net: fix probabilities in DNS SRV shuffleByWeight

Patch from msolo. Just moving it to a CL.
The test fails before and passes with the fix.

Fixes #7098

LGTM=msolo, rsc
R=rsc, iant, msolo
CC=golang-codereviews
https://golang.org/cl/88900044

src/pkg/net/dnsclient.go

@@ -191,10 +191,10 @@ func (addrs byPriorityWeight) shuffleByWeight() {
 	}
 	for sum > 0 && len(addrs) > 1 {
 		s := 0
-		n := rand.Intn(sum + 1)
+		n := rand.Intn(sum)
 		for i := range addrs {
 			s += int(addrs[i].Weight)
-			if s >= n {
+			if s > n {
 				if i > 0 {
 					t := addrs[i]
 					copy(addrs[1:i+1], addrs[0:i])

src/pkg/net/dnsclient_test.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package net
+
+import (
+	"math/rand"
+	"testing"
+)
+
+func checkDistribution(t *testing.T, data []*SRV, margin float64) {
+	sum := 0
+	for _, srv := range data {
+		sum += int(srv.Weight)
+	}
+
+	results := make(map[string]int)
+
+	count := 1000
+	for j := 0; j < count; j++ {
+		d := make([]*SRV, len(data))
+		copy(d, data)
+		byPriorityWeight(d).shuffleByWeight()
+		key := d[0].Target
+		results[key] = results[key] + 1
+	}
+
+	actual := results[data[0].Target]
+	expected := float64(count) * float64(data[0].Weight) / float64(sum)
+	diff := float64(actual) - expected
+	t.Logf("actual: %v diff: %v e: %v m: %v", actual, diff, expected, margin)
+	if diff < 0 {
+		diff = -diff
+	}
+	if diff > (expected * margin) {
+		t.Errorf("missed target weight: expected %v, %v", expected, actual)
+	}
+}
+
+func testUniformity(t *testing.T, size int, margin float64) {
+	rand.Seed(1)
+	data := make([]*SRV, size)
+	for i := 0; i < size; i++ {
+		data[i] = &SRV{Target: string('a' + i), Weight: 1}
+	}
+	checkDistribution(t, data, margin)
+}
+
+func TestUniformity(t *testing.T) {
+	testUniformity(t, 2, 0.05)
+	testUniformity(t, 3, 0.10)
+	testUniformity(t, 10, 0.20)
+	testWeighting(t, 0.05)
+}
+
+func testWeighting(t *testing.T, margin float64) {
+	rand.Seed(1)
+	data := []*SRV{
+		{Target: "a", Weight: 60},
+		{Target: "b", Weight: 30},
+		{Target: "c", Weight: 10},
+	}
+	checkDistribution(t, data, margin)
+}
+
+func TestWeighting(t *testing.T) {
+	testWeighting(t, 0.05)
+}