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96 lines
2.9 KiB
96 lines
2.9 KiB
package middleware
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// Ported from Goji's middleware, source:
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// https://github.com/zenazn/goji/tree/master/web/middleware
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import (
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"context"
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"crypto/rand"
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"encoding/base64"
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"fmt"
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"net/http"
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"os"
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"strings"
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"sync/atomic"
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)
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// Key to use when setting the request ID.
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type ctxKeyRequestID int
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// RequestIDKey is the key that holds the unique request ID in a request context.
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const RequestIDKey ctxKeyRequestID = 0
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// RequestIDHeader is the name of the HTTP Header which contains the request id.
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// Exported so that it can be changed by developers
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var RequestIDHeader = "X-Request-Id"
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var prefix string
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var reqid uint64
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// A quick note on the statistics here: we're trying to calculate the chance that
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// two randomly generated base62 prefixes will collide. We use the formula from
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// http://en.wikipedia.org/wiki/Birthday_problem
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//
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// P[m, n] \approx 1 - e^{-m^2/2n}
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//
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// We ballpark an upper bound for $m$ by imagining (for whatever reason) a server
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// that restarts every second over 10 years, for $m = 86400 * 365 * 10 = 315360000$
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//
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// For a $k$ character base-62 identifier, we have $n(k) = 62^k$
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//
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// Plugging this in, we find $P[m, n(10)] \approx 5.75%$, which is good enough for
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// our purposes, and is surely more than anyone would ever need in practice -- a
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// process that is rebooted a handful of times a day for a hundred years has less
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// than a millionth of a percent chance of generating two colliding IDs.
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func init() {
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hostname, err := os.Hostname()
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if hostname == "" || err != nil {
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hostname = "localhost"
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}
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var buf [12]byte
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var b64 string
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for len(b64) < 10 {
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rand.Read(buf[:])
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b64 = base64.StdEncoding.EncodeToString(buf[:])
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b64 = strings.NewReplacer("+", "", "/", "").Replace(b64)
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}
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prefix = fmt.Sprintf("%s/%s", hostname, b64[0:10])
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}
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// RequestID is a middleware that injects a request ID into the context of each
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// request. A request ID is a string of the form "host.example.com/random-0001",
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// where "random" is a base62 random string that uniquely identifies this go
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// process, and where the last number is an atomically incremented request
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// counter.
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func RequestID(next http.Handler) http.Handler {
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fn := func(w http.ResponseWriter, r *http.Request) {
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ctx := r.Context()
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requestID := r.Header.Get(RequestIDHeader)
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if requestID == "" {
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myid := atomic.AddUint64(&reqid, 1)
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requestID = fmt.Sprintf("%s-%06d", prefix, myid)
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}
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ctx = context.WithValue(ctx, RequestIDKey, requestID)
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next.ServeHTTP(w, r.WithContext(ctx))
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}
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return http.HandlerFunc(fn)
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}
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// GetReqID returns a request ID from the given context if one is present.
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// Returns the empty string if a request ID cannot be found.
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func GetReqID(ctx context.Context) string {
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if ctx == nil {
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return ""
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}
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if reqID, ok := ctx.Value(RequestIDKey).(string); ok {
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return reqID
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}
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return ""
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}
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// NextRequestID generates the next request ID in the sequence.
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func NextRequestID() uint64 {
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return atomic.AddUint64(&reqid, 1)
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}
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