✨(back) override channel_redis channel layer

The channel layer provided by channel_redis use msgpack to serialize and
deserialize messages. We have a problem when a video is serialized. It
is impossible then to deserialize it because in the urls the resolution
can be used as key and those keys are integer. Msgpack is unable to
deserialize this kind of key. As suggested in the issue[1] open on
channel_redis git repository we implement our own serialize and
deserialize methods.
1: django/channels_redis#287

src/backend/marsha/settings.py

@@ -171,7 +171,7 @@ class Base(Configuration):
     # }
     CHANNEL_LAYERS = {
         "default": {
-            "BACKEND": "channels_redis.core.RedisChannelLayer",
+            "BACKEND": "marsha.websocket.layers.JsonRedisChannelLayer",
             "CONFIG": {
                 "hosts": values.ListValue(
                     [("redis", 6379)], environ_name="REDIS_HOST", environ_prefix=None

src/backend/marsha/websocket/layers.py

@@ -0,0 +1,36 @@
+"""Layers used by django channels"""
+import json
+import random
+
+from django.core.serializers.json import DjangoJSONEncoder
+
+from channels_redis.core import RedisChannelLayer
+
+
+class JsonRedisChannelLayer(RedisChannelLayer):
+    """Use json to serialize and deserialize messages."""
+
+    def serialize(self, message):
+        """
+        Serializes message in json.
+        """
+        value = bytes(json.dumps(message, cls=DjangoJSONEncoder), encoding="utf-8")
+        if self.crypter:
+            value = self.crypter.encrypt(value)
+
+        # As we use an sorted set to expire messages we need to guarantee uniqueness,
+        # with 12 bytes.
+        random_prefix = random.getrandbits(8 * 12).to_bytes(12, "big")
+        return random_prefix + value
+
+    def deserialize(self, message):
+        """
+        Deserializes from a byte string.
+        """
+        # Removes the random prefix
+        message = message[12:]
+        message = message.decode("utf-8")
+
+        if self.crypter:
+            message = self.crypter.decrypt(message, self.expiry + 10)
+        return json.loads(message)

src/backend/marsha/websocket/tests/test_layers.py

@@ -0,0 +1,31 @@
+"""Test marsha layers use by django channels."""
+from django.test import TestCase
+
+from marsha.websocket.layers import JsonRedisChannelLayer
+
+
+class JsonRedisChannelLayerTest(TestCase):
+    """Test serialize and deserialize."""
+
+    def test_serialize_message(self):
+        """
+        Test default serialization method
+        """
+        message = {"a": True, "b": None, "c": {"d": []}}
+        channel_layer = JsonRedisChannelLayer()
+        serialized = channel_layer.serialize(message)
+        self.assertIsInstance(serialized, bytes)
+        self.assertEqual(serialized[12:], b'{"a": true, "b": null, "c": {"d": []}}')
+
+    def test_deserialize_message(self):
+        """
+        Test default deserialization method
+        """
+        message = (
+            b'[\x85\xf8\xdeY\xe5\xa3}is\x0f3{"a": true, "b": null, "c": {"d": []}}'
+        )
+        channel_layer = JsonRedisChannelLayer()
+        deserialized = channel_layer.deserialize(message)
+
+        self.assertIsInstance(deserialized, dict)
+        self.assertEqual(deserialized, {"a": True, "b": None, "c": {"d": []}})