Fix workers support when using Redis PubSub layer

[jalaziz]

The new Redis PubSub layer broke support for Channels workers. Add
support for workers by subscribing to non-owned channels instead of
throwing an exception.

fixes #270

[jalaziz]

While looking into this, I realized that the PubSub layer doesn't actually conform to the Channels Layer Spec.

In particular, the spec says:

Channels are a first-in, first out queue with at-most-once delivery semantics. They can have multiple writers and multiple readers; only a single reader should get each written message. Implementations must never deliver a message more than once or to more than one reader, and must drop messages if this is necessary to achieve this restriction.

While the PubSub layer is "at-most-once", it doesn't guarantee that a message will only be delivered to a single reader. This has implications for workers as having multiple workers listening to the same channel will result in duplicate messages.

A potential solution is to introduce a Redis Streams layer which allows such guarantees.

(filed #299 to track this)

[carltongibson]

@jalaziz Thanks for this. Looks good. I rebased after #298.

We need to drop PY36. Time to do that anyway... Fancy making a PR for that?

[acu192]

This looks good to me. (Except the PY36 failure, but I agree with @carltongibson that we should drop PY36 in the next release.)

[jalaziz]

I've put up #301 to drop PY36.

[carltongibson]

@jalaziz Good work. If you want to rebase here I'll take another look tomorrow 🛏️ 🎁

[jalaziz]

@jalaziz Good work. If you want to rebase here I'll take another look tomorrow 🛏️ 🎁

Was already on it :)

[carltongibson]

Super, let's have it. Thanks (again) @jalaziz! 🏅

[acu192]

Thank you @jalaziz! Great work 👍, and super valuable.

[jalaziz]

Thank you @jalaziz! Great work 👍, and super valuable.

Happy to help 😃.

On a separate note, any chance this will be released soon? Hoping the pubsub layer will fix various issues we're having with dropped and misordered messages.

[carltongibson]

I can roll a release this week I guess... — Anything else to clean up whilst we're here?

[jalaziz]

I can roll a release this week I guess... — Anything else to clean up whilst we're here?

There's the RuntimeError when the dictionary gets modified that we used to see, but that's not blocking anything. Not suer if there's anything that's easy to fix.

[carltongibson]

3.4.0 is available on PyPI now https://pypi.org/project/channels-redis/ — Thanks all, and particularly @jalaziz on this one! 🎁