tp: export ErrMissingUserURLCallback

tp/client.go

@@ -17,6 +17,13 @@ import (
 	"github.com/superfly/macaroon/bundle"
 )
 
+// ErrMissingUserURLCallback is returned when a third party asks for the user
+// to be sent to a URL in their browser, but the client has no
+// WithUserURLCallback to send them with. Callers that can fall back to an
+// interactive flow can test for it with errors.Is, including on the joined
+// error FetchDischargeTokens returns for a whole set of tickets.
+var ErrMissingUserURLCallback = errors.New("missing user-url callback")
+
 type ClientOption func(*Client)
 
 // WithHTTP specifies the HTTP client to use for requests to third parties.
@@ -321,7 +328,7 @@ func (c *Client) doUserInteractive(ctx context.Context, ui *jsonUserInteractive)
 		return "", errors.New("bad discharge response")
 	}
 	if c.userURLCallback == nil {
-		return "", errors.New("missing user-url callback")
+		return "", ErrMissingUserURLCallback
 	}
 
 	if err := c.openUserInteractiveURL(ctx, ui.UserURL); err != nil {

tp/tp_test.go

@@ -2,6 +2,7 @@ package tp
 
 import (
 	"context"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -202,6 +203,23 @@ func TestTP(t *testing.T) {
 		assert.Equal(t, []string{"fp-cav", "dis-cav"}, cavs)
 	})
 
+	t.Run("user interactive response without a callback", func(t *testing.T) {
+		handleInit = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			_, err := CaveatsFromRequest(r)
+			assert.NoError(t, err)
+
+			tp.RespondUserInteractive(w, r)
+		})
+
+		hdr := genFP(t, tp, myCaveat("fp-cav"))
+
+		// the error comes back joined with whatever the other tickets did, so
+		// this is the check a caller has to be able to make to decide whether
+		// retrying with a callback is worth it.
+		_, err = NewClient(firstPartyLocation).FetchDischargeTokens(context.Background(), hdr)
+		assert.True(t, errors.Is(err, ErrMissingUserURLCallback), "got %v", err)
+	})
+
 	t.Run("user interactive response", func(t *testing.T) {
 		userSecret := ""