David Goulet authored Jan 24, 2019 and Nick Mathewson committed Feb 21, 2019

KIST works by computing how much should be allowed to write to the kernel for
a given socket, and then it writes that amount to the outbuf.

The problem is that it could be possible that the outbuf already has lots of
data in it from a previous scheduling round (because the kernel is full/busy
and Tor was not able to flush the outbuf yet). KIST ignores that the outbuf
has been filling (is above its "highwater") and writes more anyway. The end
result is that the outbuf length would exceed INT_MAX, hence causing an
assertion error and a corresponding "Bug()" message to get printed to the
logs.

This commit makes it for KIST to take into account the outbuf length when
computing the available space.

Bug found and patch by Rob Jansen.

Closes #29168. TROVE-2019-001.

Signed-off-by: David Goulet <dgoulet@torproject.org>