makefs: don't needlessly require directories to exist

[brooks]

Summary

If a type=dir entry exists and all contents are directories, files
added with contents=, or symlinks with link= attributes then it doesn't
need to exist. Just let openat fail in that case. It's concivable
this will make debugging some cases weird, but it's sufficent to handle
the way we add /root/.ssh in CheriBSD VM images.

[markj]

To trigger the problem, a test case might do something like

./root/.ssh type=dir
./root/.ssh/id_ed25519 type=file contents=/path/to/key

where ./root/.ssh doesn't exist?

usr.sbin/makefs/zfs/fs.c

304

Just return (openat(fd, path, flags));

[brooks]

• Simplify fs_open_can_fail

[brooks]

In D38029#863849, @markj wrote:

To trigger the problem, a test case might do something like

./root/.ssh type=dir
./root/.ssh/id_ed25519 type=file contents=/path/to/key

where ./root/.ssh doesn't exist?

Yeah, that's the basic issue I ran into (in this case it was actually the authorized_keys file).