Doing this would not be hard. The underlying persistent-cache-cpp thingy
already handles TTL eviction. The time to live can be set on a per-
thumbnail basis. So, you can just ask for a thumbnail and provide an
expiry time, and the thumbnail will automatically be re-fetched once it
expires. Or you can leave the timeout off, and then the thumbnail will
hang around the cache using LRU until it isn't accessed often enough to
drop out of the cache completely (at which point it would be
automatically re-fetched as needed).

The key you would provide for this would be the URL to the image file.

If you want to thumbnail images that you have extracted yourself (where
the images do not have URL that directly points at them), you can do
that today. Doing so requires you to write them into the file system and
to then ask for a thumbnail with a URL that points at the local file.
(That's how photos are being thumbnail, for example.) The cost of this
approach is increased disk space because the thumbnailer absolutely will
not provide a thumbnail for a local image unless you can prove that you
are entitled to actually read that image. In practice, this means that,
as soon as the original image from which a thumbnail was generated is
deleted, the thumbnailer will no longer hand out a thumbnail for that
image.

All this would work for any file type that is currently supported.
(Basically all image file formats in the known universe, plus most audio
and video formats, provided the necessary gstreamer codecs are
installed.)

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

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

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