dev: Fix callback parsing

pkg/web/oauthclient/callback.go

@@ -16,10 +16,10 @@ package oauthclient
 
 import (
 	"context"
-	"encoding/json"
 	stderrors "errors"
 	"net/http"
 
+	"github.com/gorilla/schema"
 	"go.thethings.network/lorawan-stack/v3/pkg/errors"
 	"go.thethings.network/lorawan-stack/v3/pkg/webhandlers"
 	"golang.org/x/oauth2"
@@ -35,10 +35,10 @@ var (
 )
 
 type oauthAuthorizeResponse struct {
-	Error            string `form:"error" query:"error"`
-	ErrorDescription string `form:"error_description" query:"error_description"`
-	State            string `form:"state" query:"state"`
-	Code             string `form:"code" query:"code"`
+	Error            string `schema:"error"`
+	ErrorDescription string `schema:"error_description"`
+	State            string `schema:"state"`
+	Code             string `schema:"code"`
 }
 
 func (res *oauthAuthorizeResponse) ValidateContext(c context.Context) error {
@@ -58,8 +58,11 @@ func (res *oauthAuthorizeResponse) ValidateContext(c context.Context) error {
 // access token.
 func (oc *OAuthClient) HandleCallback(w http.ResponseWriter, r *http.Request) {
 	var response oauthAuthorizeResponse
-	err := json.NewDecoder(r.Body).Decode(&response)
-	if err != nil {
+	if err := r.ParseForm(); err != nil {
+		webhandlers.Error(w, r, err)
+		return
+	}
+	if err := schema.NewDecoder().Decode(&response, r.Form); err != nil {
 		webhandlers.Error(w, r, err)
 		return
 	}
@@ -77,6 +80,7 @@ func (oc *OAuthClient) HandleCallback(w http.ResponseWriter, r *http.Request) {
 		// redirect back to the client mount instead of showing an error.
 		if acErr != nil {
 			webhandlers.Error(w, r, acErr)
+			return
 		}
 		if value.AccessToken != "" {
 			config := oc.configFromContext(r.Context())