fix(oohelperd): use throw-away HTTPClient, Dialer, Resolver (#833)

This diff modifies the implementation of oohelperd in the master branch
to always use throw-away HTTPClient, Dialer, and Resolver.

The rationale of this change is to ensure we're not hitting limits of the
HTTPClient regarding the max number of connections per host.

This issue is described at https://github.com/ooni/probe/issues/2182.

While there, it feels more correct to use throw-away Dialer and Resolver.

We have a different patch for the release/3.15 branch because of
netx-related refactorings: https://github.com/ooni/probe-cli/pull/832.

internal/cmd/oohelperd/oohelperd.go

@@ -17,22 +17,22 @@ import (
 const maxAcceptableBody = 1 << 24
 
 var (
-	dialer     model.Dialer
-	endpoint   = flag.String("endpoint", ":8080", "Endpoint where to listen")
-	httpClient model.HTTPClient
-	resolver   model.Resolver
-	srvcancel  context.CancelFunc
-	srvctx     context.Context
-	srvwg      = new(sync.WaitGroup)
+	endpoint  = flag.String("endpoint", ":8080", "Endpoint where to listen")
+	srvcancel context.CancelFunc
+	srvctx    context.Context
+	srvwg     = new(sync.WaitGroup)
 )
 
 func init() {
 	srvctx, srvcancel = context.WithCancel(context.Background())
+}
+
+func newresolver() model.Resolver {
 	// Implementation note: pin to a specific resolver so we don't depend upon the
 	// default resolver configured by the box. Also, use an encrypted transport thus
 	// we're less vulnerable to any policy implemented by the box's provider.
-	resolver = netxlite.NewParallelDNSOverHTTPSResolver(log.Log, "https://8.8.8.8/dns-query")
-	httpClient = netxlite.NewHTTPClientWithResolver(log.Log, resolver)
+	resolver := netxlite.NewParallelDNSOverHTTPSResolver(log.Log, "https://8.8.8.8/dns-query")
+	return resolver
 }
 
 func shutdown(srv *http.Server) {
@@ -55,10 +55,14 @@ func main() {
 func testableMain() {
 	mux := http.NewServeMux()
 	mux.Handle("/", webconnectivity.Handler{
-		Client:            httpClient,
-		Dialer:            dialer,
 		MaxAcceptableBody: maxAcceptableBody,
-		Resolver:          resolver,
+		NewClient: func() model.HTTPClient {
+			return netxlite.NewHTTPClientWithResolver(log.Log, newresolver())
+		},
+		NewDialer: func() model.Dialer {
+			return netxlite.NewDialerWithResolver(log.Log, newresolver())
+		},
+		NewResolver: newresolver,
 	})
 	srv := &http.Server{Addr: *endpoint, Handler: mux}
 	srvwg.Add(1)