Proposal Details

The goal of this proposal is to simplify implementing fs.FS() on top of archive/tar

There was 2 "soft" rejected requests to implement fs.FS():

• proposal: archive/tar: implement fs.FS #61232
• proposal: archive/tar: add Reader FS() #74041

I could find 4 implementations that could benefit from that:

• https://pkg.go.dev/github.com/nlepage/go-tarfs: it finds the header offset by looking at how much was already read - 512 (https://github.com/nlepage/go-tarfs/blob/5be978f25f2e456c2e72636c61eb3af3f155989f/fs.go#L70), so it assumes no internal buffering
• https://pkg.go.dev/github.com/quay/claircore/pkg/tarfs: it re-implements it own parsing (https://github.com/quay/claircore/blob/v1.5.37/pkg/tarfs/parse.go)
• https://github.com/containers/image/blob/9822b6ffa5c1dc56088cbb82a2276d4f3f872a68/docker/internal/tarfile/reader.go#L242C6-L242C22 which just rescan the whole archive each time it want to find a file
• https://github.com/mholt/archives

The idea would be to add

func (tr *Reader) NextOffset() (*Header, int, error)

that also returns the offset in the io.Reader the Header was found at, so that if you Seek() to this offset and call Next() you end up with the same Header / data