pkg/system: remove Umask() utility

It was only used in a couple of places, and in most places shouldn't be used
as those locations were in unix/linux-only files, so didn't need the wrapper.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/archive/archive_linux_test.go

@@ -86,9 +86,8 @@ func checkFileMode(t *testing.T, path string, perm os.FileMode) {
 }
 
 func TestOverlayTarUntar(t *testing.T) {
-	oldmask, err := system.Umask(0)
-	assert.NilError(t, err)
-	defer system.Umask(oldmask)
+	restore := overrideUmask(0)
+	defer restore()
 
 	src, err := os.MkdirTemp("", "docker-test-overlay-tar-src")
 	assert.NilError(t, err)
@@ -125,9 +124,8 @@ func TestOverlayTarUntar(t *testing.T) {
 }
 
 func TestOverlayTarAUFSUntar(t *testing.T) {
-	oldmask, err := system.Umask(0)
-	assert.NilError(t, err)
-	defer system.Umask(oldmask)
+	restore := overrideUmask(0)
+	defer restore()
 
 	src, err := os.MkdirTemp("", "docker-test-overlay-tar-src")
 	assert.NilError(t, err)

pkg/archive/diff.go

@@ -229,13 +229,8 @@ func applyLayerHandler(dest string, layer io.Reader, options *TarOptions, decomp
 	dest = filepath.Clean(dest)
 
 	// We need to be able to set any perms
-	if runtime.GOOS != "windows" {
-		oldmask, err := system.Umask(0)
-		if err != nil {
-			return 0, err
-		}
-		defer system.Umask(oldmask)
-	}
+	restore := overrideUmask(0)
+	defer restore()
 
 	if decompress {
 		decompLayer, err := DecompressStream(layer)

pkg/archive/diff_unix.go

@@ -0,0 +1,22 @@
+//go:build !windows
+// +build !windows
+
+package archive
+
+import "golang.org/x/sys/unix"
+
+// overrideUmask sets current process's file mode creation mask to newmask
+// and returns a function to restore it.
+//
+// WARNING for readers stumbling upon this code. Changing umask in a multi-
+// threaded environment isn't safe. Don't use this without understanding the
+// risks, and don't export this function for others to use (we shouldn't even
+// be using this ourself).
+//
+// FIXME(thaJeztah): we should get rid of these hacks if possible.
+func overrideUmask(newMask int) func() {
+	oldMask := unix.Umask(newMask)
+	return func() {
+		unix.Umask(oldMask)
+	}
+}

pkg/archive/diff_windows.go

@@ -0,0 +1,6 @@
+package archive
+
+// overrideUmask is a no-op on windows.
+func overrideUmask(newmask int) func() {
+	return func() {}
+}

pkg/chrootarchive/diff_unix.go

@@ -16,7 +16,7 @@ import (
 	"github.com/containerd/containerd/pkg/userns"
 	"github.com/docker/docker/pkg/archive"
 	"github.com/docker/docker/pkg/reexec"
-	"github.com/docker/docker/pkg/system"
+	"golang.org/x/sys/unix"
 )
 
 type applyLayerResponse struct {
@@ -42,11 +42,8 @@ func applyLayer() {
 	}
 
 	// We need to be able to set any perms
-	oldmask, err := system.Umask(0)
-	defer system.Umask(oldmask)
-	if err != nil {
-		fatal(err)
-	}
+	oldmask := unix.Umask(0)
+	defer unix.Umask(oldmask)
 
 	if err := json.Unmarshal([]byte(os.Getenv("OPT")), &options); err != nil {
 		fatal(err)

pkg/system/umask.go

@@ -1,14 +0,0 @@
-//go:build !windows
-// +build !windows
-
-package system // import "github.com/docker/docker/pkg/system"
-
-import (
-	"golang.org/x/sys/unix"
-)
-
-// Umask sets current process's file mode creation mask to newmask
-// and returns oldmask.
-func Umask(newmask int) (oldmask int, err error) {
-	return unix.Umask(newmask), nil
-}

pkg/system/umask_windows.go

@@ -1,7 +0,0 @@
-package system // import "github.com/docker/docker/pkg/system"
-
-// Umask is not supported on the windows platform.
-func Umask(newmask int) (oldmask int, err error) {
-	// should not be called on cli code path
-	return 0, ErrNotSupportedPlatform
-}