jstream: implement Element.Discard

jstream/jstream.go

@@ -116,6 +116,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 )
 
 // byte blob constants
@@ -258,6 +259,43 @@ func (el Element) Stream() (*StreamReader, error) {
 	return el.sr, nil
 }
 
+// Discard reads whatever of this Element's data may be left on the StreamReader
+// it came from and discards it, making the StreamReader ready to have Next call
+// on it again.
+//
+// If the Element is a Byte Blob and is ended with io.EOF, or if the Element is
+// a Stream and is ended with ErrStreamEnded then this returns nil. If either is
+// canceled this also returns nil. All other errors are returned.
+func (el Element) Discard() error {
+	typ, err := el.Type()
+	if err != nil {
+		return err
+	}
+	switch typ {
+	case TypeByteBlob:
+		r, _ := el.Bytes()
+		_, err := io.Copy(ioutil.Discard, r)
+		if err == ErrCanceled {
+			return nil
+		}
+		return err
+	case TypeStream:
+		stream, _ := el.Stream()
+		for {
+			nextEl := stream.Next()
+			if nextEl.Err == ErrStreamEnded || nextEl.Err == ErrCanceled {
+				return nil
+			} else if nextEl.Err != nil {
+				return nextEl.Err
+			} else if err := nextEl.Discard(); err != nil {
+				return err
+			}
+		}
+	default: // TypeJSONValue
+		return nil
+	}
+}
+
 // StreamReader represents a Stream from which Elements may be read using the
 // Next method.
 type StreamReader struct {

jstream/jstream_test.go

@@ -34,11 +34,12 @@ func (cb *cancelBuffer) Read(p []byte) (int, error) {
 
 func TestEncoderDecoder(t *T) {
 	type testCase struct {
-		typ    Type
-		val    interface{}
-		bytes  []byte
-		stream []testCase
-		cancel bool
+		typ     Type
+		val     interface{}
+		bytes   []byte
+		stream  []testCase
+		cancel  bool
+		discard bool
 	}
 
 	var randTestCase func(Type, bool) testCase
@@ -57,8 +58,9 @@ func TestEncoderDecoder(t *T) {
 		}
 
 		tc := testCase{
-			typ:    typ,
-			cancel: cancelable && mtest.Rand.Intn(10) == 0,
+			typ:     typ,
+			cancel:  cancelable && mtest.Rand.Intn(10) == 0,
+			discard: mtest.Rand.Intn(20) == 0,
 		}
 
 		switch typ {
@@ -94,6 +96,11 @@ func TestEncoderDecoder(t *T) {
 		success = success && assert.NoError(t, err, l...)
 		success = success && assert.Equal(t, tc.typ, typ, l...)
 
+		if tc.discard {
+			success = success && assert.NoError(t, el.Discard())
+			return success
+		}
+
 		switch typ {
 		case TypeJSONValue:
 			var val interface{}