text: // 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).
  func ReadDelimited(r io.Reader, m proto.Message) (n int, err error) {
  	// 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
  		}
  		bytesRead += newBytesRead
  		// 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)
  }
  