Fix leaked fiber in notification_stream_handler

Of the technically four fibers spawned by `create_notification_stream`
two of them are wrapped around an ensure clause to always unsubscribe
itself from the notification job after an error, or when it simply
finishes.

The first is the heartbeat fiber, which is also actually the main
fiber of the route handler. The second is a fiber that awaits for
notification pushes from the notification job through the
`connection` channel. When an error occurs within the main heartbeat
fiber, the ensure clause is executed and the function will unsubscribe
itself from receiving any pushes from the notification job.

The problem however is that this could (will almost always actually)
occur when the notification receiver fiber is awaiting a value from
the notification job. Except the job will no longer be able to
send anything to the receiver since they were unsubscribed by the
heartbeat fiber just a moment ago.

The notification receiver fiber will now block indefinitely.

And in doing so will pretty much prevent the entire execution stack of
the fiber and the `create_notification_stream` function from getting
garbage collected.

The IO buffers for the contents of the request and
response will stay referenced, the underlying TCP/TLS sockets will
become inaccessible and leaked, the parsed structures of the YT's
massive JSON objects will stay allocated, etc.

This PR simply merges the two into a single fiber, via a select
statement ensuring that there will be no concurrency problems.

src/invidious/helpers/helpers.cr

@@ -119,11 +119,11 @@ def create_notification_stream(env, topics, connection_channel)
     end
   end
 
-  spawn do
-    begin
-      loop do
-        event = connection.receive
-
+  begin
+    heartbeat_timer = 0.seconds
+    loop do
+      select
+      when event = connection.receive
         notification = JSON.parse(event.payload)
         topic = notification["topic"].as_s
         video_id = notification["videoId"].as_s
@@ -139,24 +139,16 @@ def create_notification_stream(env, topics, connection_channel)
 
         env.response.puts "id: #{id}"
         env.response.puts "data: #{response.to_json}"
-        env.response.puts
-        env.response.flush
 
         id += 1
+      when timeout heartbeat_timer
+        # Send heartbeat on every timeout
+        env.response.puts ":keepalive #{Time.utc.to_unix}"
       end
-    rescue ex
-    ensure
-      connection_channel.send({false, connection})
-    end
-  end
 
-  begin
-    # Send heartbeat
-    loop do
-      env.response.puts ":keepalive #{Time.utc.to_unix}"
+      heartbeat_timer = ((20 + rand(11)).seconds)
       env.response.puts
       env.response.flush
-      sleep (20 + rand(11)).seconds
     end
   rescue ex
   ensure