Detect changes in supported platforms without requiring restart

[tonistiigi]

Currently, after installing qemu-user support, the daemon needs to be restarted for it to detect the support again. This is because there is a cache to avoid running the relatively costly detection algorithm again.

I think there is a version that could work for both cases. We could implement SupportedPlatfroms(resetCache) and ResetCache(platform). If you run something like buildctl debug workers we would always rescan the platforms support. If you build for a specific platform then if it is supported we would just return true, if it is not supported we would reset cache for that specific platform. Then 99% of the builds that do not return an error would still keep using cache.

related docker/buildx#208

[jingxiaolu]

@tonistiigi
Do you mean updates the code procedure as this?

CMD: buildctl debug workers

buildctl listWorkers()
	c.ListWorkers()
		c.controlClient().ListWorkers()
	
<---Buildctl
GRPC: /moby.buildkit.v1.Control/ListWorkers
--->Buildkitd 
Controller ListWorkers()
	c.opt.WorkerController.ResetCache()  // <-- new
	c.opt.WorkerController.List()
	prepare resp and return
	
Worker.Controller ResetCache()  // <-- new
	workers := c.List()
	for range workers
		worker.ResetCache()

Worker ResetCache()  // <-- new
	updatedPlatforms = binfmt_misc.SupportedPlatforms(true)
	w.WorkerOpt.Platforms = parse(updatedPlatforms)
	
binfmt_misc SupportedPlatforms(resetCache bool)		// <-- resetCache bool is newly added
	if resetCache 
		rescan platform and return

[tonistiigi]

@jingxiaolu Yes. But more importantly, there is a rescan for a specific platform in the beginning of the build request if requested platform is not already supported.

[jingxiaolu]

@tonistiigi
Currently in SupportedPlatforms() there has a once.Do() and the checkings are wrapped by it. We need remove this once.Do()?

If so, when buildkitd boots up, it may scans platforms twice here:

	if cfg.Workers.OCI.Platforms == nil {
		cfg.Workers.OCI.Platforms = binfmt_misc.SupportedPlatforms()
	}
	if cfg.Workers.Containerd.Platforms == nil {
		cfg.Workers.Containerd.Platforms = binfmt_misc.SupportedPlatforms()
	}