refactor: avoid unnecessary calls to platforms.DefaultSpec()

[profnandaa]

I came across a number of calls to platforms.DefaultSpec() that were not necessary. That prompted me to take a look various other similar places.

At least on Windows, the call is a has some cost till doing a sycall RtlGetNtVersionNumbers().

[iankingori]

This makes sense, looks like DefaultSpec was used as a type placeholder then overwritten entirely

[crazy-max]

At least on Windows, the call is a has some cost till doing a sycall RtlGetNtVersionNumbers().

I think this should be fixed upstream instead: https://github.com/containerd/platforms/blob/e3566b8ff1994b8dc88bae5768d32830e0cd0cfd/defaults_windows.go#L29 so windows.RtlGetNtVersionNumbers() is cached. WDYT @cpuguy83 @thaJeztah?

[thaJeztah]

Hm.. right; I wasn't aware this call didn't come without cost.

Assuming that versions doesn't change during the runtime of the (buildkit) daemon, perhaps the platforms module should use a sync.Once or sync.OnceValue for the version (or default spec as a whole)

[thaJeztah]

Also curious about the Normalize call; I suspect DefaultSpec() would always be normalized, so normalising should likely only be needed if a custom platform is set (not the default).

[cpuguy83]

Seems reasonable to cache it.
Also something that could probably be wrapped internally in buildkit.