syscallcompat: use early return in asUser()

internal/syscallcompat/sys_linux.go

@@ -91,38 +91,43 @@ func getSupplementaryGroups(pid uint32) (gids []int) {
 	return nil
 }
 
-// asUser runs the function `f` under the effective uid, gid, groups specified
+// asUser runs `f()` under the effective uid, gid, groups specified
 // in `context`.
+//
+// If `context` is nil, `f()` is executed directly without switching user id.
 func asUser(f func() (int, error), context *fuse.Context) (int, error) {
-	if context != nil {
+	if context == nil {
-		runtime.LockOSThread()
+		return f()
-		defer runtime.UnlockOSThread()
-
-		// Since go1.16beta1 (commit d1b1145cace8b968307f9311ff611e4bb810710c ,
-		// https://go-review.googlesource.com/c/go/+/210639 )
-		// syscall.{Setgroups,Setregid,Setreuid} affects all threads, which
-		// is exactly what we not want.
-		//
-		// We now use unix.{Setgroups,Setregid,Setreuid} instead.
-
-		err := unix.Setgroups(getSupplementaryGroups(context.Pid))
-		if err != nil {
-			return -1, err
-		}
-		defer unix.Setgroups(nil)
-
-		err = unix.Setregid(-1, int(context.Owner.Gid))
-		if err != nil {
-			return -1, err
-		}
-		defer unix.Setregid(-1, 0)
-
-		err = unix.Setreuid(-1, int(context.Owner.Uid))
-		if err != nil {
-			return -1, err
-		}
-		defer unix.Setreuid(-1, 0)
 	}
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	// Since go1.16beta1 (commit d1b1145cace8b968307f9311ff611e4bb810710c ,
+	// https://go-review.googlesource.com/c/go/+/210639 )
+	// syscall.{Setgroups,Setregid,Setreuid} affects all threads, which
+	// is exactly what we not want.
+	//
+	// We now use unix.{Setgroups,Setregid,Setreuid} instead.
+
+	err := unix.Setgroups(getSupplementaryGroups(context.Pid))
+	if err != nil {
+		return -1, err
+	}
+	defer unix.Setgroups(nil)
+
+	err = unix.Setregid(-1, int(context.Owner.Gid))
+	if err != nil {
+		return -1, err
+	}
+	defer unix.Setregid(-1, 0)
+
+	err = unix.Setreuid(-1, int(context.Owner.Uid))
+	if err != nil {
+		return -1, err
+	}
+	defer unix.Setreuid(-1, 0)
+
 	return f()
 }