After discussing this with James, here are some thoughts.

Adding direct download of arbitrary remote images opens an attack/bug
vector. For example, a caller could ask for http://<whatever> and it
turns out that the remote server (maliciously or otherwise), is very
slow, or doesn't respond at all until the request times out. This means
that one application can disable access to remote images for other
applications because the thumbnailer allows only a limited number of
outstanding HTTP requests at a time. Or the server could potentially
return garbage images each and every time, each of which would result in
an entry in our failure cache. That can have negative performance impact
on other apps.

But, I don't think all is lost. The pragmatic answer to the problem
would be for the application to download the image (or audio file, or
whatever) and simply drop it into the file system somewhere. Then pass a
URL to that file to the thumbnailer, and it will do the rest
(extraction, scaling, rotating, caching, etc.)

This isn't quite as convenient as having the thumbnailer do all of the
work, but I think it might be workable?

I'm open to suggestions though. I agree that having this feature would
be really nice. But we'd have to sort out the potential denial-of-
service/reliability issues. If the actions of one application can
disable access to thumbnails for other applications (or significantly
reduce throughput), we have a problem.

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https://bugs.launchpad.net/bugs/1532732

Title:
  Caching for arbitrary images on the web/remote hosts

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