func Lookup(ctx context.Context, proxy, path string) Repo {
	if traceRepo {
		defer logCall("Lookup(%q, %q)", proxy, path)()
	}

	return lookupCache.Do(lookupCacheKey{proxy, path}, func() Repo {
		return newCachingRepo(ctx, path, func(ctx context.Context) (Repo, error) {
			r, err := lookup(ctx, proxy, path)
			if err == nil && traceRepo {
				r = newLoggingRepo(r)
			}
			return r, err
		})
	})
}

	latestCache   par.ErrCache[struct{}, *RevInfo]
	gomodCache    par.ErrCache[string, []byte]

	once     sync.Once
	initRepo func(context.Context) (Repo, error)
	r        Repo
}

func newCachingRepo(ctx context.Context, path string, initRepo func(context.Context) (Repo, error)) *cachingRepo {
	return &cachingRepo{
		path:     path,
		initRepo: initRepo,
	}
}