net: export writeBuffers() and buffersWriter

[gobwas]

Hello!

Looking at net.Buffers.WriteTo() implementation one can find that it uses type assertion to non-exported type net.buffersWriter. The proposal is to export this type and methods which implement it.

Two things:

1. When someone wants to wrap net.Conn to get some i/o statistics, say:

type ConnWatcher struct {
    c net.Conn
    n int64
}
func (c *ConnWatcher) Write(p []byte) (int, error) {
    n, err := c.conn.Write(p)
    c.n += int64(n)
    return n, err
}

It becomes impossible to use writev() syscall – ConnWatcher has no way to implement non-exported net.buffersWriter interface.

2. When someone wants to wrap concrete *net.TCPConn (rare case, but possible) to decorate some write behaviour:

type CipherConn struct {
    *net.TCPConn                                
    Encode func([]byte) []byte
}
func (c CipherConn) Write(p []byte) (int, error) {
    return c.TCPConn.Write(c.Encode(p))
}

// Somewhere in the application.
var bs net.Buffers
var cc CipherConn
bs.WriteTo(cc) 

The last line of the example above bs.WriteTo(cc) will produce cc.TCPConn.writeBuffers(bs) call, sending raw data without encoding.

Regards,
Sergey.

[funny-falcon]

Hidden behavior that is difficult to use right.

[odeke-em]

Looks similar to #21676 in which @zombiezen talks about how frustrating it is that writeBuffers is unexported.

[gobwas]

Sorry, for open/reopen spam ) Should I close it as a duplicate or keep it opened until Go1.11?

[ianlancetaylor]

The two seem slightly different, let's keep them both for now.

[ianlancetaylor]

After more thought, closing as dup of #21676 .