System.Concurrency.Channel non-deterministic behaviour in Chez-Scheme

[CodingCellist]

When using a channel to communicate with multiple processes, things sent on the channel can get lost. Small example:

import System.Concurrency

child : Channel Nat -> IO ()
child c =
  do (S k) <- channelGet c
       | Z => putStrLn "Child thread done."
     child c

main : IO ()
main =
  do c <- makeChannel
     tids <- for [1..4] $ \_ => fork (child c)
     ignore $ for [1..8] $ \_ => channelPut c 1
     ignore $ for [1..4] $ \_ => channelPut c Z
     ignore $ traverse (\t => threadWait t) tids
     putStrLn "All threads + main done."

(a more flexible example can be found here)

Steps to Reproduce

Execute main in the code above (or from the gist), first with --cg chez then with --cg racket.

Expected Behavior

No messages are lost and all threads terminate correctly, followed by the main thread terminating.

Observed Behavior

On the Chez-Scheme backend, the behaviour is non-deterministic: Sometimes all the child threads terminate, but more often they don't and so the program never finishes because at least one of the threads in threadWait never receives the Z that was sent.

By contrast, on the Racket backend, the program seems to always behave as expected, suggesting this is a problem with the Channel implementation in support/chez/support.ss.