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>
Upstream-commit: 41eb61d5c2a44c745b11ada948034fd3bcc42db5
Component: engine

components/engine/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) {

components/engine/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 {