How do I keep reading the remaining bytes after reading the first few bytes using conn.Read() in Go?

Question

I am trying to read from a tcp connection in Go. I have a specific header format I am trying to parse first. So read the first 12 bytes which has the info associated with the header. And that tells me that the remaining message length is a certain amount of bytes. In this case 1607 bytes. I tried the following code where I read the header bytes, and then tried to read the remaining bytes.

import ("net"
        log "github.com/sirupsen/logrus"
       "bytes"
       "encoding/binary"
)

func handleRequest(conn net.Conn) {

    // Structs to make header parsing easiers
    type encapSTHdrMsgType uint16
    type encapSTHdrMsgEncap uint16
    type encapSTHdr struct {
        MsgType       encapSTHdrMsgType
        MsgEncap      encapSTHdrMsgEncap
        MsgHdrVersion uint16
        Msgflag       uint16
        Msglen        uint32
    }

    // Make a buffer to hold header data.
    headerBuf := make([]byte, 12)

    // Read the incoming header info into the buffer.
    _, err := conn.Read(headerBuf)
    if err != nil {
        log.Debug("Error reading:", err.Error())
    }

    // Header is in big endian
    var header encapSTHdr
    headerReader := bytes.NewReader(headerBuf)
    err = binary.Read(headerReader, binary.BigEndian, &header)
    if err != nil {
        log.Debugf("Could not read header bytes into the buffer: %v", err)
    }

    messageBuf := make([]byte, header.Msglen)
    messageBufLen, err := conn.Read(messageBuf)
    if err != nil {
        log.Debugf("Error reading messages: %s", err.Error())
    }
    log.Debugf("The message buffer length is: %d", messageBufLen)
    log.Debugf("The header message length is: %d", header.Msglen) 
}

When I try reading the remaining bytes by making a new buffer of the desired length of 1607, I get a new byte array with data but it is only of length 1228. Hence, I have two questions:

1) Why does conn.Read()not read the remaining bytes?

2) Now I am aware I can use a for loop until I have read the desired amount of bytes and break when done but I was wondering if there was a better way to do this?

Answer 1

Read returns when data is available, not when it fills the buffer. Use io.ReadFull() to read until the buffer is full or there's an error reading from the connection.

_, err := io.ReadFull(conn, headerBuf)

...

_, err := io.ReadFull(conn, messageBuf)