In multithreaded environment, we need to pass messages between workers.
This is done by queuing events to their respective queues. The
double-linked list is not really useful for that as it needs locking
everywhere.
This commit rewrites the event subsystem to use a single-linked list
where events are enqueued by a single atomic instruction and the queue
is processed after atomically moving the whole queue aside.
There is a simple universal IO loop, taking care of events, timers and
sockets. Primarily, one instance of a protocol should use exactly one IO
loop to do all its work, as is now done in BFD.
Contrary to previous versions, the loop is now launched and cleaned by
the nest/proto.c code, allowing for a protocol to just request its own
loop by setting the loop's lock order in config higher than the_bird.
It is not supported nor checked if any protocol changed the requested
lock order in reconfigure. No protocol should do it at all.
In general, events are code handling some some condition, which is
scheduled when such condition happened and executed independently from
I/O loop. Work-events are a subgroup of events that are scheduled
repeatedly until some (often significant) work is done (e.g. feeding
routes to protocol). All scheduled events are executed during each
I/O loop iteration.
Separate work-events from regular events to a separate queue and
rate limit their execution to a fixed number per I/O loop iteration.
That should prevent excess latency when many work-events are
scheduled at one time (e.g. simultaneous reload of many BGP sessions).
This also fixes bug that timer->recurrent was not cleared
in tm_new() and unexpected recurrence of startup timer
in BGP confused state machine and caused crash.
WALK_LIST_DELSAFE (in ev_run_list) is not safe with regard
to deletion of next node. When some events are rescheduled
during event execution, it may lead to deletion of next
node and some events are skipped. Such skipped nodes remain
in temporary list on stack and the last of them contains
'next' pointer to stack area. When this event is later
scheduled, it damages stack area trying to remove it from
the list, which leads to random crashes with funny
backtraces :-) .
