Ensure a minimum of 32KB for the buffer used in recvmsg()

See iproute2 commit bb5ae621d0c7 ("lib/libnetlink: ensure a minimum of
32KB for the buffer used in rtnl_recvmsg()").
iproute2/iproute2@bb5ae621d0c7

Linux v4.9 and newer contains the two commits mentioned in the above
commit message:
"See kernel commits 9063e21fb026 ("netlink: autosize skb lengthes") and
d35c99ff77ec ("netlink: do not enter direct reclaim from netlink_dump()")
for more details."

Also, use unix.MSG_TRUNC to peek without copying the actual payload.

Related to issues mdlayher#178.

conn_linux.go

@@ -121,28 +121,25 @@ func (c *conn) Send(m Message) error {
 
 // Receive receives one or more Messages from netlink.
 func (c *conn) Receive() ([]Message, error) {
-	b := make([]byte, os.Getpagesize())
-	for {
-		// Peek at the buffer to see how many bytes are available.
-		//
-		// TODO(mdlayher): deal with OOB message data if available, such as
-		// when PacketInfo ConnOption is true.
-		n, _, _, _, err := c.s.Recvmsg(context.Background(), b, nil, unix.MSG_PEEK)
-		if err != nil {
-			return nil, err
-		}
+	b := []byte{}
 
-		// Break when we can read all messages
-		if n < len(b) {
-			break
-		}
+	// Peek at the buffer to see how many bytes are available.
+	//
+	// TODO(mdlayher): deal with OOB message data if available, such as
+	// when PacketInfo ConnOption is true.
+	n, _, _, _, err := c.s.Recvmsg(context.Background(), b, nil, unix.MSG_PEEK|unix.MSG_TRUNC)
+	if err != nil {
+		return nil, err
+	}
 
-		// Double in size if not enough bytes
-		b = make([]byte, len(b)*2)
+	if n < 32768 {
+		n = 32768
 	}
 
+	b = make([]byte, n)
+
 	// Read out all available messages
-	n, _, _, _, err := c.s.Recvmsg(context.Background(), b, nil, 0)
+	n, _, _, _, err = c.s.Recvmsg(context.Background(), b, nil, 0)
 	if err != nil {
 		return nil, err
 	}