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>

src/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)
 	}

src/archive/tar/stat_unix.go

@@ -16,41 +16,22 @@ func init() {
 }
 
 func statUnix(fi os.FileInfo, h *Header) error {
-	switch sys := fi.Sys().(type) {
-	case *syscall.Stat_t:
-		h.Uid = int(sys.Uid)
-		h.Gid = int(sys.Gid)
-		// TODO(bradfitz): populate username & group.  os/user
-		// doesn't cache LookupId lookups, and lacks group
-		// lookup functions.
-		h.AccessTime = statAtime(sys)
-		h.ChangeTime = statCtime(sys)
-		// TODO(bradfitz): major/minor device numbers?
-		if fi.Mode().IsRegular() && sys.Nlink > 1 {
-			h.Typeflag = TypeLink
-			h.Size = 0
-			// TODO(vbatts): Linkname?
-		}
-	case *Header:
-		// for the roundtrip logic
-		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
-		}
+	sys, ok := fi.Sys().(*syscall.Stat_t)
+	if !ok {
+		return nil
+	}
+	h.Uid = int(sys.Uid)
+	h.Gid = int(sys.Gid)
+	// TODO(bradfitz): populate username & group.  os/user
+	// doesn't cache LookupId lookups, and lacks group
+	// lookup functions.
+	h.AccessTime = statAtime(sys)
+	h.ChangeTime = statCtime(sys)
+	// TODO(bradfitz): major/minor device numbers?
+	if fi.Mode().IsRegular() && sys.Nlink > 1 {
+		h.Typeflag = TypeLink
+		h.Size = 0
+		// TODO(vbatts): Linkname?
 	}
 	return nil
 }

src/archive/tar/tar_test.go

@@ -10,7 +10,6 @@ import (
 	"os"
 	"path"
 	"reflect"
-	"runtime"
 	"strings"
 	"testing"
 	"time"
@@ -136,9 +135,6 @@ type headerRoundTripTest struct {
 }
 
 func TestHeaderRoundTrip(t *testing.T) {
-	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" || runtime.GOOS == "nacl" {
-		t.Skipf("skipping on %s; issue 11426", runtime.GOOS)
-	}
 	golden := []headerRoundTripTest{
 		// regular file.
 		{