postMessage() to a reserved client throws

[jungkees]

While discussing #1031 (comment), I noticed that client.postMessage() to a reserved client in the current spec throws as the destination (ServiceWorkerContainer) object shouldn't have been created yet. I think we may need a separate message queue to hold the messages while a client is in the reserved state, and re-queue them to the client message queue of the ServiceWorkerContainer object when it's created.

[jakearchibald]

F2F: Update the spec to allow buffering. Including across redirects, unless the redirect is across origins.

[domenic]

Language about queues and moving queued up things between them is already present in HTML for MessagePorts. See e.g. the transferring steps at https://html.spec.whatwg.org/#create-a-new-messageport-object

You may want to copy that, or even use MessagePorts as some underlying mechanism like HTML does for workers (see https://html.spec.whatwg.org/#dom-worker-postmessage).

[jakearchibald]

We aren't going to expose reserved clients, so this is no longer an issue.