zstd: Spawn decoder goroutine only if needed. (#380)

zstd/decoder.go

@@ -113,9 +113,6 @@ func NewReader(r io.Reader, opts ...DOption) (*Decoder, error) {
 // Returns the number of bytes written and any error that occurred.
 // When the stream is done, io.EOF will be returned.
 func (d *Decoder) Read(p []byte) (int, error) {
-	if d.stream == nil {
-		return 0, ErrDecoderNilInput
-	}
 	var n int
 	for {
 		if len(d.current.b) > 0 {
@@ -167,18 +164,15 @@ func (d *Decoder) Reset(r io.Reader) error {
 
 	if r == nil {
 		d.current.err = ErrDecoderNilInput
+		if len(d.current.b) > 0 {
+			d.current.b = d.current.b[:0]
+		}
 		d.current.flushed = true
 		return nil
 	}
 
-	if d.stream == nil {
-		d.stream = make(chan decodeStream, 1)
-		d.streamWg.Add(1)
-		go d.startStreamDecoder(d.stream)
-	}
-
-	// If bytes buffer and < 1MB, do sync decoding anyway.
-	if bb, ok := r.(byter); ok && bb.Len() < 1<<20 {
+	// If bytes buffer and < 5MB, do sync decoding anyway.
+	if bb, ok := r.(byter); ok && bb.Len() < 5<<20 {
 		bb2 := bb
 		if debugDecoder {
 			println("*bytes.Buffer detected, doing sync decode, len:", bb.Len())
@@ -202,6 +196,12 @@ func (d *Decoder) Reset(r io.Reader) error {
 		return nil
 	}
 
+	if d.stream == nil {
+		d.stream = make(chan decodeStream, 1)
+		d.streamWg.Add(1)
+		go d.startStreamDecoder(d.stream)
+	}
+
 	// Remove current block.
 	d.current.decodeOutput = decodeOutput{}
 	d.current.err = nil
@@ -255,9 +255,6 @@ func (d *Decoder) drainOutput() {
 // The return value n is the number of bytes written.
 // Any error encountered during the write is also returned.
 func (d *Decoder) WriteTo(w io.Writer) (int64, error) {
-	if d.stream == nil {
-		return 0, ErrDecoderNilInput
-	}
 	var n int64
 	for {
 		if len(d.current.b) > 0 {