OAuth2Strategy interface to abstract the details of the login flow

This allows implementing clients the deviate from the standard OAuth2 flow

internal/auth/callback.go

@@ -0,0 +1,36 @@
+/*
+Copyright © 2023 Philipp Wolfer <phw@uploadedlobster.com>
+
+Scotty is free software: you can redistribute it and/or modify it under the
+terms of the GNU General Public License as published by the Free Software
+Foundation, either version 3 of the License, or (at your option) any later version.
+
+Scotty is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+A PARTICULAR PURPOSE. See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+Scotty. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+package auth
+
+import (
+	"fmt"
+	"net/http"
+	"net/url"
+)
+
+func RunOauth2CallbackServer(redirectURL url.URL, responseChan chan CodeResponse) {
+	http.HandleFunc(redirectURL.Path, func(w http.ResponseWriter, r *http.Request) {
+		code := r.URL.Query().Get("code")
+		state := r.URL.Query().Get("state")
+		fmt.Fprint(w, "Token received, you can close this window now.")
+		responseChan <- CodeResponse{
+			Code:  code,
+			State: state,
+		}
+	})
+
+	go http.ListenAndServe(redirectURL.Host, nil)
+}