"Can't support concurrency=greenlet with SysMonitor, only threads are supported." error on python 3.14

[DetachHead]

Describe the bug
when setting concurrency to greenlet on python 3.14, the following error occurs when running anything with coverage:

> coverage run asdf
Can't support concurrency=greenlet with SysMonitor, only threads are supported.

To Reproduce

1. create the following pyproject.toml:

   # pyproject.toml
   [project]
   name = "asdf"
   version = "0.0.1"
   dependencies = [
       "greenlet>=3.2.4",
       "coverage>=7.10.7",
   ]
   requires-python = "~=3.13.0"
   readme = "README.md"
   license = { text = "MIT" }
   
   [tool.coverage.run]
   concurrency = ["greenlet"]

2. install uv
3. run the following commands:

   uv sync
   uv run coverage run asdf
   

Answer the questions below:

1. What version of Python are you using? 3.14
2. What version of coverage.py shows the problem? 7.10.7
3. What versions of what packages do you have installed? The output of pip freeze is helpful. greenlet 3.2.4
4. What code shows the problem? N/A, happens on any command
5. What commands should we run to reproduce the problem? see above

Expected behavior
no error

Additional context
this only happens on python 3.14. works fine on python 3.13

[devdanzin]

That's because sysmon doesn't support greenlet. Does it work if you add:

[tool.coverage.run]
core = "ctrace"

Would it make sense to automatically select ctrace when sysmon doesn't support something?

Edit: it might not be obvious why this issue only happens with 3.14, the reason is that sysmon is the default for 3.14.

[DetachHead]

that works, thanks.

Would it make sense to automatically select ctrace when sysmon doesn't support something?

yeah i think that makes sense