events with broken signatures prevent backfill

[richvdh]

Example: there is an event $15155055363102ERMDq:half-shot.uk in #offtopic which has an invalid signature.

My HS does not have a copy of this event, and any attempt to scroll back in #offtopic leads my server to desperately try backfilling from every other server in the room, each of which takes ages and ultimately fails in the same way.

Eventually my HS responds to riot with an empty /messages response, which riot takes to mean we have reached the beginning of the room.

(The event in question was signed with half-shot.uk:ed25519:a_YjQO. Unfortunately, two different keys have had that ID in the past, and the key currently available is apparently different from the key used to sign the event.)

[richvdh]

The solution to this is presumably to mark the event as "rejected" and carry on, but the structure of synapse doesn't make that easy.

[richvdh]

This is fixed by #8350: we now skip over any backward extremity that we fail to backfill from, so that back-pagination can continue. (Additionally, #8349 changed things so that when there is one unverifiable event in a bunch of backfill, we still accept the rest of the events in the batch, so we still get new content for the room to allow us to continue back-paginating)