Busy/infinite Libevent loops

There is a bug causing busy loops in Libevent and infinite loops in the Shadow simulator. A connection that is marked for close, wants to flush, is held open to flush, but is rate limited (the token bucket is empty) triggers the bug.

A patch to the latest alpha (tor-0.2.3.12-alpha) is attached, and details are below.

This currently happens on read and write callbacks when the active socket is marked for close. In this case, Tor doesn't actually try to complete the read or write (it returns from those methods when marked), but instead tries to clear the connection with conn_close_if_marked(). Tor will not close a marked connection that contains data: it must be flushed first. The bug occurs when this flush operation on the marked connection can not occur because the connection is rate-limited (its write token bucket is empty).

The fix is to detect when rate limiting is preventing a marked connection from properly flushing. In this case, it should be flagged as read/write_blocked_on_bandwidth and the read/write events de-registered from Libevent. When the token bucket gets refilled, it will check the associated read/write_blocked_on_bandwidth flag, and add the read/write event back to Libevent, which will cause it to fire. This time, it will be properly flushed and closed.

The reason that both read and write events are both de-registered when the marked connection can not flush is because both result in the same behavior. Both read/write events on marked connections will never again do any actual reads/writes, and are only useful to trigger the flush and close the connection. By setting the associated read/write_blocked_on_bandwidth flag, we ensure that the event will get added back to Libevent, properly flushed, and closed.

Why is this important? Every Shadow event occurs at a discrete time instant. If Tor does not properly deregister Libevent events that fire but result in Tor essentially doing nothing, Libevent will repeatedly fire the event. In Shadow this means infinite loop, outside of Shadow this means wasted CPU cycles.