Avoid using zstd.Encoder.EncodeAll, to reduce memory usage

Followup to #25.

internal/zstd/zstd.go

@@ -17,7 +17,6 @@
 package zstd
 
 import (
-	"bytes"
 	"errors"
 	"io"
 	"runtime"
@@ -48,21 +47,22 @@ type decoderWrapper struct {
 	*zstd.Decoder
 }
 
+type encoderWrapper struct {
+	*zstd.Encoder
+	pool *sync.Pool
+}
+
 type compressor struct {
-	encoder     *zstd.Encoder
-	decoderPool sync.Pool // To hold *zstd.Decoder's.
+	encoderPool sync.Pool
+	decoderPool sync.Pool
 }
 
 func PretendInit(clobbering bool) {
 	if !clobbering && encoding.GetCompressor(Name) != nil {
 		return
 	}
 
-	enc, _ := zstd.NewWriter(nil, encoderOptions...)
-	c := &compressor{
-		encoder: enc,
-	}
-	encoding.RegisterCompressor(c)
+	encoding.RegisterCompressor(&compressor{})
 }
 
 var ErrNotInUse = errors.New("SetLevel ineffective because another zstd compressor has been registered")
@@ -71,40 +71,42 @@ var ErrNotInUse = errors.New("SetLevel ineffective because another zstd compress
 // level. NOTE: this function must only be called from an init function, and
 // is not threadsafe.
 func SetLevel(level zstd.EncoderLevel) error {
-	c, ok := encoding.GetCompressor(Name).(*compressor)
+	_, ok := encoding.GetCompressor(Name).(*compressor)
 	if !ok {
 		return ErrNotInUse
 	}
 
-	enc, err := zstd.NewWriter(nil, zstd.WithEncoderLevel(level))
-	if err != nil {
-		return err
-	}
-
-	c.encoder = enc
+	encoderOptions = append(encoderOptions, zstd.WithEncoderLevel(level))
 	return nil
 }
 
 func (c *compressor) Compress(w io.Writer) (io.WriteCloser, error) {
-	return &zstdWriteCloser{
-		enc:    c.encoder,
-		writer: w,
-	}, nil
-}
+	var err error
+	var found bool
+	var encoder *zstd.Encoder
 
-type zstdWriteCloser struct {
-	enc    *zstd.Encoder
-	writer io.Writer    // Compressed data will be written here.
-	buf    bytes.Buffer // Buffer uncompressed data here, compress on Close.
-}
+	encoder, found = c.encoderPool.Get().(*zstd.Encoder)
+	if !found {
+		encoder, err = zstd.NewWriter(w, encoderOptions...)
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		encoder.Reset(w)
+	}
+
+	wrapper := &encoderWrapper{Encoder: encoder, pool: &c.encoderPool}
+	runtime.SetFinalizer(wrapper, func(ew *encoderWrapper) {
+		ew.Reset(nil)
+		c.encoderPool.Put(ew.Encoder)
+	})
 
-func (z *zstdWriteCloser) Write(p []byte) (int, error) {
-	return z.buf.Write(p)
+	return wrapper, nil
 }
 
-func (z *zstdWriteCloser) Close() error {
-	compressed := z.enc.EncodeAll(z.buf.Bytes(), nil)
-	_, err := io.Copy(z.writer, bytes.NewReader(compressed))
+func (w *encoderWrapper) Close() error {
+	err := w.Encoder.Close()
+	w.pool.Put(w.Encoder)
 	return err
 }