Simultaneous requests for creating an instance result in recopying the image to _base before it is cached

[Alexei Karve]

Simultaneous requests for creating an instance result in unnecessary recopying of the image with ".part" extension to _base directory (one for each request) before it is cached. The fetch_func_sync needs to check again inside the synchronized whether the earlier copy was completed as indicated below in https://github.com/openstack/nova/blob/master/nova/virt/libvirt/imagebackend.py

    def cache(self, fetch_func, filename, size=None, *args, **kwargs):
        """Creates image from template.

        Ensures that template and image not already exists.
        Ensures that base directory exists.
        Synchronizes on template fetching.

        :fetch_func: Function that creates the base image
                     Should accept `target` argument.
        :filename: Name of the file in the image directory
        :size: Size of created image in bytes (optional)
        """
        @utils.synchronized(filename, external=True, lock_path=self.lock_path)
        def fetch_func_sync(target, *args, **kwargs):
            if not os.path.exists(target):
                fetch_func(target=target, *args, **kwargs)