libceph: add empty check to ceph_con_get_out_msg()

This moves the list_empty() checks from the two callers (v1 and v2)
into the base messenger.c library.  Now the v1/v2 specializations do
not need to know about con->out_queue; that implementation detail is
now hidden behind the ceph_con_get_out_msg() function.

[ idryomov: instead of changing prepare_write_message() to return
  a bool, move ceph_con_get_out_msg() call out to arrive to the same
  pattern as in messenger_v2.c ]

Signed-off-by: Max Kellermann <[email protected]>
Reviewed-by: Viacheslav Dubeyko <[email protected]>
Signed-off-by: Ilya Dryomov <[email protected]>

Author: MaxKellermann

net/ceph/messenger.c

@@ -2114,7 +2114,9 @@ struct ceph_msg *ceph_con_get_out_msg(struct ceph_connection *con)
 {
 	struct ceph_msg *msg;
 
-	BUG_ON(list_empty(&con->out_queue));
+	if (list_empty(&con->out_queue))
+		return NULL;
+
 	msg = list_first_entry(&con->out_queue, struct ceph_msg, list_head);
 	WARN_ON(msg->con != con);
 

net/ceph/messenger_v1.c

@@ -191,9 +191,9 @@ static void prepare_write_message_footer(struct ceph_connection *con,
 /*
  * Prepare headers for the next outgoing message.
  */
-static void prepare_write_message(struct ceph_connection *con)
+static void prepare_write_message(struct ceph_connection *con,
+				  struct ceph_msg *m)
 {
-	struct ceph_msg *m;
 	u32 crc;
 
 	con_out_kvec_reset(con);
@@ -209,8 +209,6 @@ static void prepare_write_message(struct ceph_connection *con)
 			&con->v1.out_temp_ack);
 	}
 
-	m = ceph_con_get_out_msg(con);
-
 	dout("prepare_write_message %p seq %lld type %d len %d+%d+%zd\n",
 	     m, con->out_seq, le16_to_cpu(m->hdr.type),
 	     le32_to_cpu(m->hdr.front_len), le32_to_cpu(m->hdr.middle_len),
@@ -1545,8 +1543,8 @@ int ceph_con_v1_try_write(struct ceph_connection *con)
 			goto more;
 		}
 		/* is anything else pending? */
-		if (!list_empty(&con->out_queue)) {
-			prepare_write_message(con);
+		if ((msg = ceph_con_get_out_msg(con)) != NULL) {
+			prepare_write_message(con, msg);
 			goto more;
 		}
 		if (con->in_seq > con->in_seq_acked) {

net/ceph/messenger_v2.c

@@ -3310,8 +3310,7 @@ static int populate_out_iter(struct ceph_connection *con)
 			pr_err("prepare_keepalive2 failed: %d\n", ret);
 			return ret;
 		}
-	} else if (!list_empty(&con->out_queue)) {
-		msg = ceph_con_get_out_msg(con);
+	} else if ((msg = ceph_con_get_out_msg(con)) != NULL) {
 		ret = prepare_message(con, msg);
 		if (ret) {
 			pr_err("prepare_message failed: %d\n", ret);