Source: decode.go in package github.com/matttproud/golang_protobuf

Copyright 2013 Matt T. Proud Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


package pbutil

import (
	"encoding/binary"
	"errors"
	"io"

	"github.com/golang/protobuf/proto"
)

var errInvalidVarint = errors.New("invalid varint32 encountered")

ReadDelimited decodes a message from the provided length-delimited stream, where the length is encoded as 32-bit varint prefix to the message body. It returns the total number of bytes read and any applicable error. This is roughly equivalent to the companion Java API's MessageLite#parseDelimitedFrom. As per the reader contract, this function calls r.Read repeatedly as required until exactly one message including its prefix is read and decoded (or an error has occurred). The function never reads more bytes from the stream than required. The function never returns an error if a message has been read and decoded correctly, even if the end of the stream has been reached in doing so. In that case, any subsequent calls return (0, io.EOF).

Per AbstractParser#parsePartialDelimitedFrom with CodedInputStream#readRawVarint32.

	var headerBuf [binary.MaxVarintLen32]byte
	var bytesRead, varIntBytes int
	var messageLength uint64
	for varIntBytes == 0 { // i.e. no varint has been decoded yet.
		if bytesRead >= len(headerBuf) {
			return bytesRead, errInvalidVarint

We have to read byte by byte here to avoid reading more bytes than required. Each read byte is appended to what we have read before.

		newBytesRead, err := r.Read(headerBuf[bytesRead : bytesRead+1])
		if newBytesRead == 0 {
			if err != nil {
				return bytesRead, err

A Reader should not return (0, nil), but if it does, it should be treated as no-op (according to the Reader contract). So let's go on...

			continue
		}

Now present everything read so far to the varint decoder and see if a varint can be decoded already.

		messageLength, varIntBytes = proto.DecodeVarint(headerBuf[:bytesRead])
	}

	messageBuf := make([]byte, messageLength)
	newBytesRead, err := io.ReadFull(r, messageBuf)
	bytesRead += newBytesRead
	if err != nil {
		return bytesRead, err
	}

	return bytesRead, proto.Unmarshal(messageBuf, m)