Fixing the sparsebundle error "Operation not supported on socket".

If you're trying to mount a .sparsebundle (typically a directory that contains a FileVault $HOME) and get an unexpected Operation not supported on socket error (aka EOPNOTSUPP / ENOTSUP for those who know what errno is) then the fix is simple: fix the permissions of the file within the .sparsebundle file. At least it worked for me. Some of the files were owned by root. So a simple

sudo chown -R $USER my.sparsebundle

did the trick.

Oddly enough I couldn't reproduce this problem by manually trying to chmod some files in the .sparsebundle. But I'm 100% sure that in my case fixing the permissions fixed the problem.