unreliable proc.on('close') event

This is a tracking issue for a potential issue noticed in https://github.com/neovim/node-client/pull/414#discussion_r1796637402

Problem

proc.on('close') does not always trigger on nvim.quit(). Based on the failures demonstrated in https://github.com/neovim/node-client/pull/418, it appears that sometimes the child (nvim) does not close the stderr pipe:

    expect(received).toStrictEqual(expected) // deep equality

    - Expected  - 1
    + Received  + 1

      Object {
        "errors": 0,
    -   "stderrClosed": true,
    +   "stderrClosed": false,
        "stdoutClosed": true,
      }

      172 |     // TODO: 'close' event sometimes does not emit. #414
      173 |     proc.on('exit', () => {
    > 174 |       expect(r).toStrictEqual({
          |                 ^
      175 |         stdoutClosed: true,
      176 |         stderrClosed: true,
      177 |         errors: 0,

      at ChildProcess.toStrictEqual (src/attach/attach.test.ts:174:17)

Solution

• document that proc.on('close') is unreliable.
• document proc.on('exit') as a workaround.
• fix the issue(?) in Nvim itself. I thought it might be related to this: https://github.com/neovim/neovim/blob/c4762b309714897615607f135aab9d7bcc763c4f/src/nvim/channel.c#L168-L171

  // Don't close on exit, in case late error messages
  if (!exiting) {
    fclose(stderr);
  }
  

  but, forcing fclose(stderr) does not pass the tests in this PR. Nor does adding fclose(stderr) anywhere else AFAICT.

Start with src/attach/attach.test.ts around the failing proc.on('exit') assertion and review the linked discussions in pull requests 414 and 418. Determine whether the issue should be resolved through node-client documentation, a reliable event-handling change, or an Nvim fix; done means the stderr/close behavior is established and the relevant test or documentation reflects it.