Coalesce UDP GSO segments as iovecs

coalesceMessages copied every additional datagram into the spare
capacity of the first buffer, which no longer exists now that element
buffers are sized to their packet; append the datagrams as iovecs
instead, which also removes the copy.

conn/bind_std.go

@@ -228,8 +228,11 @@ again:
 
 func (s *StdNetBind) putMessages(msgs *[]ipv6.Message) {
 	for i := range *msgs {
-		(*msgs)[i].OOB = (*msgs)[i].OOB[:0]
+		buffers := (*msgs)[i].Buffers
-		(*msgs)[i] = ipv6.Message{Buffers: (*msgs)[i].Buffers, OOB: (*msgs)[i].OOB}
+		for j := range buffers {
+			buffers[j] = nil
+		}
+		(*msgs)[i] = ipv6.Message{Buffers: buffers[:1], OOB: (*msgs)[i].OOB[:0]}
 	}
 	s.msgsPool.Put(msgs)
 }
@@ -491,6 +494,7 @@ func coalesceMessages(addr *net.UDPAddr, ep *StdNetEndpoint, bufs [][]byte, offs
 	var (
 		base     = -1 // index of msg we are currently coalescing into
 		gsoSize  int  // segmentation size of msgs[base]
+		totalLen int  // length of all dgrams coalesced into msgs[base]
 		dgramCnt int  // number of dgrams coalesced into msgs[base]
 		endBatch bool // tracking flag to start a new batch on next iteration of bufs
 	)
@@ -502,14 +506,14 @@ func coalesceMessages(addr *net.UDPAddr, ep *StdNetEndpoint, bufs [][]byte, offs
 		buf = buf[offset:]
 		if i > 0 {
 			msgLen := len(buf)
-			baseLenBefore := len(msgs[base].Buffers[0])
+			if msgLen+totalLen <= maxPayloadLen &&
-			freeBaseCap := cap(msgs[base].Buffers[0]) - baseLenBefore
-			if msgLen+baseLenBefore <= maxPayloadLen &&
 				msgLen <= gsoSize &&
-				msgLen <= freeBaseCap &&
 				dgramCnt < udpSegmentMaxDatagrams &&
 				!endBatch {
-				msgs[base].Buffers[0] = append(msgs[base].Buffers[0], buf...)
+				// Coalesce as an additional iovec instead of copying: element
+				// buffers are sized to their packet and have no spare capacity.
+				msgs[base].Buffers = append(msgs[base].Buffers, buf)
+				totalLen += msgLen
 				if i == len(bufs)-1 {
 					setGSO(&msgs[base].OOB, uint16(gsoSize))
 				}
@@ -530,8 +534,9 @@ func coalesceMessages(addr *net.UDPAddr, ep *StdNetEndpoint, bufs [][]byte, offs
 		endBatch = false
 		base++
 		gsoSize = len(buf)
+		totalLen = len(buf)
 		setSrcControl(&msgs[base].OOB, ep)
-		msgs[base].Buffers[0] = buf
+		msgs[base].Buffers = append(msgs[base].Buffers[:0], buf)
 		msgs[base].Addr = addr
 		dgramCnt = 1
 	}