Here's some fun. From `bufferevent.h`:
```
#define BEV_EVENT_READING 0x01 /**< error encountered while reading */
#define BEV_EVENT_WRITING 0x02 /**< error encountered while writing */
```
And from `event.h`:
```
/** Wait for a socket or FD to become readable */
#define EV_READ 0x02
/** Wait for a socket or FD to become writeable */
#define EV_WRITE 0x04
```
Library users have to be very careful to get this right; it turns out, the
library itself got this wrong in the `bufferevent_pair` code. It appears that
in most of the code, only `BEV_EVENT_FINISHED` will indicate whether it's read
or write; on error or timeout, it appears that "both" is assumed and not set in
the callback. I read through all the other places where `BEV_EVENT_FINISHED` is
passed to an event callback; it appears that the pair code is the only spot
that got it wrong.
azat: add TT_FORK to avoid breaking clean env, and rebase commit message
(copied from #359)
Fixes: #359
285188963d