Show OAuth2 errors to end users (#25261)

Partially fix #23936


![image](https://github.com/go-gitea/gitea/assets/2114189/8aa7f3ad-a5f0-42ce-a478-289a03bd08a3)


![image](https://github.com/go-gitea/gitea/assets/2114189/bb901e7d-485a-47a5-b68d-9ebe7013a6b2)


![image](https://github.com/go-gitea/gitea/assets/2114189/9a1ce0f3-f011-4baf-8e2f-cc6304bc9703)

options/locale/locale_en-US.ini

@@ -2901,7 +2901,7 @@ auths.sspi_default_language = Default user language
 auths.sspi_default_language_helper = Default language for users automatically created by SSPI auth method. Leave empty if you prefer language to be automatically detected.
 auths.tips = Tips
 auths.tips.oauth2.general = OAuth2 Authentication
-auths.tips.oauth2.general.tip = When registering a new OAuth2 authentication, the callback/redirect URL should be: <host>/user/oauth2/<Authentication Name>/callback
+auths.tips.oauth2.general.tip = When registering a new OAuth2 authentication, the callback/redirect URL should be:
 auths.tip.oauth2_provider = OAuth2 Provider
 auths.tip.bitbucket = Register a new OAuth consumer on https://bitbucket.org/account/user/<your username>/oauth-consumers/new and add the permission 'Account' - 'Read'
 auths.tip.nextcloud = Register a new OAuth consumer on your instance using the following menu "Settings -> Security -> OAuth 2.0 client"

routers/web/auth/oauth.go

@@ -4,7 +4,7 @@
 package auth
 
 import (
-	stdContext "context"
+	go_context "context"
 	"encoding/base64"
 	"errors"
 	"fmt"
@@ -12,6 +12,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"sort"
 	"strings"
 
 	"code.gitea.io/gitea/models/auth"
@@ -39,6 +40,7 @@ import (
 	"github.com/golang-jwt/jwt/v4"
 	"github.com/markbates/goth"
 	"github.com/markbates/goth/gothic"
+	go_oauth2 "golang.org/x/oauth2"
 )
 
 const (
@@ -143,7 +145,7 @@ type AccessTokenResponse struct {
 	IDToken      string    `json:"id_token,omitempty"`
 }
 
-func newAccessTokenResponse(ctx stdContext.Context, grant *auth.OAuth2Grant, serverKey, clientKey oauth2.JWTSigningKey) (*AccessTokenResponse, *AccessTokenError) {
+func newAccessTokenResponse(ctx go_context.Context, grant *auth.OAuth2Grant, serverKey, clientKey oauth2.JWTSigningKey) (*AccessTokenResponse, *AccessTokenError) {
 	if setting.OAuth2.InvalidateRefreshTokens {
 		if err := grant.IncreaseCounter(ctx); err != nil {
 			return nil, &AccessTokenError{
@@ -886,6 +888,17 @@ func SignInOAuth(ctx *context.Context) {
 func SignInOAuthCallback(ctx *context.Context) {
 	provider := ctx.Params(":provider")
 
+	if ctx.Req.FormValue("error") != "" {
+		var errorKeyValues []string
+		for k, vv := range ctx.Req.Form {
+			for _, v := range vv {
+				errorKeyValues = append(errorKeyValues, fmt.Sprintf("%s = %s", html.EscapeString(k), html.EscapeString(v)))
+			}
+		}
+		sort.Strings(errorKeyValues)
+		ctx.Flash.Error(strings.Join(errorKeyValues, "<br>"), true)
+	}
+
 	// first look if the provider is still active
 	authSource, err := auth.GetActiveOAuth2SourceByName(provider)
 	if err != nil {
@@ -894,7 +907,7 @@ func SignInOAuthCallback(ctx *context.Context) {
 	}
 
 	if authSource == nil {
-		ctx.ServerError("SignIn", errors.New("No valid provider found, check configured callback url in provider"))
+		ctx.ServerError("SignIn", errors.New("no valid provider found, check configured callback url in provider"))
 		return
 	}
 
@@ -920,6 +933,9 @@ func SignInOAuthCallback(ctx *context.Context) {
 			ctx.Redirect(setting.AppSubURL + "/user/login")
 			return
 		}
+		if err, ok := err.(*go_oauth2.RetrieveError); ok {
+			ctx.Flash.Error("OAuth2 RetrieveError: "+err.Error(), true)
+		}
 		ctx.ServerError("UserSignIn", err)
 		return
 	}

templates/admin/auth/edit.tmpl

@@ -14,8 +14,8 @@
 					<span>{{.Source.TypeName}}</span>
 				</div>
 				<div class="required inline field {{if .Err_Name}}error{{end}}">
-					<label for="name">{{.locale.Tr "admin.auths.auth_name"}}</label>
+					<label for="auth_name">{{.locale.Tr "admin.auths.auth_name"}}</label>
-					<input id="name" name="name" value="{{.Source.Name}}" autofocus required>
+					<input id="auth_name" name="name" value="{{.Source.Name}}" autofocus required>
 				</div>
 
 				<!-- LDAP and DLDAP -->
@@ -434,6 +434,17 @@
 				</div>
 			</form>
 		</div>
+
+		<h4 class="ui top attached header">
+			{{.locale.Tr "admin.auths.tips"}}
+		</h4>
+		<div class="ui attached segment">
+			<h5>GMail Settings:</h5>
+			<p>Host: smtp.gmail.com, Port: 587, Enable TLS Encryption: true</p>
+
+			<h5 class="oauth2">{{.locale.Tr "admin.auths.tips.oauth2.general"}}:</h5>
+			<p class="oauth2">{{.locale.Tr "admin.auths.tips.oauth2.general.tip"}} <b id="oauth2-callback-url"></b></p>
+		</div>
 	</div>
 
 <div class="ui g-modal-confirm delete modal">

templates/admin/auth/new.tmpl

@@ -22,8 +22,8 @@
 					</div>
 				</div>
 				<div class="required inline field {{if .Err_Name}}error{{end}}">
-					<label for="name">{{.locale.Tr "admin.auths.auth_name"}}</label>
+					<label for="auth_name">{{.locale.Tr "admin.auths.auth_name"}}</label>
-					<input id="name" name="name" value="{{.name}}" autofocus required>
+					<input id="auth_name" name="name" value="{{.name}}" autofocus required>
 				</div>
 
 				<!-- LDAP and DLDAP -->
@@ -85,8 +85,8 @@
 			<h5>GMail Settings:</h5>
 			<p>Host: smtp.gmail.com, Port: 587, Enable TLS Encryption: true</p>
 
-			<h5>{{.locale.Tr "admin.auths.tips.oauth2.general"}}:</h5>
+			<h5 class="oauth2">{{.locale.Tr "admin.auths.tips.oauth2.general"}}:</h5>
-			<p>{{.locale.Tr "admin.auths.tips.oauth2.general.tip"}}</p>
+			<p class="oauth2">{{.locale.Tr "admin.auths.tips.oauth2.general.tip"}} <b id="oauth2-callback-url"></b></p>
 
 			<h5 class="ui top attached header">{{.locale.Tr "admin.auths.tip.oauth2_provider"}}</h5>
 			<div class="ui attached segment">

templates/status/500.tmpl

@@ -1,6 +1,7 @@
 {{/* This page should only depend the minimal template functions/variables, to avoid triggering new panics.
 * base template functions: AppName, AssetUrlPrefix, AssetVersion, AppSubUrl, DefaultTheme, Str2html
 * locale
+* Flash
 * ErrorMsg
 * SignedUser (optional)
 */}}
@@ -28,6 +29,10 @@
 			</div>
 		</nav>
 		<div role="main" class="page-content status-page-500">
+			<div class="ui container" >
+				<style> .ui.message.flash-message { text-align: left; } </style>
+				{{template "base/alert" .}}
+			</div>
 			<p class="gt-mt-5 center"><img src="{{AssetUrlPrefix}}/img/500.png" alt="Internal Server Error"></p>
 			<div class="ui divider"></div>
 			<div class="ui container gt-my-5">

web_src/js/features/admin/common.js

@@ -171,6 +171,12 @@ export function initAdminCommon() {
     }
   }
 
+  if ($('.admin.authentication').length > 0) {
+    $('#auth_name').on('input', function () {
+      $('#oauth2-callback-url').text(`${window.location.origin}/user/oauth2/${encodeURIComponent($(this).val())}/callback`);
+    }).trigger('input');
+  }
+
   // Notice
   if ($('.admin.notice')) {
     const $detailModal = $('#detail-modal');