Commits on Apr 17, 2018

1. Meter accept(2) while taking into account races around thread pool mutex …

   As part of the work in puma#1278 we refactored the conditional in
   `ThreadPool#wait_until_not_full` because we found it difficult to reason about
   at the RailsConf '17 roundtable. We broke it up a bit and everyone went away
   happier. However it turns out that you need to account for both `@waiting` and
   `@todo.size` together to make a determination on whether to accept more work
   from the socket and the refactor was not equivalent to the old code.
   
   There are no guarantees that request threads that have been signalled due to
   work being added to the thread pool's queue will become active, take the mutex
   and decrement `@waiting` before the main worker's thread runs
   `ThreadPool#wait_until_not_full`. If we run that before the request thread
   decrements @waiting we end up hitting the early exit of `return @waiting > 0`,
   taking more work off the socket than the pool's capacity.
   
   The correct way to determine the number of requests we have queued in the pool
   that are outstanding is `@todo.size - @waiting`, so I've reverted to the old
   behaviour and augmented the comment above it.

   

   dannyfallon committed Apr 17, 2018
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