[Libevent-users] bufferevent high watermark bug

[Scott Kevill]

I've just started working with libevent as of v1.3b, and it appears  
to still have a bug with high watermarks for bufferevents. Chris  
Maxwell discovered and mentioned this on the list just over a year ago:

     http://monkeymail.org/archives/libevent-users/2006-February/ 
000101.html

His solution had not been implemented in any subsequent releases of  
libevent, and the bug remains.

Although his solution (removing the return in the high-watermark  
check) does work in preventing lost callbacks, I don't think it is  
the best solution (something he also wondered). The callback can  
still be called with more data buffered than the highwater mark.  
Exactly how much more depends on the arbitrary decisions made in  
evbuffer_read.

The simple change I made to fix the problem was changing:
     if (bufev->wm_read.high != 0)
         howmuch = bufev->wm_read.high;
to:
     if (bufev->wm_read.high != 0)
         howmuch = bufev->wm_read.high - EVBUFFER_LENGTH(bufev->input);

The original problem was that it was using the high watermark as the  
max *additional* bytes to read, instead of the max *total* bytes post- 
read.

It might depend on your idea of what the high watermark behaviour  
should be (it's not really documented), but to me this makes more  
sense as the caller decides how much buffering should be done.

While the above change works for my needs, it's not complete either,  
as there are situations where howmuch will go zero or negative. (eg.  
Not draining the entire buffer in the read-callback). I haven't dug  
deeper to figure out the best thing to do in this case, but the  
socket read should be skipped since there's already more buffered  
than the high watermark. The next read-callback would be with  
EVBUFFER_LENGTH(bufev->input) > bufev->wm_read.high, but if you're  
not completely draining the buffer in the callback, then you should  
probably expect that.

Comments?

Scott.
-- 
Scott Kevill
GameRanger Technologies
http://www.GameRanger.com
multiplayer online gaming services