Commits on Sep 22, 2020

1. Prevent connections from entering Reactor after shutdown begins …

   When a `Puma::Server` instance starts to shutdown, it sets the instance
   variable `@queue_requests` to `false`, then starts to shut down the
   Reactor. The intent here is that after the Reactor shuts down, threads
   won't have the option of sending their connections to the Reactor (the
   connection must either be closed, assuming we've written a response to
   the socket, or the thread must wait until the client has finished
   writing its request, then write a response, then finally close the
   connection). This works most of the time just fine.
   
   The problem is that there are races in the implementation of the
   `ThreadPool` where it's possible for a thread to see that
   `@queue_requests` is `true` before the `Server` shutdown has started,
   then later try to add the request to the Reactor, not knowing that it's
   already shutting down.
   
   Depending on the precise order of operations, one possible outcome is
   that the `ThreadPool` executes `@reactor.add` after the `@reactor` has
   closed its `@trigger` pipe, resulting in the error `Error reached top of
   thread-pool: closed stream (IOError)`. Clients experience connection
   reset errors as a result.
   
   The fix introduced in this commit is to add a `Mutex` that makes it
   impossible for a thread in the `ThreadPool` to add a client connection
   to the `@reactor` after `@queue_requests` has been set to `false`.
   
   Co-Authored-By: Leo Belyi <leonid.belyi@mycase.com>

   

   cjlarose and LeoTheMighty committed Sep 22, 2020
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