CDN debugging tools

[snarfed]

I get a steady trickle of Bridgy Fed bug reports for missing images/videos: snarfed/bridgy-fed#1715, snarfed/bridgy-fed#1709 (comment), snarfed/bridgy-fed#1515 (comment), snarfed/bridgy-fed#1349. The most common reason is that the server hosting the images is blocking or otherwise not serving Bluesky's CDN. Looks like others here have hit the same thing, eg #3042.

It'd be nice to have a way to debug this better! Eg an external-facing tool for their CDN where you can plug in a host and it'll say whether it's ever successfully fetched an image from that host, what the last response was, etc.

Other blob/CDN issues could benefit too, eg images that are over the processing pipeline's max resolution, #1777 .

[DavidBuchanan314]

The CDN does already serve basic error messages in place of an image, e.g. "Source image is unreachable" - perhaps it could be made more verbose.

[snarfed]

Oh, great point, thank you! That's definitely useful.

(Unfortunate that it's an HTTP 500, like a lot of Bluesky team errors that should really be 400s, or in this case 502, but that's a minor complaint.)