strings: use sync.Pool to cache buffer

benchmark                         old ns/op    new ns/op    delta
BenchmarkByteReplacerWriteString       3596         3094  -13.96%

benchmark                        old allocs   new allocs    delta
BenchmarkByteReplacerWriteString          1            0  -100.00%

LGTM=dvyukov
R=bradfitz, dave, dvyukov
CC=golang-codereviews
https://golang.org/cl/101330053

2 files changed, 50 insertions, 25 deletions

diff --git a/src/pkg/strings/replace.go b/src/pkg/strings/replace.go
index cb9d7b1fa4..89aca95bae 100644
--- a/src/pkg/strings/replace.go
+++ b/src/pkg/strings/replace.go
@@ -4,7 +4,10 @@
 
 package strings
 
-import "io"
+import (
+	"io"
+	"sync"
+)
 
 // A Replacer replaces a list of strings with replacements.
 type Replacer struct {
@@ -451,27 +454,31 @@ func (r *byteReplacer) Replace(s string) string {
 	return string(buf)
 }
 
-func (r *byteReplacer) WriteString(w io.Writer, s string) (n int, err error) {
-	// TODO(bradfitz): use io.WriteString with slices of s, avoiding allocation.
-	bufsize := 32 << 10
-	if len(s) < bufsize {
-		bufsize = len(s)
-	}
-	buf := make([]byte, bufsize)
+var bufferPool = sync.Pool{
+	New: func() interface{} {
+		b := make([]byte, 4096)
+		return &b
+	},
+}
 
+func (r *byteReplacer) WriteString(w io.Writer, s string) (n int, err error) {
+	bp := bufferPool.Get().(*[]byte)
+	buf := *bp
 	for len(s) > 0 {
-		ncopy := copy(buf, s[:])
-		s = s[ncopy:]
+		ncopy := copy(buf, s)
 		for i, b := range buf[:ncopy] {
 			buf[i] = r.new[b]
 		}
-		wn, err := w.Write(buf[:ncopy])
+		s = s[ncopy:]
+		var wn int
+		wn, err = w.Write(buf[:ncopy])
 		n += wn
 		if err != nil {
-			return n, err
+			break
 		}
 	}
-	return n, nil
+	bufferPool.Put(bp)
+	return
 }
 
 // byteStringReplacer is the implementation that's used when all the
diff --git a/src/pkg/strings/replace_test.go b/src/pkg/strings/replace_test.go
index 2cb318b69d..77e48b988b 100644
--- a/src/pkg/strings/replace_test.go
+++ b/src/pkg/strings/replace_test.go
@@ -308,20 +308,21 @@ func TestReplacer(t *testing.T) {
 	}
 }
 
+var algorithmTestCases = []struct {
+	r    *Replacer
+	want string
+}{
+	{capitalLetters, "*strings.byteReplacer"},
+	{htmlEscaper, "*strings.byteStringReplacer"},
+	{NewReplacer("12", "123"), "*strings.singleStringReplacer"},
+	{NewReplacer("1", "12"), "*strings.byteStringReplacer"},
+	{NewReplacer("", "X"), "*strings.genericReplacer"},
+	{NewReplacer("a", "1", "b", "12", "cde", "123"), "*strings.genericReplacer"},
+}
+
 // TestPickAlgorithm tests that NewReplacer picks the correct algorithm.
 func TestPickAlgorithm(t *testing.T) {
-	testCases := []struct {
-		r    *Replacer
-		want string
-	}{
-		{capitalLetters, "*strings.byteReplacer"},
-		{htmlEscaper, "*strings.byteStringReplacer"},
-		{NewReplacer("12", "123"), "*strings.singleStringReplacer"},
-		{NewReplacer("1", "12"), "*strings.byteStringReplacer"},
-		{NewReplacer("", "X"), "*strings.genericReplacer"},
-		{NewReplacer("a", "1", "b", "12", "cde", "123"), "*strings.genericReplacer"},
-	}
-	for i, tc := range testCases {
+	for i, tc := range algorithmTestCases {
 		got := fmt.Sprintf("%T", tc.r.Replacer())
 		if got != tc.want {
 			t.Errorf("%d. algorithm = %s, want %s", i, got, tc.want)
@@ -329,6 +330,23 @@ func TestPickAlgorithm(t *testing.T) {
 	}
 }
 
+type errWriter struct{}
+
+func (errWriter) Write(p []byte) (n int, err error) {
+	return 0, fmt.Errorf("unwritable")
+}
+
+// TestWriteStringError tests that WriteString returns an error
+// received from the underlying io.Writer.
+func TestWriteStringError(t *testing.T) {
+	for i, tc := range algorithmTestCases {
+		n, err := tc.r.WriteString(errWriter{}, "abc")
+		if n != 0 || err == nil || err.Error() != "unwritable" {
+			t.Errorf("%d. WriteStringError = %d, %v, want 0, unwritable", i, n, err)
+		}
+	}
+}
+
 // TestGenericTrieBuilding verifies the structure of the generated trie. There
 // is one node per line, and the key ending with the current line is in the
 // trie if it ends with a "+".