1. 1
2. 2
3. 3
4. 4
5. 5
6. 6
7. 7
8. 8
9. 9
10. 10
11. 11
12. 12
13. 13
14. 14
15. 15
16. 16
17. 17
18. 18
19. 19
20. 20
21. 21
22. 22
23. 23
24. 24
25. 25
26. 26
27. 27
28. 28
29. 29
30. 30
31. 31
32. 32
33. 33
34. 34
35. 35
36. 36
37. 37
38. 38
39. 39
40. 40
41. 41
42. 42
43. 43
44. 44
45. 45
46. 46
47. 47
48. 48
49. 49
50. 50
51. 51
52. 52
53. 53
54. 54
55. 55
56. 56
57. 57
58. 58
59. 59
60. 60
61. 61
62. 62
63. 63
64. 64
65. 65
66. 66
67. 67
68. 68
69. 69
70. 70
71. 71
72. 72
73. 73
74. 74
75. 75
76. 76
77. 77
78. 78
79. 79
80. 80
81. 81
82. 82
83. 83
84. 84
85. 85
86. 86
87. 87
88. 88
89. 89
90. 90
91. 91
92. 92
93. 93
94. 94
95. 95
96. 96
97. 97
98. 98
99. 99
100. 100
101. 101
102. 102
103. 103
104. 104
105. 105
106. 106
107. 107
108. 108
109. 109
110. 110
111. 111
112. 112
113. 113
114. 114
115. 115
116. 116
117. 117
118. 118
119. 119
120. 120
121. 121
122. 122
123. 123
124. 124
125. 125
126. 126
127. 127
128. 128
129. 129
130. 130
131. 131
132. 132

package middleware

import (
	"net/http"
	"strconv"
	"time"
)

const (
	errCapacityExceeded = "Server capacity exceeded."
	errTimedOut         = "Timed out while waiting for a pending request to complete."
	errContextCanceled  = "Context was canceled."
)

var (
	defaultBacklogTimeout = time.Second * 60
)

// ThrottleOpts represents a set of throttling options.
type ThrottleOpts struct {
	RetryAfterFn   func(ctxDone bool) time.Duration
	Limit          int
	BacklogLimit   int
	BacklogTimeout time.Duration
}

// Throttle is a middleware that limits number of currently processed requests
// at a time across all users. Note: Throttle is not a rate-limiter per user,
// instead it just puts a ceiling on the number of currentl in-flight requests
// being processed from the point from where the Throttle middleware is mounted.
func Throttle(limit int) func(http.Handler) http.Handler {
	return ThrottleWithOpts(ThrottleOpts{Limit: limit, BacklogTimeout: defaultBacklogTimeout})
}

// ThrottleBacklog is a middleware that limits number of currently processed
// requests at a time and provides a backlog for holding a finite number of
// pending requests.
func ThrottleBacklog(limit, backlogLimit int, backlogTimeout time.Duration) func(http.Handler) http.Handler {
	return ThrottleWithOpts(ThrottleOpts{Limit: limit, BacklogLimit: backlogLimit, BacklogTimeout: backlogTimeout})
}

// ThrottleWithOpts is a middleware that limits number of currently processed requests using passed ThrottleOpts.
func ThrottleWithOpts(opts ThrottleOpts) func(http.Handler) http.Handler {
	if opts.Limit < 1 {
		panic("chi/middleware: Throttle expects limit > 0")
	}

	if opts.BacklogLimit < 0 {
		panic("chi/middleware: Throttle expects backlogLimit to be positive")
	}

	t := throttler{
		tokens:         make(chan token, opts.Limit),
		backlogTokens:  make(chan token, opts.Limit+opts.BacklogLimit),
		backlogTimeout: opts.BacklogTimeout,
		retryAfterFn:   opts.RetryAfterFn,
	}

	// Filling tokens.
	for i := 0; i < opts.Limit+opts.BacklogLimit; i++ {
		if i < opts.Limit {
			t.tokens <- token{}
		}
		t.backlogTokens <- token{}
	}

	return func(next http.Handler) http.Handler {
		fn := func(w http.ResponseWriter, r *http.Request) {
			ctx := r.Context()

			select {

			case <-ctx.Done():
				t.setRetryAfterHeaderIfNeeded(w, true)
				http.Error(w, errContextCanceled, http.StatusTooManyRequests)
				return

			case btok := <-t.backlogTokens:
				timer := time.NewTimer(t.backlogTimeout)

				defer func() {
					t.backlogTokens <- btok
				}()

				select {
				case <-timer.C:
					t.setRetryAfterHeaderIfNeeded(w, false)
					http.Error(w, errTimedOut, http.StatusTooManyRequests)
					return
				case <-ctx.Done():
					timer.Stop()
					t.setRetryAfterHeaderIfNeeded(w, true)
					http.Error(w, errContextCanceled, http.StatusTooManyRequests)
					return
				case tok := <-t.tokens:
					defer func() {
						timer.Stop()
						t.tokens <- tok
					}()
					next.ServeHTTP(w, r)
				}
				return

			default:
				t.setRetryAfterHeaderIfNeeded(w, false)
				http.Error(w, errCapacityExceeded, http.StatusTooManyRequests)
				return
			}
		}

		return http.HandlerFunc(fn)
	}
}

// token represents a request that is being processed.
type token struct{}

// throttler limits number of currently processed requests at a time.
type throttler struct {
	tokens         chan token
	backlogTokens  chan token
	retryAfterFn   func(ctxDone bool) time.Duration
	backlogTimeout time.Duration
}

// setRetryAfterHeaderIfNeeded sets Retry-After HTTP header if corresponding retryAfterFn option of throttler is initialized.
func (t throttler) setRetryAfterHeaderIfNeeded(w http.ResponseWriter, ctxDone bool) {
	if t.retryAfterFn == nil {
		return
	}
	w.Header().Set("Retry-After", strconv.Itoa(int(t.retryAfterFn(ctxDone).Seconds())))
}