Add option to use polling instead of filesystem events when watching for filesystem changes

[qwerty01]

Describe the problem

Currently sveltekit uses cheap-watch to detect when files have changed while running the dev server (https://github.com/sveltejs/kit/blob/master/packages/kit/src/core/dev/index.js#L76). cheap-watch makes use of fs.watch, which uses a system-specific event callback when a file changes.

Unfortunately, when listening for events on a network drive (such as when using docker with a mapped folder), no events are emitted when files are changed. Other similar tools have an option to use filesystem polling instead of event callbacks, such as chokidar: https://github.com/paulmillr/chokidar#performance:

usePolling (default: false). Whether to use fs.watchFile (backed by polling), or fs.watch. If polling leads to high CPU utilization, consider setting this to false. It is typically necessary to set this to true to successfully watch files over a network

Describe the proposed solution

I believe the most flexible solution would be to switch to a different filesystem watcher that supports polling as well as filesystem events (such as chokidar). Configuring the module could then be achieved through the svelte.config.js file and a link directly to the module's documentation can be added to the current documentation for configuring watch settings.

Alternatives considered

Another option could be to use a secondary module for polling the filesystem and have a config option in svelte.config.js to either use polling or filesystem events.

Importance

would make my life easier

Additional Information

The current use case for me is to enable developing in docker. Currently, filesystem watching doesn't work at all due to network shares not triggering filesystem events, so HMR does not work. Everything works when running locally so it's not a showstopper, but it would be nice to have support for dockerized development environments.

[bluwy]

One possible solution is to use Vite's file watcher from the dev server as well, which uses chokidar. I'm not sure how that would affect HMR, but it may be doable.

[Rich-Harris]

in #3138 we switched to using Vite's chokidar instance, so you can configure the watcher via the Vite config: https://vitejs.dev/config/#server-watch