Test for data corruption in datafile decoding (#99)

* internal/data: move codec to own subpackage

* internal/data/codec: check & test nil Entry Decode

* internal/data/decoder: test for short prefix error

* internal/data/codec: test invalid key & value sizes

* internal/data/codec: check & test for truncated data

* interna/data/codec: use assert for tests

internal/data/codec/decoder.go

@@ -0,0 +1,99 @@
+package codec
+
+import (
+	"encoding/binary"
+	"io"
+
+	"github.com/pkg/errors"
+	"github.com/prologic/bitcask/internal"
+)
+
+var (
+	errInvalidKeyOrValueSize = errors.New("key/value size is invalid")
+	errCantDecodeOnNilEntry  = errors.New("can't decode on nil entry")
+	errTruncatedData         = errors.New("data is truncated")
+)
+
+// NewDecoder creates a streaming Entry decoder.
+func NewDecoder(r io.Reader, maxKeySize uint32, maxValueSize uint64) *Decoder {
+	return &Decoder{
+		r:            r,
+		maxKeySize:   maxKeySize,
+		maxValueSize: maxValueSize,
+	}
+}
+
+// Decoder wraps an underlying io.Reader and allows you to stream
+// Entry decodings on it.
+type Decoder struct {
+	r            io.Reader
+	maxKeySize   uint32
+	maxValueSize uint64
+}
+
+// Decode decodes the next Entry from the current stream
+func (d *Decoder) Decode(v *internal.Entry) (int64, error) {
+	if v == nil {
+		return 0, errCantDecodeOnNilEntry
+	}
+
+	prefixBuf := make([]byte, keySize+valueSize)
+
+	_, err := io.ReadFull(d.r, prefixBuf)
+	if err != nil {
+		return 0, err
+	}
+
+	actualKeySize, actualValueSize, err := getKeyValueSizes(prefixBuf, d.maxKeySize, d.maxValueSize)
+	if err != nil {
+		return 0, err
+	}
+
+	buf := make([]byte, uint64(actualKeySize)+actualValueSize+checksumSize)
+	if _, err = io.ReadFull(d.r, buf); err != nil {
+		return 0, errTruncatedData
+	}
+
+	decodeWithoutPrefix(buf, actualKeySize, v)
+	return int64(keySize + valueSize + uint64(actualKeySize) + actualValueSize + checksumSize), nil
+}
+
+// DecodeEntry decodes a serialized entry
+func DecodeEntry(b []byte, e *internal.Entry, maxKeySize uint32, maxValueSize uint64) error {
+	valueOffset, _, err := getKeyValueSizes(b, maxKeySize, maxValueSize)
+	if err != nil {
+		return errors.Wrap(err, "key/value sizes are invalid")
+	}
+
+	decodeWithoutPrefix(b[keySize+valueSize:], valueOffset, e)
+
+	return nil
+}
+
+func getKeyValueSizes(buf []byte, maxKeySize uint32, maxValueSize uint64) (uint32, uint64, error) {
+	actualKeySize := binary.BigEndian.Uint32(buf[:keySize])
+	actualValueSize := binary.BigEndian.Uint64(buf[keySize:])
+
+	if actualKeySize > maxKeySize || actualValueSize > maxValueSize || actualKeySize == 0 {
+
+		return 0, 0, errInvalidKeyOrValueSize
+	}
+
+	return actualKeySize, actualValueSize, nil
+}
+
+func decodeWithoutPrefix(buf []byte, valueOffset uint32, v *internal.Entry) {
+	v.Key = buf[:valueOffset]
+	v.Value = buf[valueOffset : len(buf)-checksumSize]
+	v.Checksum = binary.BigEndian.Uint32(buf[len(buf)-checksumSize:])
+}
+
+// IsCorruptedData indicates if the error correspondes to possible data corruption
+func IsCorruptedData(err error) bool {
+	switch err {
+	case errCantDecodeOnNilEntry, errInvalidKeyOrValueSize, errTruncatedData:
+		return true
+	default:
+		return false
+	}
+}