Context links give a 404 on federated rooms

[ara4n]

I clicked on https://vector.im/develop/#/room/#purple:matrix.org/$1451504230525382RHTLB:matrix.org in Safari, which was logged into a non-guest account not joined to #purple:matrix.org already. It showed me a 'join this room' prompt; i clicked join; it got stuck on an infinite spinner in the RoomPreviewBar. Switching away from the room and back again fixed it, but then I had to re-load the URL to actually get the context link up.

[ara4n]

one other datapoint: the account in question was on arasphere.net rather than matrix.org.

[richvdh]

This is a combination of #979, together with the fact that synapse does not make requests over federation when it receives a /context request.

@erikjohnston: any feeling for how easy it will be to fix the latter?

[richvdh]

see also #1532

[ara4n]

i've finally filed a synapse issue for this one.

[ara4n]

Meanwhile #2925 also gets in the way of permalinks working.

[turt2live]

matrix-org/matrix-react-sdk#2250 fixes this, although only to the point of being able to join. Due to the synapse bug users will be hit with "Failed to load the timeline position" until their server backpaginates enough of the history.

[turt2live]

Closing this as matrix-org/matrix-react-sdk#2250 addresses the Riot part of the problem. To actually fix the remainder of the issue, matrix-org/synapse#3848 should be fixed. Closing as the remainder is a synapse problem - please reopen if there's disagreement.