god fucking dammit this part of the spec sucks ASS
aand very nicely fixed a race condition with simultaneous url previews, it only gets requested once and all requests get the same response when it's finished :3
I love how promises make this so elegant
https://git.pixie.town/f0x/synapse-media-proxy/commit/9b55f320fa13da434841222a873b7a2c0390f432
all fixed though in a series of commits across synapse-media-proxy and @f0x52/autodiscover-server-configuration