)
+        raise ValueError(f"Tried to JSON serialize unsupported type {type(field)}: {field}")
+
 
 class Client:
     def __init__(self, socket_path: str, is_json=False) -> None:

test/test_ipc.py

@@ -0,0 +1,51 @@
+# Copyright (c) 2022 Jaakko Sirén
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+import pytest
+
+from libqtile.ipc import _IPC
+
+
+def test_ipc_json_encoder_supports_sets():
+    serialized = _IPC.pack({"foo": set()}, is_json=True)
+    assert serialized == b'{"foo": []}'
+
+
+def test_ipc_json_throws_error_on_unsupported_field():
+    class NonSerializableType:
+        ...
+
+    with pytest.raises(
+        ValueError,
+        match=(
+            "Tried to JSON serialize unsupported type <class '"
+            "test.test_ipc.test_ipc_json_throws_error_on_unsupported_field.<locals>.NonSerializableType"
+            "'>.*"
+        ),
+    ):
+        _IPC.pack({"foo": NonSerializableType()}, is_json=True)
+
+
+def test_ipc_marshall_error_on_unsupported_field():
+    class NonSerializableType:
+        ...
+
+    with pytest.raises(ValueError, match="unmarshallable object"):
+        _IPC.pack({"foo": NonSerializableType()})