embed: guarantee `(*FS).Open`'s returning value implements `io.Seeker`

[hajimehoshi]

From the current document, there is no guarantee whether (*embed.FS).Open's returning value implements io.Seeker or not.

Open opens the named file for reading and returns it as an fs.File.

IIUC the returned value always implements io.Seeker in the current implementation, and actually it is fine to assume this (e.g., #44175 (comment)). Would it be possible to make the current behavior explicit in the document?

[hajimehoshi]

CC @ianlancetaylor

[josharian]

Embedded directories do not implement Seek. (This bit me yesterday.)

[hajimehoshi]

Ah, I see. So I want a guarantee that (*embed.FS).Open returns a fs.File that implements io.Seeker when the returned value is a file, not a directory.

[josharian]

Yep. Or we could make directories seekers too, I guess.

[ianlancetaylor]

I think it would be fine to document that the result of embed.FS.Open, when opening a file that is not a directory, will implement io.Seeker.

[ianlancetaylor]

This doesn't have to be a proposal, so taking it out of the proposal process.