refactor(netxlite): use *Netx for the system resolver

This code modifies how we construct netxlite's system resolver such that
public functions use the *Netx equivalents.

The general idea for which I am pushing here is to have additional
clarity about dependencies, to better analyze the requirements of non
measuring code for ooni/probe#2531.

internal/netxlite/dnsovergetaddrinfo.go

@@ -27,9 +27,14 @@ type dnsOverGetaddrinfoTransport struct {
 	provider *MaybeCustomUnderlyingNetwork
 }
 
+func (netx *Netx) newDNSOverGetaddrinfoTransport() model.DNSTransport {
+	return &dnsOverGetaddrinfoTransport{provider: netx.tproxyNilSafeProvider()}
+}
+
 // NewDNSOverGetaddrinfoTransport creates a new dns-over-getaddrinfo transport.
 func NewDNSOverGetaddrinfoTransport() model.DNSTransport {
-	return &dnsOverGetaddrinfoTransport{}
+	netx := &Netx{Underlying: nil}
+	return netx.newDNSOverGetaddrinfoTransport()
 }
 
 var _ model.DNSTransport = &dnsOverGetaddrinfoTransport{}

internal/netxlite/netx.go

@@ -18,17 +18,8 @@ type Netx struct {
 }
 
 // tproxyNilSafeProvider wraps the [model.UnderlyingNetwork] using a [tproxyNilSafeProvider].
-func (n *Netx) tproxyNilSafeProvider() *MaybeCustomUnderlyingNetwork {
-	return &MaybeCustomUnderlyingNetwork{n.Underlying}
-}
-
-// NewStdlibResolver is like [netxlite.NewStdlibResolver] but the constructed [model.Resolver]
-// uses the [model.UnderlyingNetwork] configured inside the [Netx] structure.
-func (n *Netx) NewStdlibResolver(logger model.DebugLogger, wrappers ...model.DNSTransportWrapper) model.Resolver {
-	unwrapped := &resolverSystem{
-		t: WrapDNSTransport(&dnsOverGetaddrinfoTransport{provider: n.tproxyNilSafeProvider()}, wrappers...),
-	}
-	return WrapResolver(logger, unwrapped)
+func (netx *Netx) tproxyNilSafeProvider() *MaybeCustomUnderlyingNetwork {
+	return &MaybeCustomUnderlyingNetwork{netx.Underlying}
 }
 
 // NewDialerWithResolver is like [netxlite.NewDialerWithResolver] but the constructed [model.Dialer]

internal/netxlite/resolvercore.go

@@ -27,8 +27,15 @@ var ErrNoDNSTransport = errors.New("operation requires a DNS transport")
 // with an internal "stdlib" resolver type. The list of optional wrappers
 // allow to wrap the underlying getaddrinfo transport. Any nil wrapper
 // will be silently ignored by the code that performs the wrapping.
+func (netx *Netx) NewStdlibResolver(logger model.DebugLogger, wrappers ...model.DNSTransportWrapper) model.Resolver {
+	return WrapResolver(logger, netx.newUnwrappedStdlibResolver(wrappers...))
+}
+
+// NewStdlibResolver is equivalent to creating an empty [*Netx]
+// and callings its NewStdlibResolver method.
 func NewStdlibResolver(logger model.DebugLogger, wrappers ...model.DNSTransportWrapper) model.Resolver {
-	return WrapResolver(logger, NewUnwrappedStdlibResolver(wrappers...))
+	netx := &Netx{Underlying: nil}
+	return netx.NewStdlibResolver(logger, wrappers...)
 }
 
 // NewParallelDNSOverHTTPSResolver creates a new DNS over HTTPS resolver
@@ -40,13 +47,18 @@ func NewParallelDNSOverHTTPSResolver(logger model.DebugLogger, URL string) model
 	return WrapResolver(logger, NewUnwrappedParallelResolver(txp))
 }
 
+func (netx *Netx) newUnwrappedStdlibResolver(wrappers ...model.DNSTransportWrapper) model.Resolver {
+	return &resolverSystem{
+		t: WrapDNSTransport(netx.newDNSOverGetaddrinfoTransport(), wrappers...),
+	}
+}
+
 // NewUnwrappedStdlibResolver returns a new, unwrapped resolver using the standard
 // library (i.e., getaddrinfo if possible and &net.Resolver{} otherwise). As the name
 // implies, this function returns an unwrapped resolver.
 func NewUnwrappedStdlibResolver(wrappers ...model.DNSTransportWrapper) model.Resolver {
-	return &resolverSystem{
-		t: WrapDNSTransport(NewDNSOverGetaddrinfoTransport(), wrappers...),
-	}
+	netx := &Netx{Underlying: nil}
+	return netx.newUnwrappedStdlibResolver(wrappers...)
 }
 
 // NewSerialUDPResolver creates a new Resolver using DNS-over-UDP