ORC Segfault when storing ref Exception from seperate thread (Nim 2.2.2)

[miguelmartin75]

Nim Version

I tested with 2.2.4 and devel, and the following issue is patched. I am reporting to document my findings.

devel:

Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2025-04-30
Copyright (c) 2006-2025 by Andreas Rumpf

git hash: b5b7a127fd92349a1517d2c7e7a4f25a532fac59
active boot switches: -d:release

2.2.4:

Nim Compiler Version 2.2.4 [Linux: amd64]
Compiled at 2025-04-30
Copyright (c) 2006-2025 by Andreas Rumpf

git hash: f7145dd26efeeeb6eeae6fff649db244d81b212d
active boot switches: -d:release

For 2.2.2, the issue is present:

Nim Compiler Version 2.2.2 [Linux: amd64]
Compiled at 2025-04-11
Copyright (c) 2006-2025 by Andreas Rumpf

git hash: 6c34f62785263ad412f662f3e4e4bf8d8751d113
active boot switches: -d:release

Description

Here is a reproducible snippet, call this thread_orc_segfault.nim:

type
  MyResult = object
    value*: int
    exception*: ref Exception

proc threadFn(result: ptr MyResult) =
  proc foo(x: int): int =
    doAssert x != 42, "It can't be!"
    return x * 2

  try:
    result[].value = foo(42)
  except:
    result[].exception = getCurrentException()

let nThreads = 1

var
  outputs: seq[MyResult]
  threads: seq[Thread[ptr MyResult]]

threads.setLen(nThreads)
outputs.setLen(nThreads)

for i in 0..<nThreads:
  createThread(threads[i], threadFn, (outputs[i].addr))

for i in 0..<nThreads:
  joinThread(threads[i])

for i, o in outputs:
  if o.exception != nil:
    echo "outputs[", i, "]="
    echo o.exception.msg
    echo o.exception.getStackTrace()

Note nThreads can be set to anything >=1.

Compile and run with: nim c --mm:orc -r thread_orc_segfault.nim or nim c --mm:orc -d:useMalloc -r thread_orc_segfault.nim

Current Output

$ nim c --mm:orc -d:useMalloc -r thread_orc_segfault.nim
Hint: used config file '/home/mig/repos/Nim/config/nim.cfg' [Conf]
Hint: used config file '/home/mig/repos/Nim/config/config.nims' [Conf]
....................................................................
CC: ../Nim/lib/system/exceptions.nim
CC: ../Nim/lib/std/private/digitsutils.nim
CC: ../Nim/lib/std/assertions.nim
CC: ../Nim/lib/system/dollars.nim
CC: ../Nim/lib/std/typedthreads.nim
CC: ../Nim/lib/system.nim
CC: thread_orc_segfault.nim
Hint:  [Link]
Hint: mm: orc; threads: on; opt: none (DEBUG BUILD, `-d:release` generates faster code)
27157 lines; 0.279s; 30.926MiB peakmem; proj: /home/mig/repos/nim-issues/thread_orc_segfault.nim; out: /home/mig/repos/nim-issues/thread_orc_segfault [SuccessX]
Hint: /home/mig/repos/nim-issues/thread_orc_segfault [Exec]
outputs[0]=
/home/mig/repos/nim-issues/thread_orc_segfault.nim(8, 5) `x != 42` It can't be!
/home/mig/repos/nim-issues/thread_orc_segfault.nim(12) threadFn
/home/mig/repos/nim-issues/thread_orc_segfault.nim(8) foo
/home/mig/repos/Nim/lib/std/assertions.nim(41) failedAssertImpl
/home/mig/repos/Nim/lib/std/assertions.nim(36) raiseAssert
/home/mig/repos/Nim/lib/system/fatal.nim(53) sysFatal

Traceback (most recent call last)
/home/mig/repos/nim-issues/thread_orc_segfault.nim(19) thread_orc_segfault
/home/mig/repos/Nim/lib/system/orc.nim(521) nimDecRefIsLastCyclicDyn
/home/mig/repos/Nim/lib/system/orc.nim(503) rememberCycle
/home/mig/repos/Nim/lib/system/orc.nim(151) unregisterCycle
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
Segmentation fault (core dumped)
Error: execution of an external program failed: '/home/mig/repos/nim-issues/thread_orc_segfault'

Expected Output

No segfault.

Known Workarounds

For 2.2.2:

1. Use --mm:arc (with or without -d:useMalloc for this example), or
2. --mm:atomicArc

Or upgrade to 2.2.4 or devel

Additional Information

No response

[miguelmartin75]

In real code, I am still getting a segfault for 2.2.4 - I need a better example.

[darkestpigeon]

I don't think this is supposed to work without --mm:atomicArc. References are tied to the garbage collector, hence to the thread.