Make "/user/login" page redirect if the current user has signed in (#29583)

Fix #29582 and maybe more.
Maybe fix #29116

(cherry picked from commit df1268ca08aaacae54c775a8eec34006dfe365e0)

routers/web/auth/auth_test.go

@@ -0,0 +1,43 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package auth
+
+import (
+	"net/http"
+	"net/url"
+	"testing"
+
+	"code.gitea.io/gitea/modules/test"
+	"code.gitea.io/gitea/services/contexttest"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestUserLogin(t *testing.T) {
+	ctx, resp := contexttest.MockContext(t, "/user/login")
+	SignIn(ctx)
+	assert.Equal(t, http.StatusOK, resp.Code)
+
+	ctx, resp = contexttest.MockContext(t, "/user/login")
+	ctx.IsSigned = true
+	SignIn(ctx)
+	assert.Equal(t, http.StatusSeeOther, resp.Code)
+	assert.Equal(t, "/", test.RedirectURL(resp))
+
+	ctx, resp = contexttest.MockContext(t, "/user/login?redirect_to=/other")
+	ctx.IsSigned = true
+	SignIn(ctx)
+	assert.Equal(t, "/other", test.RedirectURL(resp))
+
+	ctx, resp = contexttest.MockContext(t, "/user/login")
+	ctx.Req.AddCookie(&http.Cookie{Name: "redirect_to", Value: "/other-cookie"})
+	ctx.IsSigned = true
+	SignIn(ctx)
+	assert.Equal(t, "/other-cookie", test.RedirectURL(resp))
+
+	ctx, resp = contexttest.MockContext(t, "/user/login?redirect_to="+url.QueryEscape("https://example.com"))
+	ctx.IsSigned = true
+	SignIn(ctx)
+	assert.Equal(t, "/", test.RedirectURL(resp))
+}