chrootarchive: Pass root via fd

[cgwalters]

I'd like to support passing a file descriptor root for the container storage, and not an absolute path.

In the bootc codebase (partially a philosophy inherited from ostree) we've heavily invested in fd-relative accesses, primarily because it's common for us to operate in different namespaces/roots, and fd-relative access avoids a lot of possible footguns when dealing with absolute paths. It's also more efficient, avoiding the need for the kernel to traverse full paths a lot.

This is just one of a few preparatory changes necessary in making it work to do:

podman --root=/proc/self/fd/3 --runroot=... pull busybox

Note that as part of doing this, I refactored code here to move the "split root and dest" logic into the caller... there was some logically dead code here because

[cgwalters]

See https://github.com/containers/bootc/pull/731 particularly https://github.com/containers/bootc/pull/724/files#diff-30bb05ef4e35724331b84e50edacf12aaf7eb21e3a5576a341d556accb0d1400R75 which is doing a complex dance right now to bridge from our internal fds to the absolute paths this package currently requires.

[cgwalters]

Fixed up the cross-compilation issues.

[cgwalters]

Actually...is this macos/windows code ever used?

[rhatdan]

Most likely not.

[cgwalters]

This triggered a memory for me that we also ship oc image extract...I went to look at what it does, and it forked from moby.

I am not sure if oc image extract is currently load bearing on windows/macos? Not much happening there but it looks like it compiles...

[cgwalters]

Tossed up https://github.com/containers/storage/pull/2051

[cgwalters]

OK I reworked this in a way which works on Linux (tested) and FreeBSD (not tested, but in theory), and leaves other platforms unchanged (not tested).

[cgwalters]

A note: If we take this out farther, what would make sense is for c/storage to hold a fd (not an absolute path) for its storage root, then this code would need more refactoring as dest wouldn't necessarily be prepended with the root by default. That'd probably be a distinct new API calling into the shared bits, we just wouldn't need to do the "strip off the root" on the Unix paths in that case.

[rhatdan]

@giuseppe @nalind PTAL

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[rhatdan]

/lgtm