Java中是否存在用于协议缓冲区分隔I/O函数的C++等价物？

Question

我正在尝试从C++和Java中的文件读取/写入多个Protocol Buffers消息.谷歌建议在消息之前写长度前缀,但默认情况下没办法(我可以看到).

但是,2.1.0版中的Java API收到了一组"Delimited"I/O函数,显然可以完成这项工作:

parseDelimitedFrom
mergeDelimitedFrom
writeDelimitedTo

有C++等价物吗？如果没有,那么Java API附加的大小前缀是什么,所以我可以用C++解析这些消息？

---

更新:

这些现在存在于google/protobuf/util/delimited_message_util.hv3.3.0中.

Answer 1

我在这里参加派对有点晚了,但是下面的实现包括其他答案中缺少的一些优化,并且在64MB输入后不会失败(尽管它仍然对每条消息强制执行64MB限制,而不是在整个流上).

(我是C++和Java protobuf库的作者,但我不再为Google工作了.很抱歉,这段代码从未进入官方库.这就是它的样子.)

bool writeDelimitedTo(
    const google::protobuf::MessageLite& message,
    google::protobuf::io::ZeroCopyOutputStream* rawOutput) {
  // We create a new coded stream for each message.  Don't worry, this is fast.
  google::protobuf::io::CodedOutputStream output(rawOutput);

  // Write the size.
  const int size = message.ByteSize();
  output.WriteVarint32(size);

  uint8_t* buffer = output.GetDirectBufferForNBytesAndAdvance(size);
  if (buffer != NULL) {
    // Optimization:  The message fits in one buffer, so use the faster
    // direct-to-array serialization path.
    message.SerializeWithCachedSizesToArray(buffer);
  } else {
    // Slightly-slower path when the message is multiple buffers.
    message.SerializeWithCachedSizes(&output);
    if (output.HadError()) return false;
  }

  return true;
}

bool readDelimitedFrom(
    google::protobuf::io::ZeroCopyInputStream* rawInput,
    google::protobuf::MessageLite* message) {
  // We create a new coded stream for each message.  Don't worry, this is fast,
  // and it makes sure the 64MB total size limit is imposed per-message rather
  // than on the whole stream.  (See the CodedInputStream interface for more
  // info on this limit.)
  google::protobuf::io::CodedInputStream input(rawInput);

  // Read the size.
  uint32_t size;
  if (!input.ReadVarint32(&size)) return false;

  // Tell the stream not to read beyond that size.
  google::protobuf::io::CodedInputStream::Limit limit =
      input.PushLimit(size);

  // Parse the message.
  if (!message->MergeFromCodedStream(&input)) return false;
  if (!input.ConsumedEntireMessage()) return false;

  // Release the limit.
  input.PopLimit(limit);

  return true;
}

Answer 2

好的,所以我无法找到实现我需要的顶级C++函数,但是在MessageLite接口中,有些人通过Java API参考了解了以下内容:

void writeDelimitedTo(OutputStream output)
/*  Like writeTo(OutputStream), but writes the size of 
    the message as a varint before writing the data.   */

所以Java大小前缀是(Protocol Buffers)varint!

有了这些信息,我开始挖掘C++ API并找到了CodedStream标头,其中包含:

bool CodedInputStream::ReadVarint32(uint32 * value)
void CodedOutputStream::WriteVarint32(uint32 value)

使用这些,我应该能够使用我自己的C++函数来完成这项工作.

他们应该真的将它添加到主Message API中; 考虑到Java,它缺少功能,Marc Gravell的优秀protobuf-net C#端口(通过SerializeWithLengthPrefix和DeserializeWithLengthPrefix)也是如此.

Answer 3

我使用CodedOutputStream/ArrayOutputStream解决了同样的问题,用于编写消息(带有大小)和CodedInputStream/ArrayInputStream来读取消息(带有大小).

例如,以下伪代码写入消息后面的消息大小:

const unsigned bufLength = 256;
unsigned char buffer[bufLength];
Message protoMessage;

google::protobuf::io::ArrayOutputStream arrayOutput(buffer, bufLength);
google::protobuf::io::CodedOutputStream codedOutput(&arrayOutput);

codedOutput.WriteLittleEndian32(protoMessage.ByteSize());
protoMessage.SerializeToCodedStream(&codedOutput);

写作时,还应检查您的缓冲区是否足够大以适应消息(包括大小).在阅读时,您应该检查您的缓冲区是否包含整个消息(包括大小).

如果他们为C++ API添加类似于Java API提供的便捷方法,那肯定会很方便.

Answer 4

干得好:

#include <google/protobuf/io/zero_copy_stream_impl.h>
#include <google/protobuf/io/coded_stream.h>

using namespace google::protobuf::io;

class FASWriter 
{
    std::ofstream mFs;
    OstreamOutputStream *_OstreamOutputStream;
    CodedOutputStream *_CodedOutputStream;
public:
    FASWriter(const std::string &file) : mFs(file,std::ios::out | std::ios::binary)
    {
        assert(mFs.good());

        _OstreamOutputStream = new OstreamOutputStream(&mFs);
        _CodedOutputStream = new CodedOutputStream(_OstreamOutputStream);
    }

    inline void operator()(const ::google::protobuf::Message &msg)
    {
        _CodedOutputStream->WriteVarint32(msg.ByteSize());

        if ( !msg.SerializeToCodedStream(_CodedOutputStream) )
            std::cout << "SerializeToCodedStream error " << std::endl;
    }

    ~FASWriter()
    {
        delete _CodedOutputStream;
        delete _OstreamOutputStream;
        mFs.close();
    }
};

class FASReader
{
    std::ifstream mFs;

    IstreamInputStream *_IstreamInputStream;
    CodedInputStream *_CodedInputStream;
public:
    FASReader(const std::string &file), mFs(file,std::ios::in | std::ios::binary)
    {
        assert(mFs.good());

        _IstreamInputStream = new IstreamInputStream(&mFs);
        _CodedInputStream = new CodedInputStream(_IstreamInputStream);      
    }

    template<class T>
    bool ReadNext()
    {
        T msg;
        unsigned __int32 size;

        bool ret;
        if ( ret = _CodedInputStream->ReadVarint32(&size) )
        {   
            CodedInputStream::Limit msgLimit = _CodedInputStream->PushLimit(size);
            if ( ret = msg.ParseFromCodedStream(_CodedInputStream) )
            {
                _CodedInputStream->PopLimit(msgLimit);      
                std::cout << mFeed << " FASReader ReadNext: " << msg.DebugString() << std::endl;
            }
        }

        return ret;
    }

    ~FASReader()
    {
        delete _CodedInputStream;
        delete _IstreamInputStream;
        mFs.close();
    }
};

Answer 5

IsteamInputStream对于与std :: istream一起使用时容易发生的eofs和其他错误非常脆弱.在此之后,protobuf流被永久性损坏,并且任何已经使用的缓冲数据被破坏.对protobuf中的传统流进行读取有适当的支持.

google::protobuf::io::CopyingInputStream与CopyingInputStreamAdapter一起实现和使用它.对输出变体执行相同的操作.

实际上,解析调用最终会在google::protobuf::io::CopyingInputStream::Read(void* buffer, int size)给定缓冲区的位置.剩下要做的就是以某种方式阅读它.

以下是与Asio同步流(SyncReadStream/SyncWriteStream)一起使用的示例:

#include <google/protobuf/io/zero_copy_stream_impl_lite.h>

using namespace google::protobuf::io;


template <typename SyncReadStream>
class AsioInputStream : public CopyingInputStream {
    public:
        AsioInputStream(SyncReadStream& sock);
        int Read(void* buffer, int size);
    private:
        SyncReadStream& m_Socket;
};


template <typename SyncReadStream>
AsioInputStream<SyncReadStream>::AsioInputStream(SyncReadStream& sock) :
    m_Socket(sock) {}


template <typename SyncReadStream>
int
AsioInputStream<SyncReadStream>::Read(void* buffer, int size)
{
    std::size_t bytes_read;
    boost::system::error_code ec;
    bytes_read = m_Socket.read_some(boost::asio::buffer(buffer, size), ec);

    if(!ec) {
        return bytes_read;
    } else if (ec == boost::asio::error::eof) {
        return 0;
    } else {
        return -1;
    }
}


template <typename SyncWriteStream>
class AsioOutputStream : public CopyingOutputStream {
    public:
        AsioOutputStream(SyncWriteStream& sock);
        bool Write(const void* buffer, int size);
    private:
        SyncWriteStream& m_Socket;
};


template <typename SyncWriteStream>
AsioOutputStream<SyncWriteStream>::AsioOutputStream(SyncWriteStream& sock) :
    m_Socket(sock) {}


template <typename SyncWriteStream>
bool
AsioOutputStream<SyncWriteStream>::Write(const void* buffer, int size)
{   
    boost::system::error_code ec;
    m_Socket.write_some(boost::asio::buffer(buffer, size), ec);
    return !ec;
}

用法:

AsioInputStream<boost::asio::ip::tcp::socket> ais(m_Socket); // Where m_Socket is a instance of boost::asio::ip::tcp::socket
CopyingInputStreamAdaptor cis_adp(&ais);
CodedInputStream cis(&cis_adp);

Message protoMessage;
uint32_t msg_size;

/* Read message size */
if(!cis.ReadVarint32(&msg_size)) {
    // Handle error
 }

/* Make sure not to read beyond limit of message */
CodedInputStream::Limit msg_limit = cis.PushLimit(msg_size);
if(!msg.ParseFromCodedStream(&cis)) {
    // Handle error
}

/* Remove limit */
cis.PopLimit(msg_limit);

Answer 6

我在C++和Python中遇到了同样的问题.

对于C++版本,我使用了在此线程上发布的代码Kenton Varda以及他发送给protobuf团队的pull请求中的代码(因为此处发布的版本不会处理EOF,而他发送给github的那个).

#include <google/protobuf/message_lite.h>
#include <google/protobuf/io/zero_copy_stream.h>
#include <google/protobuf/io/coded_stream.h>


bool writeDelimitedTo(const google::protobuf::MessageLite& message,
    google::protobuf::io::ZeroCopyOutputStream* rawOutput)
{
    // We create a new coded stream for each message.  Don't worry, this is fast.
    google::protobuf::io::CodedOutputStream output(rawOutput);

    // Write the size.
    const int size = message.ByteSize();
    output.WriteVarint32(size);

    uint8_t* buffer = output.GetDirectBufferForNBytesAndAdvance(size);
    if (buffer != NULL)
    {
        // Optimization:  The message fits in one buffer, so use the faster
        // direct-to-array serialization path.
        message.SerializeWithCachedSizesToArray(buffer);
    }

    else
    {
        // Slightly-slower path when the message is multiple buffers.
        message.SerializeWithCachedSizes(&output);
        if (output.HadError())
            return false;
    }

    return true;
}

bool readDelimitedFrom(google::protobuf::io::ZeroCopyInputStream* rawInput, google::protobuf::MessageLite* message, bool* clean_eof)
{
    // We create a new coded stream for each message.  Don't worry, this is fast,
    // and it makes sure the 64MB total size limit is imposed per-message rather
    // than on the whole stream.  (See the CodedInputStream interface for more
    // info on this limit.)
    google::protobuf::io::CodedInputStream input(rawInput);
    const int start = input.CurrentPosition();
    if (clean_eof)
        *clean_eof = false;


    // Read the size.
    uint32_t size;
    if (!input.ReadVarint32(&size))
    {
        if (clean_eof)
            *clean_eof = input.CurrentPosition() == start;
        return false;
    }
    // Tell the stream not to read beyond that size.
    google::protobuf::io::CodedInputStream::Limit limit = input.PushLimit(size);

    // Parse the message.
    if (!message->MergeFromCodedStream(&input)) return false;
    if (!input.ConsumedEntireMessage()) return false;

    // Release the limit.
    input.PopLimit(limit);

    return true;
}

这是我的python2实现:

from google.protobuf.internal import encoder
from google.protobuf.internal import decoder

#I had to implement this because the tools in google.protobuf.internal.decoder
#read from a buffer, not from a file-like objcet
def readRawVarint32(stream):
    mask = 0x80 # (1 << 7)
    raw_varint32 = []
    while 1:
        b = stream.read(1)
        #eof
        if b == "":
            break
        raw_varint32.append(b)
        if not (ord(b) & mask):
            #we found a byte starting with a 0, which means it's the last byte of this varint
            break
    return raw_varint32

def writeDelimitedTo(message, stream):
    message_str = message.SerializeToString()
    delimiter = encoder._VarintBytes(len(message_str))
    stream.write(delimiter + message_str)

def readDelimitedFrom(MessageType, stream):
    raw_varint32 = readRawVarint32(stream)
    message = None

    if raw_varint32:
        size, _ = decoder._DecodeVarint32(raw_varint32, 0)

        data = stream.read(size)
        if len(data) < size:
            raise Exception("Unexpected end of file")

        message = MessageType()
        message.ParseFromString(data)

    return message

#In place version that takes an already built protobuf object
#In my tests, this is around 20% faster than the other version 
#of readDelimitedFrom()
def readDelimitedFrom_inplace(message, stream):
    raw_varint32 = readRawVarint32(stream)

    if raw_varint32:
        size, _ = decoder._DecodeVarint32(raw_varint32, 0)

        data = stream.read(size)
        if len(data) < size:
            raise Exception("Unexpected end of file")

        message.ParseFromString(data)

        return message
    else:
        return None

它可能不是最好看的代码,我确信它可以被重构一下,但至少应该向您展示一种方法.

现在最大的问题是:它很慢.

即使使用python-protobuf的C++实现,它也比纯C++慢一个数量级.我有一个基准测试,我从文件中读取每个约30个字节的10M protobuf消息.它在C++中需要~0.9s,在python中需要35s.

使其快一点的一种方法是重新实现varint解码器,使其从文件中读取并一次解码,而不是从文件中读取然后按照此代码进行解码.(分析表明在varint编码器/解码器中花费了大量时间).但不用说,单靠这一点还不足以缩小python版本和C++版本之间的差距.

任何想让它变得更快的想法都非常受欢迎:)

Answer 7

为了完整起见，我在这里发布了一个与 protobuf 和 Python3 主版本一起使用的最新版本

对于 C++ 版本，使用 delimited_message_utils.h 中的 utils 就足够了，这里是 MWE

#include <google/protobuf/io/zero_copy_stream_impl.h>
#include <google/protobuf/util/delimited_message_util.h>

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

template <typename T>
bool writeManyToFile(std::deque<T> messages, std::string filename) {
    int outfd = open(filename.c_str(), O_WRONLY | O_CREAT | O_TRUNC);
    google::protobuf::io::FileOutputStream fout(outfd);

    bool success;
    for (auto msg: messages) {
        success = google::protobuf::util::SerializeDelimitedToZeroCopyStream(
            msg, &fout);
        if (! success) {
            std::cout << "Writing Failed" << std::endl;
            break;
        }
    }
    fout.Close();
    close(outfd);
    return success;
}

template <typename T>
std::deque<T> readManyFromFile(std::string filename) {
    int infd = open(filename.c_str(), O_RDONLY);

    google::protobuf::io::FileInputStream fin(infd);
    bool keep = true;
    bool clean_eof = true;
    std::deque<T> out;

    while (keep) {
        T msg;
        keep = google::protobuf::util::ParseDelimitedFromZeroCopyStream(
            &msg, &fin, nullptr);
        if (keep)
            out.push_back(msg);
    }
    fin.Close();
    close(infd);
    return out;
}

对于Python3版本，基于@fireboot的答案，唯一需要修改的是raw_varint32的解码

def getSize(raw_varint32):
    result = 0
    shift = 0
    b = six.indexbytes(raw_varint32, 0)
    result |= ((ord(b) & 0x7f) << shift)
    return result

def readDelimitedFrom(MessageType, stream):
    raw_varint32 = readRawVarint32(stream)
    message = None

    if raw_varint32:
        size = getSize(raw_varint32)

        data = stream.read(size)
        if len(data) < size:
            raise Exception("Unexpected end of file")

        message = MessageType()
        message.ParseFromString(data)

    return message