util/ctxkey: add package for type-safe context keys (#10841)

The lack of type-safety in context.WithValue leads to the common pattern
of defining of package-scoped type to ensure global uniqueness:

	type fooKey struct{}

	func withFoo(ctx context, v Foo) context.Context {
		return context.WithValue(ctx, fooKey{}, v)
	}

	func fooValue(ctx context) Foo {
		v, _ := ctx.Value(fooKey{}).(Foo)
		return v
	}

where usage becomes:

	ctx = withFoo(ctx, foo)
	foo := fooValue(ctx)

With many different context keys, this can be quite tedious.

Using generics, we can simplify this as:

	var fooKey = ctxkey.New("mypkg.fooKey", Foo{})

where usage becomes:

	ctx = fooKey.WithValue(ctx, foo)
	foo := fooKey.Value(ctx)

See https://go.dev/issue/49189

Updates #cleanup

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

0 files changed, 0 insertions, 0 deletions