sctp: add support for MSG_MORE

This patch is to add support for MSG_MORE on sctp.

It adds force_delay in sctp_datamsg to save MSG_MORE, and sets it after
creating datamsg according to the send flag. sctp_packet_can_append_data
then uses it to decide if the chunks of this msg will be sent at once or
delay it.

Note that unlike [1], this patch saves MSG_MORE in datamsg, instead of
in assoc. As sctp enqueues the chunks first, then dequeue them one by
one. If it's saved in assoc,the current msg's send flag (MSG_MORE) may
affect other chunks' bundling.

Since last patch, sctp flush out queue once assoc state falls into
SHUTDOWN_PENDING, the close block problem mentioned in [1] has been
solved as well.

[1] https://patchwork.ozlabs.org/patch/372404/Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/structs.h

@@ -497,6 +497,7 @@ struct sctp_datamsg {
 	/* Did the messenge fail to send? */
 	int send_error;
 	u8 send_failed:1,
+	   force_delay:1,
 	   can_delay;	    /* should this message be Nagle delayed */
 };
 

net/sctp/output.c

@@ -704,18 +704,15 @@ static sctp_xmit_t sctp_packet_can_append_data(struct sctp_packet *packet,
 	 * unacknowledged.
 	 */
 
-	if (sctp_sk(asoc->base.sk)->nodelay)
-		/* Nagle disabled */
+	if ((sctp_sk(asoc->base.sk)->nodelay || inflight == 0) &&
+	    !chunk->msg->force_delay)
+		/* Nothing unacked */
 		return SCTP_XMIT_OK;
 
 	if (!sctp_packet_empty(packet))
 		/* Append to packet */
 		return SCTP_XMIT_OK;
 
-	if (inflight == 0)
-		/* Nothing unacked */
-		return SCTP_XMIT_OK;
-
 	if (!sctp_state(asoc, ESTABLISHED))
 		return SCTP_XMIT_OK;
 

net/sctp/socket.c

@@ -1964,6 +1964,7 @@ static int sctp_sendmsg(struct sock *sk, struct msghdr *msg, size_t msg_len)
 		err = PTR_ERR(datamsg);
 		goto out_free;
 	}
+	datamsg->force_delay = !!(msg->msg_flags & MSG_MORE);
 
 	/* Now send the (possibly) fragmented message. */
 	list_for_each_entry(chunk, &datamsg->chunks, frag_list) {