system.once is not thread safe

[timotheecour]

system.once isn't thread safe:

template once*(body: untyped): untyped =
  var alreadyExecuted {.global.} = false
  if not alreadyExecuted:
    alreadyExecuted = true
    body

[EDIT] Example

when defined caseSub:
  const N = 10
  var thr: array[N, Thread[int]]
  var count = 0
  proc threadFunc(a: int) {.thread.} =
    once: count.inc
    doAssert count == 1
  proc main =
    for i in 0..<N:
      createThread(thr[i], threadFunc, i)
    joinThreads(thr)
  main()

else:
  import os, strformat
  proc main =
    const nim = getCurrentCompilerExe()
    const exe = "/tmp/z01b"
    const file = currentSourcePath
    # -d:danger
    let cmd = fmt"{nim} c -d:caseSub --threads -o:{exe} {file}"
    echo cmd
    doAssert execShellCmd(cmd) == 0
    for i in 0..<10000:
      let ret = execShellCmd(exe)
      doAssert ret == 0, $i
  main()

Current output

fails after a few iterations: count == 1 [AssertionDefect]

Expected output

works

see also

• https://stackoverflow.com/questions/8412630/how-to-execute-a-piece-of-code-only-once
• http://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11/

[timotheecour]

probably needs import rlocks or locks?

EDIT: note from @xflywind : see
https://github.com/golang/go/blob/master/src/sync/once.go

[timotheecour]

update: added a minimal repro, see top msg /cc @xflywind