Channels do not deep copy with gc:orc contrary to the docs

[yglukhov]

Nim Version

Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2025-04-09
Copyright (c) 2006-2025 by Andreas Rumpf

git hash: 29a2e25
active boot switches: -d:release

Description

type
  Msg = object
    content: Content
  Content = ref object
    value: int

var chan: Channel[Msg]

var contentAddress: int

# This proc will be run in another thread using the threads module.
proc firstWorker() =
  let content = Content(value: 5)
  contentAddress = cast[int](content)
  echo "Sending content: ", contentAddress
  chan.send(Msg(content: content))

# Initialize the channel.
chan.open()

# Launch the worker.
var worker1: Thread[void]
createThread(worker1, firstWorker)

# Block until the message arrives, then print it out.
let c = chan.recv()
echo "Received content: ", cast[int](c.content)
doAssert(contentAddress != cast[int](c.content)) # FAILS HERE!!!

# Wait for the thread to exit
worker1.joinThread()

Current Output

Sending content: 139901182169160
Received content: 139901182169160
/home/y/proj/test/tests/test2.nim(160) test2
/home/y/proj/nim-devel/lib/std/assertions.nim(41) failedAssertImpl
/home/y/proj/nim-devel/lib/std/assertions.nim(36) raiseAssert
/home/y/proj/nim-devel/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: /home/y/proj/test/tests/test2.nim(160, 9) `contentAddress != cast[int](c.content)`  [AssertionDefect]

Expected Output

No assertsion failure

Known Workarounds

No response

Additional Information

Behaves correctly with --gc:refc

[Araq]

Docs need to be updated. One point of ORC and the shared MM is that we can send without deepcopies.

[yglukhov]

But then how the reference counting will work for ref objects shared between two threads? It is not atomic, is it?

[Araq]

With --mm:atomicArc it is. If it's not you move the references around and then you need to ensure uniqueness for this to work. IME it's easy to do that in practice but almost impossible to come up with good compiler support for it. Well there is isolate...