bgpd: Ensure send order is 100% consistent

When BGP is sending updates to peers on a neighbor up event
it was noticed that the bgp updates being sent were in reverse
order being sent to the first peer.

Imagine r1 -- r2 -- r3.  r1 and r2 are ebgp peers and
r2 and r3 are ebgp peers.  r1's interface to r2 is currently
shutdown.  Prior to this fix the send order would look like this:

r1 -> r2 send of routes to r2 and then they would be installed in order
received:

10.0.0.12 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.11 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.10 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.9 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.8 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.7 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.6 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.5 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.4 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.3 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.2 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.1 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20

r2 would then send these routes to r3 and then they would be installed
in order received:

10.0.0.1 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.2 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.3 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.4 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.5 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.6 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.7 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.8 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.9 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.10 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.11 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.12 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20

Not that big of a deal right?  Well imagine a situation where r1 is
originating several ten's of thousands of routes.  It sends routes to r2
r2 is processing routes but in reverse order and at the same time it
is sending routes to r3, in the correct order of the bgp table.

r3 will have the early 10.0.0.1/32 routes installed and start forwarding
while r2 will not have those routes installed yet( since they were at the
end and zebra is slightly slower for processing routes than bgp is ).

Ensure that the order sent is a true FIFO.  What is happening is that
there is an update fifo which stores all routes.  And off that FIFO
is a bgp advertise attribute list which stores the list of prefixes
which share the same attribute that allow for more efficient packing
this list was being stored in reverse order causing the problem for
the initial send.  When adding items to this list put them at the
end so we keep the fifo order that is traversed when we walk through
the bgp table.

After the fix:

r2 installation order:

10.0.0.0 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.1 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.2 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.3 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.4 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.5 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.6 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.7 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.8 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.9 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.10 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.11 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20
10.0.0.12 nhid 39 via 192.168.8.2 dev leaf2-eth5 proto bgp metric 20

r3 installation order:

10.0.0.0 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.1 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.2 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.3 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.4 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.5 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.6 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.7 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.8 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.9 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.10 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.11 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20
10.0.0.12 nhid 12 via 192.168.61.2 dev spine2-eth1 proto bgp metric 20

Signed-off-by: Donald Sharp <[email protected]>

bgpd/bgp_advertise.c

@@ -83,10 +83,25 @@ void bgp_advertise_free(struct bgp_advertise *adv)
 void bgp_advertise_add(struct bgp_advertise_attr *baa,
 		       struct bgp_advertise *adv)
 {
-	adv->next = baa->adv;
-	if (baa->adv)
-		baa->adv->prev = adv;
-	baa->adv = adv;
+	struct bgp_advertise *spot, *prev = NULL;
+
+	spot = baa->adv;
+
+	while (spot) {
+		prev = spot;
+		spot = spot->next;
+	}
+
+	if (prev) {
+		prev->next = adv;
+		adv->prev = prev;
+	} else
+		adv->prev = NULL;
+
+	adv->next = NULL;
+
+	if (!baa->adv)
+		baa->adv = adv;
 }
 
 void bgp_advertise_delete(struct bgp_advertise_attr *baa,