Method Read

value.go:1537–1574

Read reads the value log at a given location. TODO: Make this read private.

(vp valuePointer, s *y.Slice)

// Read reads the value log at a given location.
// TODO: Make this read private.
func (vlog *valueLog) Read(vp valuePointer, s *y.Slice) ([]byte, func(), error) {
	buf, lf, err := vlog.readValueBytes(vp, s)
	// log file is locked so, decide whether to lock immediately or let the caller to
	// unlock it, after caller uses it.
	cb := vlog.getUnlockCallback(lf)
	if err != nil {
		return nil, cb, err
	}

	if vlog.opt.VerifyValueChecksum {
		hash := crc32.New(y.CastagnoliCrcTable)
		if _, err := hash.Write(buf[:len(buf)-crc32.Size]); err != nil {
			runCallback(cb)
			return nil, nil, errors.Wrapf(err, "failed to write hash for vp %+v", vp)
		}
		// Fetch checksum from the end of the buffer.
		checksum := buf[len(buf)-crc32.Size:]
		if hash.Sum32() != y.BytesToU32(checksum) {
			runCallback(cb)
			return nil, nil, errors.Wrapf(y.ErrChecksumMismatch, "value corrupted for vp: %+v", vp)
		}
	}
	var h header
	headerLen := h.Decode(buf)
	kv := buf[headerLen:]
	if lf.encryptionEnabled() {
		kv, err = lf.decryptKV(kv, vp.Offset)
		if err != nil {
			return nil, cb, err
		}
	}
	if uint32(len(kv)) < h.klen+h.vlen {
		vlog.db.opt.Logger.Errorf("Invalid read: vp: %+v", vp)
		return nil, nil, errors.Errorf("Invalid read: Len: %d read at:[%d:%d]",
			len(kv), h.klen, h.klen+h.vlen)
	}
	return kv[h.klen : h.klen+h.vlen], cb, nil
}

// getUnlockCallback will returns a function which unlock the logfile if the logfile is mmaped.
// otherwise, it unlock the logfile and return nil.