refactor(dap): ensure run_last fetches the latest configuration by name

[kwdiwt]

The current implementation of run_last caches the configuration object from the previous session. This leads to "stale" runs if the user modifies their launch.json or if a dynamic configuration provider updates its values (e.g., changing an environment variable from "RUN_MODE": "a" to "b").

Example Case: When debugging a Python module with a launch.json like this:

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "main",
      "type": "debugpy",
      "request": "launch",
      "module": "mobile.main",
      "env": {
        "RUN_MODE": "a",
      }
   }
}

If the user changes "RUN_MODE" to "b", the updated run_last will correctly pick up the new environment variable without requiring the user to re-select the configuration manually.

[mfussenegger]

This would break re-running dynamically generated configurations that are directly used via dap.run(config). It isn't ensured that a configuration that was run is also part of the config providers result set.
A common example use-case for that is debugging individual unit tests.

The change here could be tweaked to use the last_run config as fallback same as before, but even then I'm not sure the logic change here is desired.
(And it would also need to run in a coroutine to honor the providers contract)

Leaving it open for a while to gather more feedback.

[kwdiwt]

or add M.run_last_by_name ?

[mfussenegger]

or add M.run_last_by_name ?

I'm not to keen on adding different versions of the function. If so I'd rather tend to a opts parameter that adds options to control the behavior. There have also been requests for showing a list of recent configurations, or to avoid re-evaluation of function properties.

But before that I'd like to see more interest in this.

In the meantime you can as workaround also add a custom version in your dotfiles with something like:

local dap = require("dap")
local last_config = nil

---@param session Session
dap.listeners.after.event_initialized["store_config"] = function(session)
  last_config = session.config
end

function M.run_last()
  if last_config then
    -- resolve config like in this PR
  else
    dap.continue()
  end
end