How to use pick_process API (or otherwise toggle between processes) with nvim-dap and debugpy?

[philippguevorguian]

As far as I understand, nvim-dap has an API for selecting a process in a menu. I've not seen it documented here or used in debugpy configs (only found examples with JS). Is this possible to do? The launch.json I use allows me to start multiprocessing python scripts with modules and correctly represents the state of one of the processes, and I can step over / into on that process until I am sufficiently out of sync with the other process and get an error. So my question is if there is a way already implement to switch between processes or any examples of how to set this up in my nvim-dap configuration?

If this is a more appropriate question for the debugpy repo, let me know. Thanks !

[mfussenegger]

debugpy afaik doesn't support attaching to a process pid.

You either need to launch the application directly for debugging or use a listen approach documented in https://github.com/microsoft/debugpy?tab=readme-ov-file#debugging-a-script-file

Not sure what you mean with toggling between processes. If you start multiple debug sessions you can switch sessions via the sessions widget.

[philippguevorguian]

Let me clarify, I'm launching the script directly with the configuration below, which starts two processes. I'm able to set a breakpoint at which both processes stop. However, when I step, the debugger only steps for one of the processes. I'm not sufficiently familiar with the internals here, am I able to switch between these processes using the sessions API such that I can step with the other process?

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "titan",
            "type": "debugpy",
            "request": "launch",
            "module": "torch.distributed.run",
            "console": "integratedTerminal",
            "justMyCode": false,
            "args": [
                "--nproc_per_node",
                "2",
                "--rdzv_backend",
                "c10d",
                "--rdzv_endpoint",
                "localhost:10015",
                "--local-ranks-filter",
                "0",
                "--role",
                "rank",
                "--tee",
                "3",
                "train.py",
                "--job.config_file",
                "./train_configs/debug_model.toml"
            ],
            "env": {
                "CUDA_VISIBLE_DEVICES": "0,1",
                "WORLD_SIZE": "2"
            }
        }
    ]
}

[mfussenegger]

am I able to switch between these processes using the sessions API such that I can step with the other process?

You should be. I currently use a command like this:

local widgets = require('dap.ui.widgets')
local sessions_bar = widgets.sidebar(widgets.sessions, {}, '5 sp | setlocal winfixheight')
vim.api.nvim_create_user_command("DapSessions", sessions_bar.toggle, { nargs = 0 })

You can then run :DapSessions and should see one session per sub-process. If you put the cursor over one of the sessions you can press <CR> to set it as active session.