Threads can get all sorts of interesting sometimes. And fun to debug. Essentially, whenever the user would restart the Petite process (using the Stop button) a new output thread would be spawned. There wasn’t any problem with the output being duplicated or lost–I already had locking in place to prevent that–but what would happen is that the threads would interleave the output, scrambling output nicely.

Things like this got all sorts of possible:

Expected:

~ (let loop ([i 0])
    (when (< i 10)
      (display i)
      (loop (+ i 1))))
0123456789

Actual:

~ (let loop ([i 0])
    (when (< i 10)
      (display i)
      (loop (+ i 1))))
0123<span style="color: #ff0000;">46</span>789<span style="color: #ff0000;">5</span>

(And it only got worse from there.)

It took a bit longer than I would have liked to track that down, but I finally managed to fix it so that only a single thread would be started no matter how many times the user restarts Petite.