There's no guarantee that the problem that valgrind is reporting is the cause
of your bug of course.
Anyhow, the code in question looks to be this, in threadpool.nim:
proc nimSpawn3(fn: WorkerProc; data: pointer) {.compilerProc.} =
# implementation of 'spawn' that is used by the code generator.
while true:
if selectWorker(readyWorker, fn, data): return
vs this:
proc slave(w: ptr Worker) {.thread.} =
isSlave = true
while true:
when declared(atomicStoreN):
atomicStoreN(addr(w.ready), true, ATOMIC_SEQ_CST)
else:
w.ready = true
readyWorker = w
