net: remove dependency on math/rand

Like we did for sync, let the runtime give net random numbers,
to avoid forcing an import of math/rand for DNS.

Change-Id: Iab3e64121d687d288a3961a8ccbcebe589047253
Reviewed-on: https://go-review.googlesource.com/c/go/+/241258
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/build/deps_test.go

@@ -318,7 +318,6 @@ var depsRules = `
 	# so large dependencies must be kept out.
 	# This is a long-looking list but most of these
 	# are small with few dependencies.
-	# math/rand should probably be removed at some point.
 	CGO,
 	golang.org/x/net/dns/dnsmessage,
 	golang.org/x/net/lif,
@@ -327,11 +326,11 @@ var depsRules = `
 	internal/poll,
 	internal/singleflight,
 	internal/race,
-	math/rand,
 	os
 	< net;
 
 	fmt, unicode !< net;
+	math/rand !< net; # net uses runtime instead
 
 	# NET is net plus net-helper packages.
 	FMT, net
@@ -479,7 +478,7 @@ var depsRules = `
 	CGO, OS, fmt
 	< os/signal/internal/pty;
 
-	NET, testing
+	NET, testing, math/rand
 	< golang.org/x/net/nettest;
 
 	FMT, container/heap, math/rand

src/net/dnsclient.go

@@ -5,12 +5,25 @@
 package net
 
 import (
-	"math/rand"
 	"sort"
 
 	"golang.org/x/net/dns/dnsmessage"
 )
 
+// provided by runtime
+func fastrand() uint32
+
+func randInt() int {
+	x, y := fastrand(), fastrand()    // 32-bit halves
+	u := uint(x)<<31 ^ uint(int32(y)) // full uint, even on 64-bit systems; avoid 32-bit shift on 32-bit systems
+	i := int(u >> 1)                  // clear sign bit, even on 32-bit systems
+	return i
+}
+
+func randIntn(n int) int {
+	return randInt() % n
+}
+
 // reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
 // address addr suitable for rDNS (PTR) record lookup or an error if it fails
 // to parse the IP address.
@@ -162,7 +175,7 @@ func (addrs byPriorityWeight) shuffleByWeight() {
 	}
 	for sum > 0 && len(addrs) > 1 {
 		s := 0
-		n := rand.Intn(sum)
+		n := randIntn(sum)
 		for i := range addrs {
 			s += int(addrs[i].Weight)
 			if s > n {
@@ -206,7 +219,7 @@ func (s byPref) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
 // sort reorders MX records as specified in RFC 5321.
 func (s byPref) sort() {
 	for i := range s {
-		j := rand.Intn(i + 1)
+		j := randIntn(i + 1)
 		s[i], s[j] = s[j], s[i]
 	}
 	sort.Sort(s)

src/net/dnsclient_test.go

@@ -5,7 +5,6 @@
 package net
 
 import (
-	"math/rand"
 	"testing"
 )
 
@@ -17,7 +16,7 @@ func checkDistribution(t *testing.T, data []*SRV, margin float64) {
 
 	results := make(map[string]int)
 
-	count := 1000
+	count := 10000
 	for j := 0; j < count; j++ {
 		d := make([]*SRV, len(data))
 		copy(d, data)
@@ -39,7 +38,6 @@ func checkDistribution(t *testing.T, data []*SRV, margin float64) {
 }
 
 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' + rune(i)), Weight: 1}
@@ -55,7 +53,6 @@ func TestDNSSRVUniformity(t *testing.T) {
 }
 
 func testWeighting(t *testing.T, margin float64) {
-	rand.Seed(1)
 	data := []*SRV{
 		{Target: "a", Weight: 60},
 		{Target: "b", Weight: 30},

src/net/dnsclient_unix.go

@@ -18,7 +18,6 @@ import (
 	"context"
 	"errors"
 	"io"
-	"math/rand"
 	"os"
 	"sync"
 	"time"
@@ -47,7 +46,7 @@ var (
 )
 
 func newRequest(q dnsmessage.Question) (id uint16, udpReq, tcpReq []byte, err error) {
-	id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
+	id = uint16(randInt())
 	b := dnsmessage.NewBuilder(make([]byte, 2, 514), dnsmessage.Header{ID: id, RecursionDesired: true})
 	b.EnableCompression()
 	if err := b.StartQuestions(); err != nil {

src/runtime/stubs.go

@@ -130,6 +130,9 @@ func fastrandn(n uint32) uint32 {
 //go:linkname sync_fastrand sync.fastrand
 func sync_fastrand() uint32 { return fastrand() }
 
+//go:linkname net_fastrand net.fastrand
+func net_fastrand() uint32 { return fastrand() }
+
 // in internal/bytealg/equal_*.s
 //go:noescape
 func memequal(a, b unsafe.Pointer, size uintptr) bool