Redirect to the only OAuth2 provider when no other login methods and fix various problems

routers/web/auth/auth.go

@@ -218,6 +218,19 @@ func performAutoLogin(ctx *context.Context) bool {
 	return false
 }
 
+func performAutoLoginOAuth2(ctx *context.Context) bool {
+	providers, ok := ctx.Data["OAuth2Providers"].([]oauth2.Provider)
+	if ok && len(providers) == 1 {
+		skipToOAuthURL := setting.AppSubURL + "/user/oauth2/" + url.PathEscape(providers[0].DisplayName())
+		if redirectTo := ctx.FormString("redirect_to"); redirectTo != "" {
+			skipToOAuthURL += "?" + url.Values{"redirect_to": []string{redirectTo}}.Encode()
+		}
+		ctx.Redirect(skipToOAuthURL)
+		return true
+	}
+	return false
+}
+
 func prepareSignInPageData(ctx *context.Context) {
 	ctx.Data["Title"] = ctx.Tr("sign_in")
 	ctx.Data["OAuth2Providers"], _ = oauth2.GetOAuth2Providers(ctx, optional.Some(true))
@@ -242,6 +255,18 @@ func SignIn(ctx *context.Context) {
 		return
 	}
 	prepareSignInPageData(ctx)
+
+	// If only 1 OAuth provider is present and other login methods are disabled, redirect to the OAuth provider.
+	if !ctx.FormBool("skipAutoLogin") &&
+		!setting.Service.EnablePasswordSignInForm &&
+		!setting.Service.EnableOpenIDSignIn &&
+		!setting.Service.EnablePasskeyAuth &&
+		!auth.IsSSPIEnabled(ctx) {
+		if performAutoLoginOAuth2(ctx) {
+			return
+		}
+	}
+
 	ctx.HTML(http.StatusOK, tplSignIn)
 }
 

routers/web/auth/auth_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	auth_model "code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/session"
 	"code.gitea.io/gitea/modules/setting"
@@ -134,4 +135,22 @@ func TestWebAuthOAuth2(t *testing.T) {
 		u.RawQuery = ""
 		assert.Equal(t, "https://example.com/oidc-logout", u.String())
 	})
+
+	t.Run("SignInAutoRedirectSingleProvider", func(t *testing.T) {
+		unittest.PrepareTestEnv(t)
+		addOAuth2Source(t, "dummy-auth-source", oauth2.Source{})
+		defer test.MockVariableValue(&setting.Service.EnablePasswordSignInForm, false)()
+		defer test.MockVariableValue(&setting.Service.EnableOpenIDSignIn, false)()
+		defer test.MockVariableValue(&setting.Service.EnablePasskeyAuth, false)()
+
+		ctx, resp := contexttest.MockContext(t, "/user/login?redirect_to=/other")
+		SignIn(ctx)
+		assert.Equal(t, http.StatusSeeOther, resp.Code)
+		expectedURL := "/user/oauth2/dummy-auth-source?redirect_to=" + url.QueryEscape("/other")
+		assert.Equal(t, expectedURL, test.RedirectURL(resp))
+
+		ctx, resp = contexttest.MockContext(t, "/user/login?redirect_to=/other&skipAutoLogin=true")
+		SignIn(ctx)
+		assert.Equal(t, http.StatusOK, resp.Code)
+	})
 }