archive/zip: Writer should only use extended timestamps for non-zero times

[dsnet]

4c79ed5 caused a regression where a round-trip zip file does not preserve the same ModTime. The reason is because the logic for using extra timestamp is always applied in the Writer no matter what.

This is an issue for when the user doesn't set the ModifiedDate and ModifiedTime fields at all. Causing the Writer to convert a zero MS-DOS timestamp (gibberish) to a Unix timestamp (now gibberish) and back again. The fix is to only apply Extra timestamp only if the MS-DOS timestamp is non-zero.

See:

go/src/archive/zip/writer.go

Lines 102 to 109 in 4c79ed5

	mt := uint32(h.FileHeader.ModTime().Unix())
	var mbuf [9]byte // 2x uint16 + uint8 + uint32
	eb := writeBuf(mbuf[:])
	eb.uint16(exttsExtraId)
	eb.uint16(5) // size = uint8 + uint32
	eb.uint8(1) // flags = modtime
	eb.uint32(mt) // ModTime
	h.Extra = append(h.Extra, mbuf[:]...)

/cc @mattn

[mattn]

What do you mean same? Timestamp in the zip file was always preserved without timezone. It was really problem.

[dsnet]

This fails on tip, but passes on Go 1.7

https://play.golang.org/p/-ckATAnaEn

[mattn]

Thanks for looking this. Are you trying to fix to avoid preserve ModifiedTime=0 ModifiedDate=0 if the file already having timestamp?

[gopherbot]

CL https://golang.org/cl/30811 mentions this issue.

[gopherbot]

CL https://golang.org/cl/34651 mentions this issue.

[gopherbot]

Change https://golang.org/cl/44394 mentions this issue: archive/zip: Store the main timestamp as local time.

[gopherbot]

Change https://golang.org/cl/36078 mentions this issue: archive/zip: handle mtime in NTFS/UNIX/ExtendedTS extra fields