archive/zip: pool flate readers

Similar to the flate Writer pools already used,
this adds pooling for flate Readers.

compress/flate allows re-using of Readers, see
https://codereview.appspot.com/97140043/

In a real-world scenario when reading ~ 500 small files from a ZIP
archive this gives a speedup of 1.5x-2x.

Fixes #14289

Change-Id: I2d98ad983e95ab7d97e06fd0145f619b4f47caa4
Reviewed-on: https://go-review.googlesource.com/19416
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/archive/zip/register.go

@@ -64,6 +64,44 @@ func (w *pooledFlateWriter) Close() error {
 	return err
 }
 
+var flateReaderPool sync.Pool
+
+func newFlateReader(r io.Reader) io.ReadCloser {
+	fr, ok := flateReaderPool.Get().(io.ReadCloser)
+	if ok {
+		fr.(flate.Resetter).Reset(r, nil)
+	} else {
+		fr = flate.NewReader(r)
+	}
+	return &pooledFlateReader{fr: fr}
+}
+
+type pooledFlateReader struct {
+	mu sync.Mutex // guards Close and Read
+	fr io.ReadCloser
+}
+
+func (r *pooledFlateReader) Read(p []byte) (n int, err error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.fr == nil {
+		return 0, errors.New("Read after Close")
+	}
+	return r.fr.Read(p)
+}
+
+func (r *pooledFlateReader) Close() error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	var err error
+	if r.fr != nil {
+		err = r.fr.Close()
+		flateReaderPool.Put(r.fr)
+		r.fr = nil
+	}
+	return err
+}
+
 var (
 	mu sync.RWMutex // guards compressor and decompressor maps
 
@@ -74,7 +112,7 @@ var (
 
 	decompressors = map[uint16]Decompressor{
 		Store:   ioutil.NopCloser,
-		Deflate: flate.NewReader,
+		Deflate: newFlateReader,
 	}
 )