Add IsHedgedRequest (#19)

hedged.go

@@ -107,7 +107,7 @@ func (ht *hedgedTransport) RoundTrip(req *http.Request) (*http.Response, error)
 	for sent := 0; len(errOverall.Errors) < ht.upto; sent++ {
 		if sent < ht.upto {
 			idx := sent
-			subReq, cancel := reqWithCtx(req, mainCtx)
+			subReq, cancel := reqWithCtx(req, mainCtx, idx != 0)
 			cancels[idx] = cancel
 
 			runInPool(func() {
@@ -174,12 +174,23 @@ type indexedResp struct {
 	Resp  *http.Response
 }
 
-func reqWithCtx(r *http.Request, ctx context.Context) (*http.Request, func()) {
+func reqWithCtx(r *http.Request, ctx context.Context, isHedged bool) (*http.Request, func()) {
 	ctx, cancel := context.WithCancel(ctx)
+	if isHedged {
+		ctx = context.WithValue(ctx, hedgedRequest{}, struct{}{})
+	}
 	req := r.WithContext(ctx)
 	return req, cancel
 }
 
+type hedgedRequest struct{}
+
+// IsHedgedRequest reports when a request is hedged.
+func IsHedgedRequest(r *http.Request) bool {
+	val := r.Context().Value(hedgedRequest{})
+	return val != nil
+}
+
 // atomicCounter is a false sharing safe counter.
 type atomicCounter struct {
 	count uint64

hedged_test.go

@@ -3,6 +3,7 @@ package hedgedhttp_test
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -425,6 +426,42 @@ func TestCancelByClient(t *testing.T) {
 	}
 }
 
+func TestIsHedged(t *testing.T) {
+	var gotRequests int
+
+	rt := testRoundTripper(func(req *http.Request) (*http.Response, error) {
+		if gotRequests == 0 {
+			if hedgedhttp.IsHedgedRequest(req) {
+				t.Fatal("first request is hedged")
+			}
+		} else {
+			if !hedgedhttp.IsHedgedRequest(req) {
+				t.Fatalf("%d request is not hedged", gotRequests)
+			}
+		}
+		gotRequests++
+		return nil, errors.New("just an error")
+	})
+
+	req, err := http.NewRequest("GET", "http://no-matter-what", http.NoBody)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	const upto = 7
+	_, _ = hedgedhttp.NewRoundTripper(10*time.Millisecond, upto, rt).RoundTrip(req)
+
+	if gotRequests != upto {
+		t.Fatalf("want %v, got %v", upto, gotRequests)
+	}
+}
+
+type testRoundTripper func(req *http.Request) (*http.Response, error)
+
+func (t testRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	return t(req)
+}
+
 func checkAllMetricsAreZero(t *testing.T, metrics *hedgedhttp.Stats) {
 	expectExactMetricsAndSnapshot(t, metrics, hedgedhttp.StatsSnapshot{})
 }