No "slot" exists that allows read-only access to any file

That's an interesting problem to think through. We actually don't want to have an interface that has access to just any file without context, but I can see your use case for having some support for that ability.

FYI, this also came up in the context of virus scanners.

For now, I would settle for a read-only 'home' interface to get it basically working. I don't want to allow writes.

That said, I think there is a strong use case for "read any file", too. What if I'm working with files in another user's $HOME? Or in /opt? Or /media? Or on a shared network drive? (I could continue.)

Restricting the snap to read non-hidden files in $HOME arbitrarily limits its usefulness.

To be clear, I don't think that's a typical interface at all. It sounds like a different feature altogether. We don't want to have an interface like "read-any-file".

OK, I can certainly understand why that isn't generally desired. But there are plenty of use cases for snaps that should be allowed to read more than non-hidden files in $HOME. (What if I want to snap a SSH client, so I need the user's private key? Or an application that works with GPG keys?)

But for more general purpose utilities, why wouldn't you allow a permission like "read any file"? It's a legitimate need for some types of applications (dangerous or not). What if I'm snapping an data processing app and I want it to read arbitrary files on the system?

As for the alternative of a more granular 'home' interface, I have to ask: why just $HOME? Must I symlink from my $HOME to the places I actually want to be? Or do I have to copy things across filesystems to use them with snaps? It doesn't make sense to me. There should be a way for me to grant permission for a snap to access particular arbitrary files. Perhaps if one of the command-line arguments exactly matches a readable file, permission could be granted to the current process. Or I should be able to specify a list of allowed paths.

So a couple of user stories:

(1) As a user, I want to grant a snap permission to access certain files (either explicit, such as an access list, or implicit, such as by passing it to the snap on the command-line).

(2) As a developer, I want to specify a need to access certain files (such as ~/.ssh/*) so that my app can function properly.

The alternatives are ugly:
 - cat /media/usb/my-file | my-snap
 - grant 'network' privileges and ssh/scp/sftp/nfs/smb to the local host to grab arbitrary files (ouch)
 - grant 'system-trace' privileges and hack something together