Refactor external URL detection (#29973)

Follow #29960, `IsExternalURL` is not needed anymore. 
Add some tests for `RedirectToCurrentSite`

services/context/context_test.go

@@ -6,9 +6,11 @@ package context
 import (
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"testing"
 
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -22,3 +24,28 @@ func TestRemoveSessionCookieHeader(t *testing.T) {
 	assert.Len(t, w.Header().Values("Set-Cookie"), 1)
 	assert.Contains(t, "other=bar", w.Header().Get("Set-Cookie"))
 }
+
+func TestRedirectToCurrentSite(t *testing.T) {
+	defer test.MockVariableValue(&setting.AppURL, "http://localhost:3000/sub/")()
+	defer test.MockVariableValue(&setting.AppSubURL, "/sub")()
+	cases := []struct {
+		location string
+		want     string
+	}{
+		{"/", "/sub/"},
+		{"http://localhost:3000/sub?k=v", "http://localhost:3000/sub?k=v"},
+		{"http://other", "/sub/"},
+	}
+	for _, c := range cases {
+		t.Run(c.location, func(t *testing.T) {
+			req := &http.Request{URL: &url.URL{Path: "/"}}
+			resp := httptest.NewRecorder()
+			base, baseCleanUp := NewBaseContext(resp, req)
+			defer baseCleanUp()
+			ctx := NewWebContext(base, nil, nil)
+			ctx.RedirectToCurrentSite(c.location)
+			redirect := test.RedirectURL(resp)
+			assert.Equal(t, c.want, redirect)
+		})
+	}
+}