net: add UDPConn.WriterTo (experimental)

Allows alloc-free writes. Previously 1 alloc of 32 bytes per packet.

Change-Id: I2e9b9626799ccb7d28136059b8ad52024c97f05e

Author: josharian

src/net/udpsock.go

@@ -6,6 +6,7 @@ package net
 
 import (
 	"context"
+	"io"
 	"syscall"
 )
 
@@ -171,6 +172,37 @@ func (c *UDPConn) WriteTo(b []byte, addr Addr) (int, error) {
 	return n, err
 }
 
+// WriterTo returns an io.Writer that writes UDP packets to addr.
+// This is more efficient than WriteTo when many packets will be sent to the same addr.
+func (c *UDPConn) WriterTo(addr Addr) (io.Writer, error) {
+	a, ok := addr.(*UDPAddr)
+	if !ok {
+		return nil, &OpError{Op: "write", Net: c.fd.net, Source: c.fd.laddr, Addr: addr, Err: syscall.EINVAL}
+	}
+	sa, err := a.sockaddr(c.fd.family)
+	if err != nil {
+		return nil, err
+	}
+	return &udpWriterTo{c, a, sa}, nil
+}
+
+type udpWriterTo struct {
+	conn *UDPConn
+	addr *UDPAddr
+	sa   syscall.Sockaddr
+}
+
+func (c *udpWriterTo) Write(b []byte) (int, error) {
+	if !c.conn.ok() {
+		return 0, syscall.EINVAL
+	}
+	n, err := c.writeTo(b, c.sa)
+	if err != nil {
+		err = &OpError{Op: "write", Net: c.conn.fd.net, Source: c.conn.fd.laddr, Addr: c.addr.opAddr(), Err: err}
+	}
+	return n, err
+}
+
 // WriteMsgUDP writes a message to addr via c if c isn't connected, or
 // to c's remote address if c is connected (in which case addr must be
 // nil). The payload is copied from b and the associated out-of-band

src/net/udpsock_posix.go

@@ -80,6 +80,13 @@ func (c *UDPConn) writeTo(b []byte, addr *UDPAddr) (int, error) {
 	return c.fd.writeTo(b, sa)
 }
 
+func (c *udpWriterTo) writeTo(b []byte, sa syscall.Sockaddr) (int, error) {
+	if c.conn.fd.isConnected {
+		return 0, ErrWriteToConnected
+	}
+	return c.conn.fd.writeTo(b, c.sa)
+}
+
 func (c *UDPConn) writeMsg(b, oob []byte, addr *UDPAddr) (n, oobn int, err error) {
 	if c.fd.isConnected && addr != nil {
 		return 0, 0, ErrWriteToConnected