Update database channel backend to pass conformance

channels/database_layer.py

@@ -3,6 +3,7 @@ import json
 import random
 import string
 import time
+import base64
 
 from django.apps.registry import Apps
 from django.db import DEFAULT_DB_ALIAS, IntegrityError, connections, models
@@ -21,7 +22,8 @@ class DatabaseChannelLayer(object):
 
     Also uses JSON for serialization, as we don't want to make Django depend
     on msgpack for the built-in backend. The JSON format uses \uffff as first
-    character to signify a byte string rather than a text string.
+    character to signify a b64 byte string rather than a text string. Ugly, but
+    it's not a valid Unicode character, so it should be safe enough.
     """
 
     def __init__(self, db_alias=DEFAULT_DB_ALIAS, expiry=60):
@@ -75,7 +77,7 @@ class DatabaseChannelLayer(object):
 
     ### ASGI Group extension ###
 
-    def group_add(self, group, channel, expiry=None):
+    def group_add(self, group, channel):
         """
         Adds the channel to the named group for at least 'expiry'
         seconds (expiry defaults to message expiry if not provided).
@@ -83,7 +85,6 @@ class DatabaseChannelLayer(object):
         self.group_model.objects.update_or_create(
             group=group,
             channel=channel,
-            defaults={"expiry": now() + datetime.timedelta(seconds=expiry or self.expiry)},
         )
 
     def group_discard(self, group, channel):
@@ -110,10 +111,10 @@ class DatabaseChannelLayer(object):
     ### Serialization ###
 
     def serialize(self, message):
-        return json.dumps(message)
+        return AsgiJsonEncoder().encode(message)
 
     def deserialize(self, message):
-        return json.loads(message)
+        return AsgiJsonDecoder().decode(message)
 
     ### Database state mgmt ###
 
@@ -157,7 +158,6 @@ class DatabaseChannelLayer(object):
         class Group(models.Model):
             group = models.CharField(max_length=200)
             channel = models.CharField(max_length=200)
-            expiry = models.DateTimeField(db_index=True)
 
             class Meta:
                 apps = Apps()
@@ -174,9 +174,60 @@ class DatabaseChannelLayer(object):
         """
         Cleans out expired groups and messages.
         """
-        # Include a 10-second grace period because that solves some clock sync
-        self.channel_model.objects.filter(expiry__lt=now() - datetime.timedelta(seconds=10)).delete()
-        self.group_model.objects.filter(expiry__lt=now() - datetime.timedelta(seconds=10)).delete()
+        # Include a 1-second grace period for clock sync drift
+        target = now() - datetime.timedelta(seconds=1)
+        # First, go through old messages and pick out channels that got expired
+        old_messages = self.channel_model.objects.filter(expiry__lt=target)
+        channels_to_ungroup = old_messages.values_list("channel", flat=True).distinct()
+        old_messages.delete()
+        # Now, remove channel membership from channels that expired
+        self.group_model.objects.filter(channel__in=channels_to_ungroup).delete()
 
     def __str__(self):
         return "%s(alias=%s)" % (self.__class__.__name__, self.connection.alias)
+
+
+class AsgiJsonEncoder(json.JSONEncoder):
+    """
+    Special encoder that transforms bytestrings into unicode strings
+    prefixed with u+ffff
+    """
+
+    def transform(self, o):
+        if isinstance(o, (list, tuple)):
+            return [self.transform(x) for x in o]
+        elif isinstance(o, dict):
+            return {
+                self.transform(k): self.transform(v)
+                for k, v in o.items()
+            }
+        elif isinstance(o, six.binary_type):
+            return u"\uffff" + base64.b64encode(o).decode("ascii")
+        else:
+            return o
+
+    def encode(self, o):
+        return super(AsgiJsonEncoder, self).encode(self.transform(o))
+
+
+class AsgiJsonDecoder(json.JSONDecoder):
+    """
+    Special encoder that transforms bytestrings into unicode strings
+    prefixed with u+ffff
+    """
+
+    def transform(self, o):
+        if isinstance(o, (list, tuple)):
+            return [self.transform(x) for x in o]
+        elif isinstance(o, dict):
+            return {
+                self.transform(k): self.transform(v)
+                for k, v in o.items()
+            }
+        elif isinstance(o, six.text_type) and o and o[0] == u"\uffff":
+            return base64.b64decode(o[1:].encode("ascii"))
+        else:
+            return o
+
+    def decode(self, o):
+        return self.transform(super(AsgiJsonDecoder, self).decode(o))

channels/tests/test_database_layer.py

@@ -3,4 +3,4 @@ from asgiref.conformance import make_tests
 from ..database_layer import DatabaseChannelLayer
 
 channel_layer = DatabaseChannelLayer(expiry=1)
-DatabaseLayerTests = make_tests(channel_layer, expiry_delay=1.1)
+DatabaseLayerTests = make_tests(channel_layer, expiry_delay=2.1)

docs/asgi.rst

@@ -367,7 +367,8 @@ clients reconnect will immediately resolve the problem.
 
 If a channel layer implements the ``groups`` extension, it must persist group
 membership until at least the time when the member channel has a message
-expire due to non-consumption.
+expire due to non-consumption. It should drop membership after a while to
+prevent collision of old messages with new clients with the same random ID.
 
 
 Message Formats