net/dnscache: initialize the single Resolver more directly

net/dnscache/dnscache.go

@@ -17,20 +17,22 @@ import (
 	"golang.org/x/sync/singleflight"
 )
 
-var single = func() *Resolver {
+var single = &Resolver{
-	r := &Resolver{
+	Forward: &net.Resolver{PreferGo: preferGoResolver()},
-		Forward: &net.Resolver{PreferGo: true},
 }
 
+func preferGoResolver() bool {
 	// There does not appear to be a local resolver running
 	// on iOS, and NetworkExtension is good at isolating DNS.
 	// So do not use the Go resolver on macOS/iOS.
 	if runtime.GOOS == "darwin" {
-		r.Forward.PreferGo = false
+		return false
 	}
 
-	return r
+	// Otherwise, the Go resolver is fine and slightly preferred
-}()
+	// since it's lighter, not using cgo calls & threads.
+	return true
+}
 
 // Get returns a caching Resolver singleton.
 func Get() *Resolver { return single }