gc:arc cannot fully support threadpool with FlowVar

[khchen]

Example

import threadpool, os

proc thread(): string =
  os.sleep(1000)
  return "ok"

var fv = spawn thread()
sync()
echo ^fv

Current Output

Error: cannot bind another '=destroy' to: FlowVarObj; previous declaration was constructed here implicitly: d:\nim\lib\pure\concurrency\threadpool.nim(211, 48)

Expected Output

ok

$ nim -v
Nim Compiler Version 1.1.1 [Windows: amd64]
Compiled at 2020-03-27```

[cooldome]

I do have a fix for this one

[diegomrsantos]

Seems there is a regression. Running the same code as reported outputs the following:

Current Output

ok
Traceback (most recent call last)
.choosenim/toolchains/nim-#devel/lib/pure/concurrency/threadpool.nim(216) test
.choosenim/toolchains/nim-#devel/lib/system/orc.nim(486) nimDecRefIsLastCyclicDyn
.choosenim/toolchains/nim-#devel/lib/system/orc.nim(469) rememberCycle
.choosenim/toolchains/nim-#devel/lib/system/orc.nim(147) unregisterCycle
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Nim Compiler Version 1.7.3 [MacOSX: amd64]
Compiled at 2022-11-09
Copyright (c) 2006-2022 by Andreas Rumpf

git hash: cdb136f
active boot switches: -d:release

[Araq]

Report it properly. A test program that fails, a new issue, more information, things like that.

[diegomrsantos]

I can create a new issue if that's better, but I used the same code as initially reported and posted the current output.

[Araq]

Ah ok.

[ringabout]

I can create a new issue if that's better, but I used the same code as initially reported and posted the current output.

Duplicate of #14901