SymlinkOrCopyStorage: new custom storage system
This patch has been created as part of my bachelor thesis  – as part of the daemon I wrote. I'm using django-storages to send files to FTP, Amazon S3 and Amazon CloudFront.
I needed to efficiently create a mirror of a directory tree (so that "origin pull" CDNs can automatically pull files). The trick was that some files could be modified, and some could be identical to the original. Of course it doesn't make sense to store the exact same data twice on the file system. So I created SymlinkOrCopyStorage.
SymlinkOrCopyStorage allows you to symlink a file when it's identical to the original file and to copy the file if it's modified. Of course, it's impossible to know if a file is modified just by looking at the file, without knowing what the original file was. That's what the symlinkWithin parameter is for. It accepts one or more paths (if multiple, they should be concatenated using a colon (:)). Files that will be saved using SymlinkOrCopyStorage are then checked on their location: if they are within one of the symlinkWithin directories, they will be symlinked, otherwise they will be copied.
The rationale is that unmodified files will exist in their original location, e.g. /htdocs/example.com/image.jpg and modified files will be stored in a temporary directory, e.g. /tmp/image.jpg.
I'm looking forward to your comments, questions and criticism :)