MSC2753: Peeking via sync (take 2)

[ara4n]

Rendered

A first cut at an alternative solution to peeking via /sync, using a /peek method rather than messing around with filters.

Hopefully obsoletes matrix-org/matrix-spec-proposals#1776. Credit to @Half-Shot for suggesting this approach at https://github.com/matrix-org/matrix-doc/pull/1776#pullrequestreview-189824984.

[bwindels]

Overall, this looks like a nice API to work with as a client developer. Have left some comments for the specific use cases mentioned though, which each seem to have their own intuitive scope and lifetime for a peek.

[erikjohnston]

This seems relatively sane, modulo concerns with clients accidentally not cleaning up after themselves. (Plus to what extent this interacts with the request format for paginated sync).

I wholeheartedly approve of putting these rooms in a dedicated peek section in /sync response

[carlbordum]

Hi, we are building something slightly-novel on matrix. Just wanted to let you know that this MSC would be an improvement for us :+1: