Respect tar entries modes when rewriting them on Windows

Previously, only perm-related bits where preserved when rewriting
FileMode in tar entries on Windows. This had the nasty side effect of
having tarsum returning different values when executing from a tar filed
produced on Windows or Linux.

This fix the issue, and pave the way for incremental build context
to work in hybrid contexts.

Signed-off-by: Simon Ferquel <simon.ferquel@docker.com>

builder/remotecontext/tarsum_test.go

@@ -4,7 +4,6 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
-	"runtime"
 	"testing"
 
 	"github.com/docker/docker/builder"
@@ -48,7 +47,7 @@ func TestHashFile(t *testing.T) {
 	contextDir, cleanup := createTestTempDir(t, "", "builder-tarsum-test")
 	defer cleanup()
 
-	createTestTempFile(t, contextDir, filename, contents, 0777)
+	createTestTempFile(t, contextDir, filename, contents, 0755)
 
 	tarSum := makeTestTarsumContext(t, contextDir)
 
@@ -63,9 +62,6 @@ func TestHashFile(t *testing.T) {
 	}
 
 	expected := "1149ab94af7be6cc1da1335e398f24ee1cf4926b720044d229969dfc248ae7ec"
-	if runtime.GOOS == "windows" {
-		expected = "55dfeb344351ab27f59aa60ebb0ed12025a2f2f4677bf77d26ea7a671274a9ca"
-	}
 
 	if actual := sum; expected != actual {
 		t.Fatalf("invalid checksum. expected %s, got %s", expected, actual)
@@ -77,12 +73,12 @@ func TestHashSubdir(t *testing.T) {
 	defer cleanup()
 
 	contextSubdir := filepath.Join(contextDir, "builder-tarsum-test-subdir")
-	err := os.Mkdir(contextSubdir, 0700)
+	err := os.Mkdir(contextSubdir, 0755)
 	if err != nil {
 		t.Fatalf("Failed to make directory: %s", contextSubdir)
 	}
 
-	testFilename := createTestTempFile(t, contextSubdir, filename, contents, 0777)
+	testFilename := createTestTempFile(t, contextSubdir, filename, contents, 0755)
 
 	tarSum := makeTestTarsumContext(t, contextDir)
 
@@ -103,9 +99,6 @@ func TestHashSubdir(t *testing.T) {
 	}
 
 	expected := "d7f8d6353dee4816f9134f4156bf6a9d470fdadfb5d89213721f7e86744a4e69"
-	if runtime.GOOS == "windows" {
-		expected = "74a3326b8e766ce63a8e5232f22e9dd895be647fb3ca7d337e5e0a9b3da8ef28"
-	}
 
 	if actual := sum; expected != actual {
 		t.Fatalf("invalid checksum. expected %s, got %s", expected, actual)

pkg/archive/archive_windows.go

@@ -42,11 +42,14 @@ func CanonicalTarNameForPath(p string) (string, error) {
 // chmodTarEntry is used to adjust the file permissions used in tar header based
 // on the platform the archival is done.
 func chmodTarEntry(perm os.FileMode) os.FileMode {
-	perm &= 0755
+	//perm &= 0755 // this 0-ed out tar flags (like link, regular file, directory marker etc.)
+	permPart := perm & os.ModePerm
+	noPermPart := perm &^ os.ModePerm
 	// Add the x bit: make everything +x from windows
-	perm |= 0111
+	permPart |= 0111
+	permPart &= 0755
 
-	return perm
+	return noPermPart | permPart
 }
 
 func setHeaderForSpecialDevice(hdr *tar.Header, name string, stat interface{}) (err error) {

pkg/archive/archive_windows_test.go

@@ -82,6 +82,8 @@ func TestChmodTarEntry(t *testing.T) {
 		{0644, 0755},
 		{0755, 0755},
 		{0444, 0555},
+		{0755 | os.ModeDir, 0755 | os.ModeDir},
+		{0755 | os.ModeSymlink, 0755 | os.ModeSymlink},
 	}
 	for _, v := range cases {
 		if out := chmodTarEntry(v.in); out != v.expected {