feat(auth/httptransport): add ability to customize transport (#10023)

This was a known limitation of the current implementation that had been forgotten to be implemented. See removal of todo in related PR.

Updates: #9812
Updates: #9814
Related: googleapis/google-api-go-client#2541

auth/httptransport/httptransport.go

@@ -44,6 +44,9 @@ type Options struct {
 	// Headers are extra HTTP headers that will be appended to every outgoing
 	// request.
 	Headers http.Header
+	// BaseRoundTripper overrides the base transport used for serving requests.
+	// If specified ClientCertProvider is ignored.
+	BaseRoundTripper http.RoundTripper
 	// Endpoint overrides the default endpoint to be used for a service.
 	Endpoint string
 	// APIKey specifies an API key to be used as the basis for authentication.
@@ -181,7 +184,11 @@ func NewClient(opts *Options) (*http.Client, error) {
 	if err != nil {
 		return nil, err
 	}
-	trans, err := newTransport(defaultBaseTransport(clientCertProvider, dialTLSContext), opts)
+	baseRoundTripper := opts.BaseRoundTripper
+	if baseRoundTripper == nil {
+		baseRoundTripper = defaultBaseTransport(clientCertProvider, dialTLSContext)
+	}
+	trans, err := newTransport(baseRoundTripper, opts)
 	if err != nil {
 		return nil, err
 	}

auth/httptransport/httptransport_test.go

@@ -350,10 +350,46 @@ func TestNewClient_APIKey(t *testing.T) {
 	}
 }
 
+func TestNewClient_BaseRoundTripper(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		got := r.Header.Get("Foo")
+		if want := "foo"; got != want {
+			t.Errorf("got %q, want %q", got, want)
+		}
+		got = r.Header.Get("Bar")
+		if want := "bar"; got != want {
+			t.Errorf("got %q, want %q", got, want)
+		}
+	}))
+	defer ts.Close()
+	client, err := NewClient(&Options{
+		BaseRoundTripper: &rt{key: "Bar", value: "bar"},
+		Headers:          http.Header{"Foo": []string{"foo"}},
+		APIKey:           "key",
+	})
+	if err != nil {
+		t.Fatalf("NewClient() = %v", err)
+	}
+	if _, err := client.Get(ts.URL); err != nil {
+		t.Fatalf("client.Get() = %v", err)
+	}
+}
+
 type staticTP string
 
 func (tp staticTP) Token(context.Context) (*auth.Token, error) {
 	return &auth.Token{
 		Value: string(tp),
 	}, nil
 }
+
+type rt struct {
+	key   string
+	value string
+}
+
+func (r *rt) RoundTrip(req *http.Request) (*http.Response, error) {
+	req2 := req.Clone(req.Context())
+	req2.Header.Add(r.key, r.value)
+	return http.DefaultTransport.RoundTrip(req2)
+}