strings: fix flaky TestBuilderGrow test

Fixes #24647

Change-Id: I79c2b45cf7fc9c0ed0c7a665472556bd248e7584
Reviewed-on: https://go-review.googlesource.com/104235
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

diff --git a/src/strings/builder_test.go b/src/strings/builder_test.go
index 29d49e98a72effd9c9507edd78df48e9296dbe4c..949f214619d5e4a9d2d63d709e0c873d47421f38 100644 (file)
--- a/src/strings/builder_test.go
+++ b/src/strings/builder_test.go
@@ -6,7 +6,6 @@ package strings_test
 
 import (
        "bytes"
-       "runtime"
        . "strings"
        "testing"
 )
@@ -86,15 +85,21 @@ func TestBuilderReset(t *testing.T) {
 
 func TestBuilderGrow(t *testing.T) {
        for _, growLen := range []int{0, 100, 1000, 10000, 100000} {
-               var b Builder
-               b.Grow(growLen)
                p := bytes.Repeat([]byte{'a'}, growLen)
-               allocs := numAllocs(func() { b.Write(p) })
-               if allocs > 0 {
-                       t.Errorf("growLen=%d: allocation occurred during write", growLen)
+               allocs := testing.AllocsPerRun(100, func() {
+                       var b Builder
+                       b.Grow(growLen) // should be only alloc, when growLen > 0
+                       b.Write(p)
+                       if b.String() != string(p) {
+                               t.Fatalf("growLen=%d: bad data written after Grow", growLen)
+                       }
+               })
+               wantAllocs := 1
+               if growLen == 0 {
+                       wantAllocs = 0
                }
-               if b.String() != string(p) {
-                       t.Errorf("growLen=%d: bad data written after Grow", growLen)
+               if g, w := int(allocs), wantAllocs; g != w {
+                       t.Errorf("growLen=%d: got %d allocs during Write; want %v", growLen, g, w)
                }
        }
 }
@@ -168,13 +173,14 @@ func TestBuilderWriteByte(t *testing.T) {
 
 func TestBuilderAllocs(t *testing.T) {
        var b Builder
-       b.Grow(5)
+       const msg = "hello"
+       b.Grow(len(msg) * 2) // because AllocsPerRun does an extra "warm-up" iteration
        var s string
-       allocs := numAllocs(func() {
+       allocs := int(testing.AllocsPerRun(1, func() {
                b.WriteString("hello")
                s = b.String()
-       })
-       if want := "hello"; s != want {
+       }))
+       if want := msg + msg; s != want {
                t.Errorf("String: got %#q; want %#q", s, want)
        }
        if allocs > 0 {
@@ -194,15 +200,6 @@ func TestBuilderAllocs(t *testing.T) {
        }
 }
 
-func numAllocs(fn func()) uint64 {
-       defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
-       var m1, m2 runtime.MemStats
-       runtime.ReadMemStats(&m1)
-       fn()
-       runtime.ReadMemStats(&m2)
-       return m2.Mallocs - m1.Mallocs
-}
-
 func TestBuilderCopyPanic(t *testing.T) {
        tests := []struct {
                name      string