archive/tar: move round-trip reading into common os file

Fixes #11426

Change-Id: I77368b0e852149ed4533e139cc43887508ac7f78
Reviewed-on: https://go-review.googlesource.com/11662
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

archive/tar/common.go

@@ -249,6 +249,30 @@ func FileInfoHeader(fi os.FileInfo, link string) (*Header, error) {
 	if fm&os.ModeSticky != 0 {
 		h.Mode |= c_ISVTX
 	}
+	// If possible, populate additional fields from OS-specific
+	// FileInfo fields.
+	if sys, ok := fi.Sys().(*Header); ok {
+		// This FileInfo came from a Header (not the OS). Use the
+		// original Header to populate all remaining fields.
+		h.Uid = sys.Uid
+		h.Gid = sys.Gid
+		h.Uname = sys.Uname
+		h.Gname = sys.Gname
+		h.AccessTime = sys.AccessTime
+		h.ChangeTime = sys.ChangeTime
+		if sys.Xattrs != nil {
+			h.Xattrs = make(map[string]string)
+			for k, v := range sys.Xattrs {
+				h.Xattrs[k] = v
+			}
+		}
+		if sys.Typeflag == TypeLink {
+			// hard link
+			h.Typeflag = TypeLink
+			h.Size = 0
+			h.Linkname = sys.Linkname
+		}
+	}
 	if sysStat != nil {
 		return h, sysStat(fi, h)
 	}