bytes: deflake TestGrow by using testing.AllocsPerRun

Fixes #36695

Change-Id: I4392246015252018b49f321a5a839cc68cc611d7
GitHub-Last-Rev: c2fb1f7
GitHub-Pull-Request: #36732
Reviewed-on: https://go-review.googlesource.com/c/go/+/216237
Reviewed-by: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: howjmay

src/bytes/buffer_test.go

@@ -8,7 +8,6 @@ import (
 	. "bytes"
 	"io"
 	"math/rand"
-	"runtime"
 	"testing"
 	"unicode/utf8"
 )
@@ -495,20 +494,20 @@ func TestGrow(t *testing.T) {
 	x := []byte{'x'}
 	y := []byte{'y'}
 	tmp := make([]byte, 72)
-	for _, startLen := range []int{0, 100, 1000, 10000, 100000} {
-		xBytes := Repeat(x, startLen)
-		for _, growLen := range []int{0, 100, 1000, 10000, 100000} {
+	for _, growLen := range []int{0, 100, 1000, 10000, 100000} {
+		for _, startLen := range []int{0, 100, 1000, 10000, 100000} {
+			xBytes := Repeat(x, startLen)
+
 			buf := NewBuffer(xBytes)
 			// If we read, this affects buf.off, which is good to test.
 			readBytes, _ := buf.Read(tmp)
-			buf.Grow(growLen)
 			yBytes := Repeat(y, growLen)
+			allocs := testing.AllocsPerRun(100, func() {
+				buf.Grow(growLen)
+				buf.Write(yBytes)
+			})
 			// Check no allocation occurs in write, as long as we're single-threaded.
-			var m1, m2 runtime.MemStats
-			runtime.ReadMemStats(&m1)
-			buf.Write(yBytes)
-			runtime.ReadMemStats(&m2)
-			if runtime.GOMAXPROCS(-1) == 1 && m1.Mallocs != m2.Mallocs {
+			if allocs != 0 {
 				t.Errorf("allocation occurred during write")
 			}
 			// Check that buffer has correct data.