> > > > > > In this technical note, I write down the workings of the midas event buffer code

Event buffer readers and writers need to communicate following information:

- writer to reader: when new events are written to the buffer, send a message to wakeup any readers that have matching requests
- reader to writer: when buffer is full and we are waiting for free space, readers need to send us a message after they free up some space

Writer to reader path:
writer - bm_send_event() -> bm_wake_up_client_locked() -> for all readers with matching requests -> if "read_wait" is set, send "B" message
reader - ... -> bm_read_buffer() -> bm_wait_for_more_events() -> set "read_wait" to TRUE -> ss_suspend(1000, MSG_BM) - wait for "B" message, also poll the 
buffer every 1000 msec -> when new event is received, clear "read_wait".

Reader to writer path:
writer - bm_send_event() -> bm_wait_for_free_space() -> set "write_wait" to the amount of free space requested -> ss_suspend(1000, MSG_BM) - wait for "B" 
message, also poll every 1000 msec -> clear "write_wait" to zero.
reader - ... -> bm_read_buffer() -> (also via bm_fill_read_cache() -> bm_wakeup_producers_locked() -> if we are a GET_ALL reader -> if buffer is 50% empty -> 
if write_wait < free_space -> send "B" message.

N.B.1. There is a fly in the ointment for the writer-to-reader path: a function called
bm_mark_read_waiting() is called from several places to set and clear the reader "read_wait" flag.
I think this causes several logic errors: "read_wait" is mistakenly set for buffers where we have
not requested anything; and "read_wait" is forcibly cleared when we are actually waiting
for a "B" message, with "read_wait" cleared, this message will not be sent. But because we  also
poll for new events (nominally every 1000 msec, with cm_watchdog() cutting it down to average 500 msec),
we will not see this fault unless we look carefully for any extra delays between sending and receiving events.

N.B.2. There is a mistake in bm_wakeup_producers(), it should not send "B" messages to clients with zero "write_wait". (fixed in the new code).

and that's all she wrote,
K.O.