encoding/json: use sync.Pool

Benchmark is within the noise. I had to run this a dozen times
each before & after (on wall power, without a browser running)
before I could get halfway consistent numbers, and even then
they jumped all over the place, with the new one sometimes
being better. But these are the best of a dozen each.

Slowdown is expected anyway, since I imagine channels are
optimized more.

benchmark                 old ns/op    new ns/op    delta
BenchmarkCodeEncoder       26556987     27291072   +2.76%
BenchmarkEncoderEncode         1069         1071   +0.19%

benchmark                  old MB/s     new MB/s  speedup
BenchmarkCodeEncoder          73.07        71.10    0.97x

benchmark                old allocs   new allocs    delta
BenchmarkEncoderEncode            2            2    0.00%

benchmark                 old bytes    new bytes    delta
BenchmarkEncoderEncode          221          221    0.00%

Update #4720

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/37720047

src/pkg/encoding/json/encode.go

@@ -241,24 +241,15 @@ type encodeState struct {
 	scratch      [64]byte
 }
 
-// TODO(bradfitz): use a sync.Cache here
-var encodeStatePool = make(chan *encodeState, 8)
+var encodeStatePool sync.Pool
 
 func newEncodeState() *encodeState {
-	select {
-	case e := <-encodeStatePool:
+	if v := encodeStatePool.Get(); v != nil {
+		e := v.(*encodeState)
 		e.Reset()
 		return e
-	default:
-		return new(encodeState)
-	}
-}
-
-func putEncodeState(e *encodeState) {
-	select {
-	case encodeStatePool <- e:
-	default:
 	}
+	return new(encodeState)
 }
 
 func (e *encodeState) marshal(v interface{}) (err error) {

src/pkg/encoding/json/stream.go

@@ -173,7 +173,7 @@ func (enc *Encoder) Encode(v interface{}) error {
 	if _, err = enc.w.Write(e.Bytes()); err != nil {
 		enc.err = err
 	}
-	putEncodeState(e)
+	encodeStatePool.Put(e)
 	return err
 }