Repository: fxamacker/cbor (go)

Issue:
feature: Add Skip() to Decoder
**Is your feature request related to a problem? Please describe.**

It can be useful to skip decoding a CBOR data item as discussed in comments for issue https://github.com/fxamacker/cbor/issues/362.

**Describe the solution you'd like**

Add Skip() to the decoder to skip next data item.

**Describe alternatives you've considered**

As mentioned by @fxamacker 
>  Until I add a Skip feature, a less efficient way is to decode the messages you want to skip to cbor.RawMessage. cbor.RawMessage copies data item into a byte slice without parsing the data item.

```go
var throwAway cbor.RawMessage
err := decodoer.Decode(&throwAway)
```

Pull request description:
Add Decoder.Skip() to skip CBOR data item in CBOR Sequences (RFC 8742)
When decoding CBOR Sequences ([RFC 8742](https://www.rfc-editor.org/rfc/rfc8742.html)), it can be useful to skip to the next CBOR data item without decoding.

This PR adds `Decoder.Skip()` which will skip to next CBOR data item (if there is any) otherwise it will return error such as `io.EOF`, `io.UnexpectedEOF`, etc.

Special thanks to @espoal and @x448 for their comments on this feature.

Closes #366

Patch:
diff --git a/stream.go b/stream.go
index 9337940..c559f52 100644
--- a/stream.go
+++ b/stream.go
@@ -58,6 +58,38 @@ func (dec *Decoder) Decode(v interface{}) error {
 	}
 }
 
+// Skip skips to the next CBOR data item (if there is any),
+// otherwise it returns error such as io.EOF, io.UnexpectedEOF, etc.
+func (dec *Decoder) Skip() error {
+	if len(dec.buf) == dec.off {
+		if n, err := dec.read(); n == 0 {
+			return err
+		}
+	}
+	for {
+		dec.d.reset(dec.buf[dec.off:])
+		err := dec.d.valid(true)
+		if err == nil {
+			// Only increment dec.off if current CBOR data item is valid.
+			// If current data item is incomplete (io.ErrUnexpectedEOF),
+			// we want to try again after reading more data.
+			dec.off += dec.d.off
+			dec.bytesRead += dec.d.off
+			return nil
+		}
+		if err != io.ErrUnexpectedEOF {
+			return err
+		}
+		// Need to read more data.
+		if n, err := dec.read(); n == 0 {
+			if err == io.EOF {
+				err = io.ErrUnexpectedEOF
+			}
+			return err
+		}
+	}
+}
+
 // NumBytesRead returns the number of bytes read.
 func (dec *Decoder) NumBytesRead() int {
 	return dec.bytesRead


Test patch:
diff --git a/stream_test.go b/stream_test.go
index 6dfe339..211d79d 100644
--- a/stream_test.go
+++ b/stream_test.go
@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"io"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 )
@@ -193,7 +194,7 @@ func TestDecoderReadError(t *testing.T) {
 }
 
 func TestDecoderInvalidData(t *testing.T) {
-	data := []byte{0x01, 0x83, 0x01, 0x02}
+	data := []byte{0x01, 0x3e}
 	decoder := NewDecoder(bytes.NewReader(data))
 
 	var v1 interface{}
@@ -206,8 +207,125 @@ func TestDecoderInvalidData(t *testing.T) {
 	err = decoder.Decode(&v2)
 	if err == nil {
 		t.Errorf("Decode() didn't return error when decoding invalid data item")
-	} else if err != io.ErrUnexpectedEOF {
-		t.Errorf("Decode() error %q, want %q", err, io.ErrUnexpectedEOF)
+	} else if !strings.Contains(err.Error(), "cbor: invalid additional information") {
+		t.Errorf("Decode() error %q, want \"cbor: invalid additional information\"", err)
+	}
+}
+
+func TestDecoderSkip(t *testing.T) {
+	var buf bytes.Buffer
+	for i := 0; i < 5; i++ {
+		for _, tc := range unmarshalTests {
+			buf.Write(tc.cborData)
+		}
+	}
+	decoder := NewDecoder(&buf)
+	bytesRead := 0
+	for i := 0; i < 5; i++ {
+		for _, tc := range unmarshalTests {
+			if err := decoder.Skip(); err != nil {
+				t.Fatalf("Skip() returned error %v", err)
+			}
+			bytesRead += len(tc.cborData)
+			if decoder.NumBytesRead() != bytesRead {
+				t.Errorf("NumBytesRead() = %v, want %v", decoder.NumBytesRead(), bytesRead)
+			}
+		}
+	}
+	for i := 0; i < 2; i++ {
+		// no more data
+		err := decoder.Skip()
+		if err != io.EOF {
+			t.Errorf("Skip() returned error %v, want io.EOF (no more data)", err)
+		}
+	}
+}
+
+func TestDecoderSkipInvalidDataError(t *testing.T) {
+	var buf bytes.Buffer
+	for _, tc := range unmarshalTests {
+		buf.Write(tc.cborData)
+	}
+	buf.WriteByte(0x3e)
+
+	decoder := NewDecoder(&buf)
+	bytesRead := 0
+	for i := 0; i < len(unmarshalTests); i++ {
+		tc := unmarshalTests[i]
+		if err := decoder.Skip(); err != nil {
+			t.Fatalf("Skip() returned error %v", err)
+		}
+		bytesRead += len(tc.cborData)
+		if decoder.NumBytesRead() != bytesRead {
+			t.Errorf("NumBytesRead() = %v, want %v", decoder.NumBytesRead(), bytesRead)
+		}
+	}
+	for i := 0; i < 2; i++ {
+		// last data item is invalid
+		err := decoder.Skip()
+		if err == nil {
+			t.Fatalf("Skip() didn't return error")
+		} else if !strings.Contains(err.Error(), "cbor: invalid additional information") {
+			t.Errorf("Skip() error %q, want \"cbor: invalid additional information\"", err)
+		}
+	}
+}
+
+func TestDecoderSkipUnexpectedEOFError(t *testing.T) {
+	var buf bytes.Buffer
+	for _, tc := range unmarshalTests {
+		buf.Write(tc.cborData)
+	}
+	buf.Truncate(buf.Len() - 1)
+
+	decoder := NewDecoder(&buf)
+	bytesRead := 0
+	for i := 0; i < len(unmarshalTests)-1; i++ {
+		tc := unmarshalTests[i]
+		if err := decoder.Skip(); err != nil {
+			t.Fatalf("Skip() returned error %v", err)
+		}
+		bytesRead += len(tc.cborData)
+		if decoder.NumBytesRead() != bytesRead {
+			t.Errorf("NumBytesRead() = %v, want %v", decoder.NumBytesRead(), bytesRead)
+		}
+	}
+	for i := 0; i < 2; i++ {
+		// last data item is invalid
+		err := decoder.Skip()
+		if err != io.ErrUnexpectedEOF {
+			t.Errorf("Skip() returned error %v, want io.ErrUnexpectedEOF (truncated data)", err)
+		}
+	}
+}
+
+func TestDecoderSkipReadError(t *testing.T) {
+	var buf bytes.Buffer
+	for _, tc := range unmarshalTests {
+		buf.Write(tc.cborData)
+	}
+	buf.Truncate(buf.Len() - 1)
+
+	readerErr := errors.New("reader error")
+
+	decoder := NewDecoder(NewErrorReader(buf.Bytes(), readerErr))
+	bytesRead := 0
+	for i := 0; i < len(unmarshalTests)-1; i++ {
+		tc := unmarshalTests[i]
+		if err := decoder.Skip(); err != nil {
+			t.Fatalf("Skip() returned error %v", err)
+		}
+		bytesRead += len(tc.cborData)
+		if decoder.NumBytesRead() != bytesRead {
+			t.Errorf("NumBytesRead() = %v, want %v", decoder.NumBytesRead(), bytesRead)
+		}
+	}
+	for i := 0; i < 2; i++ {
+		// truncated data because Reader returned error
+		err := decoder.Skip()
+		if err != readerErr {
+			t.Errorf("Skip() returned error %v, want reader error", err)
+		}
 	}
 }