Cable message encoding

* Introduce a connection coder responsible for encoding Cable messages
  as WebSocket messages, defaulting to `ActiveSupport::JSON` and duck-
  typing to any object responding to `#encode` and `#decode`.
* Consolidate encoding responsibility to the connection. No longer
  explicitly JSON-encode from channels or other sources. Pass Cable
  messages as Hashes to `#transmit` and rely on it to encode.
* Introduce stream encoders responsible for decoding pubsub messages.
  Preserve the currently raw encoding, but make it easy to use JSON.
  Same duck type as the connection encoder.
* Revert recent data normalization/quoting (#23649) which treated
  `identifier` and `data` values as nested JSON objects rather than as
  opaque JSON-encoded strings. That dealt us an awkward hand where we'd
  decode JSON strings… or not, but always encode as JSON. Embedding
  JSON object values directly is preferably, no extra JSON encoding,
  but that should be a purposeful protocol version change rather than
  ambiguously, inadvertently supporting multiple message formats.

2 files changed, 24 insertions, 14 deletions

diff --git a/actioncable/lib/action_cable/channel/base.rb b/actioncable/lib/action_cable/channel/base.rb
index 464d0581dd..845b747fc5 100644
--- a/actioncable/lib/action_cable/channel/base.rb
+++ b/actioncable/lib/action_cable/channel/base.rb
@@ -198,7 +198,7 @@ module ActionCable
 
           payload = { channel_class: self.class.name, data: data, via: via }
           ActiveSupport::Notifications.instrument("transmit.action_cable", payload) do
-            connection.transmit ActiveSupport::JSON.encode(identifier: @identifier, message: data)
+            connection.transmit identifier: @identifier, message: data
           end
         end
 
@@ -274,7 +274,7 @@ module ActionCable
             logger.info "#{self.class.name} is transmitting the subscription confirmation"
 
             ActiveSupport::Notifications.instrument("transmit_subscription_confirmation.action_cable", channel_class: self.class.name) do
-              connection.transmit ActiveSupport::JSON.encode(identifier: @identifier, type: ActionCable::INTERNAL[:message_types][:confirmation])
+              connection.transmit identifier: @identifier, type: ActionCable::INTERNAL[:message_types][:confirmation]
               @subscription_confirmation_sent = true
             end
           end
@@ -289,7 +289,7 @@ module ActionCable
           logger.info "#{self.class.name} is transmitting the subscription rejection"
 
           ActiveSupport::Notifications.instrument("transmit_subscription_rejection.action_cable", channel_class: self.class.name) do
-            connection.transmit ActiveSupport::JSON.encode(identifier: @identifier, type: ActionCable::INTERNAL[:message_types][:rejection])
+            connection.transmit identifier: @identifier, type: ActionCable::INTERNAL[:message_types][:rejection]
           end
         end
     end
diff --git a/actioncable/lib/action_cable/channel/streams.rb b/actioncable/lib/action_cable/channel/streams.rb
index 23d7320a28..f654ce0bfa 100644
--- a/actioncable/lib/action_cable/channel/streams.rb
+++ b/actioncable/lib/action_cable/channel/streams.rb
@@ -46,9 +46,7 @@ module ActionCable
     #     def subscribed
     #       @room = Chat::Room[params[:room_number]]
     #
-    #       stream_for @room, -> (encoded_message) do
-    #         message = ActiveSupport::JSON.decode(encoded_message)
-    #
+    #       stream_for @room, coder: ActiveSupport::JSON do |message|
     #         if message['originated_at'].present?
     #           elapsed_time = (Time.now.to_f - message['originated_at']).round(2)
     #
@@ -71,16 +69,23 @@ module ActionCable
 
       # Start streaming from the named <tt>broadcasting</tt> pubsub queue. Optionally, you can pass a <tt>callback</tt> that'll be used
       # instead of the default of just transmitting the updates straight to the subscriber.
-      def stream_from(broadcasting, callback = nil)
+      # Pass `coder: ActiveSupport::JSON` to decode messages as JSON before passing to the callback.
+      # Defaults to `coder: nil` which does no decoding, passes raw messages.
+      def stream_from(broadcasting, callback = nil, coder: nil, &block)
         broadcasting = String(broadcasting)
         # Don't send the confirmation until pubsub#subscribe is successful
         defer_subscription_confirmation!
 
-        callback ||= default_stream_callback(broadcasting)
-        streams << [ broadcasting, callback ]
+        if handler = callback || block
+          handler = -> message { handler.(coder.decode(message)) } if coder
+        else
+          handler = default_stream_handler(broadcasting, coder: coder)
+        end
+
+        streams << [ broadcasting, handler ]
 
         connection.server.event_loop.post do
-          pubsub.subscribe(broadcasting, callback, lambda do
+          pubsub.subscribe(broadcasting, handler, lambda do
             transmit_subscription_confirmation
             logger.info "#{self.class.name} is streaming from #{broadcasting}"
           end)
@@ -90,8 +95,11 @@ module ActionCable
       # Start streaming the pubsub queue for the <tt>model</tt> in this channel. Optionally, you can pass a
       # <tt>callback</tt> that'll be used instead of the default of just transmitting the updates straight
       # to the subscriber.
-      def stream_for(model, callback = nil)
-        stream_from(broadcasting_for([ channel_name, model ]), callback)
+      #
+      # Pass `coder: ActiveSupport::JSON` to decode messages as JSON before passing to the callback.
+      # Defaults to `coder: nil` which does no decoding, passes raw messages.
+      def stream_for(model, callback = nil, coder: nil, &block)
+        stream_from(broadcasting_for([ channel_name, model ]), callback || block, coder: coder)
       end
 
       # Unsubscribes all streams associated with this channel from the pubsub queue.
@@ -109,9 +117,11 @@ module ActionCable
           @_streams ||= []
         end
 
-        def default_stream_callback(broadcasting)
+        def default_stream_handler(broadcasting, coder:)
+          coder ||= ActiveSupport::JSON
+
           -> (message) do
-            transmit ActiveSupport::JSON.decode(message), via: "streamed from #{broadcasting}"
+            transmit coder.decode(message), via: "streamed from #{broadcasting}"
           end
         end
     end