proposal: os: add methods to safely convert between files and roots

[Jorropo]

Proposal Details

I have code which opens both files and directories as files to perform various actions such as fetching and comparing generations (I need to upstream to linux once I write tests) or other metadata with SyscallConn.

Trying to use #67002 to make it less conservative with some TOCTOU edge cases prove challenging since *os.Root does not provide needed SyscallConn to fetch the generation of directories and more architecturally the current codepath is to open everything as a *os.File and then use statx to decide what to do next.

---

package os

// AsRoot open the File as a Root if it is a directory otherwise it errors.
// The Root is returned with a new lifetime such that each need to be closed independently.
func (*File) AsRoot() (*Root, error)

// AsFile opens the Root as a File.
// The File is returned with a new lifetime such that each need to be closed independently.
func (*Root) AsFile() (*File, error)

[Jorropo]

I discussed this with @neild already and:
I think it might be possible to convert *os.File → *os.Root by using SyscallConn then calling os.OpenRoot with fmt.Sprintf("/proc/self/fd/%d") on Linux.
Not portable tho.

He pointed out *os.Root → *os.File should be possible by calling (*os.Root).Open(root, ".").

[Jorropo]

I am not sure if lifetimes should be independent or not.
I guess from a point of implementing in the std this is the easiest, but as a consumer I would sometime want the opposite.