Hello again with futures!

I started working on futures-based workers and got quickly stuck with a
dead-lock I think does not originate in my code (although it is two
semaphores, 8 futures, so I'll refrain from strong opinions here).

I implemented a very simple futures-friendly queue using mutable pairs
and created a minimal-deadlocking-example[1]. I am running racket 3m which includes fixes for the futures-related bugs I discovered

Sometimes the code just runs fine and shows the numbers of worker
iterations performed in different futures (as traced by the 'fid'
argument). But sometimes it locks in a state where there is one last
number in the queue (0 - zero) and yet the fsemaphore-count for the
count fsemaphore returns 0. Which means the semaphore was decremented
twice somewhere. The code is really VERY simple and I do not see a
race-condition within the code, that would allow any code path to
decrement the fsema-count fsemaphore twice once the worker future
receives 0.

I am able to reproduce the behavior with racket3m running under gdb and
get the stack traces for all the threads pretty consistently. The
deadlock is apparently at:

  2    Thread 0x7ffff7fca700 (LWP 46368) "mfqueue.rkt"
futex_wait_cancelable (private=<optimized out>, expected=0,
futex_word=0x5555559d8e78) at ../sysdeps/nptl/futex-internal.h:183

But that is just where the issue is showing up. The real question is how
the counter gets decremented twice (given that fsemaphores should be

Any hints would be VERY appreciated!


[1] http://pasterack.org/pastes/28883

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