zstd: free Decoder resources when Reset is called with a nil io.Reader (

#305)

* zstd: free Decoder resources when Reset is called with a nil io.Reader
* Return a sentinel error when trying to read from a nil reader.
* Return the same error if the Decoder was created with a nil reader.

Fixes #296.

zstd/decoder.go

@@ -85,6 +85,10 @@ func NewReader(r io.Reader, opts ...DOption) (*Decoder, error) {
 	d.current.output = make(chan decodeOutput, d.o.concurrent)
 	d.current.flushed = true
 
+	if r == nil {
+		d.current.err = ErrDecoderNilInput
+	}
+
 	// Transfer option dicts.
 	d.dicts = make(map[uint32]dict, len(d.o.dicts))
 	for _, dc := range d.o.dicts {
@@ -111,7 +115,7 @@ func NewReader(r io.Reader, opts ...DOption) (*Decoder, error) {
 // When the stream is done, io.EOF will be returned.
 func (d *Decoder) Read(p []byte) (int, error) {
 	if d.stream == nil {
-		return 0, errors.New("no input has been initialized")
+		return 0, ErrDecoderNilInput
 	}
 	var n int
 	for {
@@ -152,12 +156,20 @@ func (d *Decoder) Read(p []byte) (int, error) {
 
 // Reset will reset the decoder the supplied stream after the current has finished processing.
 // Note that this functionality cannot be used after Close has been called.
+// Reset can be called with a nil reader to release references to the previous reader.
+// After being called with a nil reader, no other operations than Reset or DecodeAll or Close
+// should be used.
 func (d *Decoder) Reset(r io.Reader) error {
 	if d.current.err == ErrDecoderClosed {
 		return d.current.err
 	}
+
+	d.drainOutput()
+
 	if r == nil {
-		return errors.New("nil Reader sent as input")
+		d.current.err = ErrDecoderNilInput
+		d.current.flushed = true
+		return nil
 	}
 
 	if d.stream == nil {
@@ -166,8 +178,6 @@ func (d *Decoder) Reset(r io.Reader) error {
 		go d.startStreamDecoder(d.stream)
 	}
 
-	d.drainOutput()
-
 	// If bytes buffer and < 1MB, do sync decoding anyway.
 	if bb, ok := r.(*bytes.Buffer); ok && bb.Len() < 1<<20 {
 		if debug {
@@ -249,7 +259,7 @@ func (d *Decoder) drainOutput() {
 // Any error encountered during the write is also returned.
 func (d *Decoder) WriteTo(w io.Writer) (int64, error) {
 	if d.stream == nil {
-		return 0, errors.New("no input has been initialized")
+		return 0, ErrDecoderNilInput
 	}
 	var n int64
 	for {

zstd/decoder_test.go

@@ -1434,6 +1434,48 @@ func TestPredefTables(t *testing.T) {
 	}
 }
 
+func TestResetNil(t *testing.T) {
+	dec, err := NewReader(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer dec.Close()
+
+	_, err = ioutil.ReadAll(dec)
+	if err != ErrDecoderNilInput {
+		t.Fatalf("Expected ErrDecoderNilInput when decoding from a nil reader, got %v", err)
+	}
+
+	emptyZstdBlob := []byte{40, 181, 47, 253, 32, 0, 1, 0, 0}
+
+	dec.Reset(bytes.NewBuffer(emptyZstdBlob))
+
+	result, err := ioutil.ReadAll(dec)
+	if err != nil && err != io.EOF {
+		t.Fatal(err)
+	}
+	if len(result) != 0 {
+		t.Fatalf("Expected to read 0 bytes, actually read %d", len(result))
+	}
+
+	dec.Reset(nil)
+
+	_, err = ioutil.ReadAll(dec)
+	if err != ErrDecoderNilInput {
+		t.Fatalf("Expected ErrDecoderNilInput when decoding from a nil reader, got %v", err)
+	}
+
+	dec.Reset(bytes.NewBuffer(emptyZstdBlob))
+
+	result, err = ioutil.ReadAll(dec)
+	if err != nil && err != io.EOF {
+		t.Fatal(err)
+	}
+	if len(result) != 0 {
+		t.Fatalf("Expected to read 0 bytes, actually read %d", len(result))
+	}
+}
+
 func timeout(after time.Duration) (cancel func()) {
 	c := time.After(after)
 	cc := make(chan struct{})

zstd/zstd.go

@@ -73,6 +73,10 @@ var (
 	// ErrDecoderClosed will be returned if the Decoder was used after
 	// Close has been called.
 	ErrDecoderClosed = errors.New("decoder used after Close")
+
+	// ErrDecoderNilInput is returned when a nil Reader was provided
+	// and an operation other than Reset/DecodeAll/Close was attempted.
+	ErrDecoderNilInput = errors.New("nil input provided as reader")
 )
 
 func println(a ...interface{}) {