- [X] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? ----- ## Overview Cache a configured LXD environment as an instance instead of an image. ## Details On the first run of an application, an instance will be launched from an image (i.e. an image from https://cloud-images.ubuntu.com). The instance is then setup. After setup, a copy of this instance is saved (or cached) as a 'base instance'. This is done to reduce setup time on subsequent runs. When the application requests a new instance on a subsequent run, the base instance will be copied to create the new instance. This instance is run through a small subset of the setup, which is referred to as 'warmup'. ## Previous behavior  ## New behavior The flowchart below describes the logic inside the `launch()` call. The flowchart has become crowded as it's evolved - it's a result of the fixed drawing size in Google Doc's embedded drawings.  ## Notes on the PR - `craft_providers/lxd/launcher.py` was completely refactored, since the underlying mechanism was redone. It's may be easier to review the new file instead of the diff. - The unit tests did not need to be changed dramatically (thanks to https://github.com/canonical/craft-providers/pull/178). However I added more tests. - The integration tests are mostly unchanged too, aside from swapping `base image` assertions for `base instance` assertions. - I've added a deprecation warning for the parameter `use_snapshots` in favor of `use_base_instance`. For now, the old parameter will still work but will log a warning. ## Performance improvement The scenarios below were measure how long the call to `lxd.launch()` takes when building a simple snap on a typical developer laptop. | scenario | previous time | new time | speed up | |-----|------|----------|-----| |clean environment|01:58.415|01:24:483|34 seconds| |base instance exists|00:24.721|00:04.886|19 seconds| |instance exists|00:04:145|00:04.194|no change| (CRAFT-1339)