Move context from modules to services (#29440)

Since `modules/context` has to depend on `models` and many other
packages, it should be moved from `modules/context` to
`services/context` according to design principles. There is no logic
code change on this PR, only move packages.

- Move `code.gitea.io/gitea/modules/context` to
`code.gitea.io/gitea/services/context`
- Move `code.gitea.io/gitea/modules/contexttest` to
`code.gitea.io/gitea/services/contexttest` because of depending on
context
- Move `code.gitea.io/gitea/modules/upload` to
`code.gitea.io/gitea/services/context/upload` because of depending on
context

services/context/xsrf.go

@@ -0,0 +1,99 @@
+// Copyright 2012 Google Inc. All Rights Reserved.
+// Copyright 2014 The Macaron Authors
+// Copyright 2020 The Gitea Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// SPDX-License-Identifier: Apache-2.0
+
+package context
+
+import (
+	"bytes"
+	"crypto/hmac"
+	"crypto/sha1"
+	"crypto/subtle"
+	"encoding/base64"
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// CsrfTokenTimeout represents the duration that XSRF tokens are valid.
+// It is exported so clients may set cookie timeouts that match generated tokens.
+const CsrfTokenTimeout = 24 * time.Hour
+
+// CsrfTokenRegenerationInterval is the interval between token generations, old tokens are still valid before CsrfTokenTimeout
+var CsrfTokenRegenerationInterval = 10 * time.Minute
+
+var csrfTokenSep = []byte(":")
+
+// GenerateCsrfToken returns a URL-safe secure XSRF token that expires in CsrfTokenTimeout hours.
+// key is a secret key for your application.
+// userID is a unique identifier for the user.
+// actionID is the action the user is taking (e.g. POSTing to a particular path).
+func GenerateCsrfToken(key, userID, actionID string, now time.Time) string {
+	nowUnixNano := now.UnixNano()
+	nowUnixNanoStr := strconv.FormatInt(nowUnixNano, 10)
+	h := hmac.New(sha1.New, []byte(key))
+	h.Write([]byte(strings.ReplaceAll(userID, ":", "_")))
+	h.Write(csrfTokenSep)
+	h.Write([]byte(strings.ReplaceAll(actionID, ":", "_")))
+	h.Write(csrfTokenSep)
+	h.Write([]byte(nowUnixNanoStr))
+	tok := fmt.Sprintf("%s:%s", h.Sum(nil), nowUnixNanoStr)
+	return base64.RawURLEncoding.EncodeToString([]byte(tok))
+}
+
+func ParseCsrfToken(token string) (issueTime time.Time, ok bool) {
+	data, err := base64.RawURLEncoding.DecodeString(token)
+	if err != nil {
+		return time.Time{}, false
+	}
+
+	pos := bytes.LastIndex(data, csrfTokenSep)
+	if pos == -1 {
+		return time.Time{}, false
+	}
+	nanos, err := strconv.ParseInt(string(data[pos+1:]), 10, 64)
+	if err != nil {
+		return time.Time{}, false
+	}
+	return time.Unix(0, nanos), true
+}
+
+// ValidCsrfToken returns true if token is a valid and unexpired token returned by Generate.
+func ValidCsrfToken(token, key, userID, actionID string, now time.Time) bool {
+	issueTime, ok := ParseCsrfToken(token)
+	if !ok {
+		return false
+	}
+
+	// Check that the token is not expired.
+	if now.Sub(issueTime) >= CsrfTokenTimeout {
+		return false
+	}
+
+	// Check that the token is not from the future.
+	// Allow 1-minute grace period in case the token is being verified on a
+	// machine whose clock is behind the machine that issued the token.
+	if issueTime.After(now.Add(1 * time.Minute)) {
+		return false
+	}
+
+	expected := GenerateCsrfToken(key, userID, actionID, issueTime)
+
+	// Check that the token matches the expected value.
+	// Use constant time comparison to avoid timing attacks.
+	return subtle.ConstantTimeCompare([]byte(token), []byte(expected)) == 1
+}