1package middleware
2
3// Ported from Goji's middleware, source:
4// https://github.com/zenazn/goji/tree/master/web/middleware
5
6import (
7	"context"
8	"crypto/rand"
9	"encoding/base64"
10	"fmt"
11	"net/http"
12	"os"
13	"strings"
14	"sync/atomic"
15)
16
17// Key to use when setting the request ID.
18type ctxKeyRequestID int
19
20// RequestIDKey is the key that holds the unique request ID in a request context.
21const RequestIDKey ctxKeyRequestID = 0
22
23// RequestIDHeader is the name of the HTTP Header which contains the request id.
24// Exported so that it can be changed by developers
25var RequestIDHeader = "X-Request-Id"
26
27var prefix string
28var reqid uint64
29
30// A quick note on the statistics here: we're trying to calculate the chance that
31// two randomly generated base62 prefixes will collide. We use the formula from
32// http://en.wikipedia.org/wiki/Birthday_problem
33//
34// P[m, n] \approx 1 - e^{-m^2/2n}
35//
36// We ballpark an upper bound for $m$ by imagining (for whatever reason) a server
37// that restarts every second over 10 years, for $m = 86400 * 365 * 10 = 315360000$
38//
39// For a $k$ character base-62 identifier, we have $n(k) = 62^k$
40//
41// Plugging this in, we find $P[m, n(10)] \approx 5.75%$, which is good enough for
42// our purposes, and is surely more than anyone would ever need in practice -- a
43// process that is rebooted a handful of times a day for a hundred years has less
44// than a millionth of a percent chance of generating two colliding IDs.
45
46func init() {
47	hostname, err := os.Hostname()
48	if hostname == "" || err != nil {
49		hostname = "localhost"
50	}
51	var buf [12]byte
52	var b64 string
53	for len(b64) < 10 {
54		rand.Read(buf[:])
55		b64 = base64.StdEncoding.EncodeToString(buf[:])
56		b64 = strings.NewReplacer("+", "", "/", "").Replace(b64)
57	}
58
59	prefix = fmt.Sprintf("%s/%s", hostname, b64[0:10])
60}
61
62// RequestID is a middleware that injects a request ID into the context of each
63// request. A request ID is a string of the form "host.example.com/random-0001",
64// where "random" is a base62 random string that uniquely identifies this go
65// process, and where the last number is an atomically incremented request
66// counter.
67func RequestID(next http.Handler) http.Handler {
68	fn := func(w http.ResponseWriter, r *http.Request) {
69		ctx := r.Context()
70		requestID := r.Header.Get(RequestIDHeader)
71		if requestID == "" {
72			myid := atomic.AddUint64(&reqid, 1)
73			requestID = fmt.Sprintf("%s-%06d", prefix, myid)
74		}
75		ctx = context.WithValue(ctx, RequestIDKey, requestID)
76		next.ServeHTTP(w, r.WithContext(ctx))
77	}
78	return http.HandlerFunc(fn)
79}
80
81// GetReqID returns a request ID from the given context if one is present.
82// Returns the empty string if a request ID cannot be found.
83func GetReqID(ctx context.Context) string {
84	if ctx == nil {
85		return ""
86	}
87	if reqID, ok := ctx.Value(RequestIDKey).(string); ok {
88		return reqID
89	}
90	return ""
91}
92
93// NextRequestID generates the next request ID in the sequence.
94func NextRequestID() uint64 {
95	return atomic.AddUint64(&reqid, 1)
96}
97