Improve URL validation for external wiki and external issues (#4710)

* Improve URL validation for external wiki  and external issues

* Do not allow also localhost address for external URLs

modules/validation/helpers_test.go

@@ -0,0 +1,90 @@
+// Copyright 2018 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package validation
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+func Test_IsValidURL(t *testing.T) {
+	cases := []struct {
+		description string
+		url         string
+		valid       bool
+	}{
+		{
+			description: "Empty URL",
+			url:         "",
+			valid:       false,
+		},
+		{
+			description: "Loobpack IPv4 URL",
+			url:         "http://127.0.1.1:5678/",
+			valid:       true,
+		},
+		{
+			description: "Loobpack IPv6 URL",
+			url:         "https://[::1]/",
+			valid:       true,
+		},
+		{
+			description: "Missing semicolon after schema",
+			url:         "http//meh/",
+			valid:       false,
+		},
+	}
+
+	for _, testCase := range cases {
+		t.Run(testCase.description, func(t *testing.T) {
+			assert.Equal(t, testCase.valid, IsValidURL(testCase.url))
+		})
+	}
+}
+
+func Test_IsValidExternalURL(t *testing.T) {
+	setting.AppURL = "https://try.gitea.io/"
+
+	cases := []struct {
+		description string
+		url         string
+		valid       bool
+	}{
+		{
+			description: "Current instance URL",
+			url:         "https://try.gitea.io/test",
+			valid:       true,
+		},
+		{
+			description: "Loobpack IPv4 URL",
+			url:         "http://127.0.1.1:5678/",
+			valid:       false,
+		},
+		{
+			description: "Current instance API URL",
+			url:         "https://try.gitea.io/api/v1/user/follow",
+			valid:       false,
+		},
+		{
+			description: "Local network URL",
+			url:         "http://192.168.1.2/api/v1/user/follow",
+			valid:       true,
+		},
+		{
+			description: "Local URL",
+			url:         "http://LOCALHOST:1234/whatever",
+			valid:       false,
+		},
+	}
+
+	for _, testCase := range cases {
+		t.Run(testCase.description, func(t *testing.T) {
+			assert.Equal(t, testCase.valid, IsValidExternalURL(testCase.url))
+		})
+	}
+}