mount-util: add helper that ensures something is a mount point

src/nspawn/nspawn.c

@@ -3688,11 +3688,9 @@ static int outer_child(
                 return r;
 
         /* Make sure we always have a mount that we can move to root later on. */
-        if (!path_is_mount_point(directory, NULL, 0)) {
-                r = mount_nofollow_verbose(LOG_ERR, directory, directory, NULL, MS_BIND|MS_REC, NULL);
-                if (r < 0)
-                        return r;
-        }
+        r = make_mount_point(directory);
+        if (r < 0)
+                return r;
 
         if (dissected_image) {
                 /* Now we know the uid shift, let's now mount everything else that might be in the image. */

src/shared/mount-util.c

@@ -986,3 +986,23 @@ int mount_image_in_namespace(
 
         return mount_in_namespace(target, propagate_path, incoming_path, src, dest, read_only, make_file_or_directory, options, true);
 }
+
+int make_mount_point(const char *path) {
+        int r;
+
+        assert(path);
+
+        /* If 'path' is already a mount point, does nothing and returns 0. If it is not it makes it one, and returns 1. */
+
+        r = path_is_mount_point(path, NULL, 0);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to determine whether '%s' is a mount point: %m", path);
+        if (r > 0)
+                return 0;
+
+        r = mount_nofollow_verbose(LOG_DEBUG, path, path, NULL, MS_BIND|MS_REC, NULL);
+        if (r < 0)
+                return r;
+
+        return 1;
+}

src/shared/mount-util.h

@@ -103,3 +103,5 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(char*, umount_and_rmdir_and_free);
 
 int bind_mount_in_namespace(pid_t target, const char *propagate_path, const char *incoming_path, const char *src, const char *dest, bool read_only, bool make_file_or_directory);
 int mount_image_in_namespace(pid_t target, const char *propagate_path, const char *incoming_path, const char *src, const char *dest, bool read_only, bool make_file_or_directory, const MountOptions *options);
+
+int make_mount_point(const char *path);