It is possible to run into an IndexError in the subprocess module's _communicate function.

```
    return run(
  File "subprocess.py", line 491, in run
  File "subprocess.py", line 1024, in communicate
  File "subprocess.py", line 1418, in _communicate
IndexError: list index out of range
```

The lines in question are: 

```
            if stdout is not None:
                stdout = stdout[0]
            if stderr is not None:
                stderr = stderr[0]
```

I believe this is due to the fact there is no safety checking to make sure that self._stdout_buff and self._stderr_buff have any content in them after being set to empty lists. 

The fix I suggest is to change the checks from `if stdout is not None` to simply `if stdout` to make sure it is a populated list. 

Example fixed code: 

```
            if stdout:
                stdout = stdout[0]
            if stderr:
                stderr = stderr[0]
```

If a more stringent check is required, we could expand that out to check type and length, such as `isinstance(stdout, list) and len(stdout) > 0:` but that is more then necessary currently.

The presumption I suppose is that these statements only execute if self.stdout_thread and/or self.stderr_thread completes successfully. I suppose that the read could fail or get canceled via CancelSynchronousIo(). Of course in that case you have a bigger problem than an IndexError.

On a related note, _communicate() needs significant changes in Windows. See bpo-43346 if you're interested.

New changeset b4fc44bb2d209182390b4f9fdf074a46b0165a2f by Chris Griffith in branch 'master':
bpo-43423 Fix IndexError in subprocess _communicate function (GH-24777)
https://github.com/python/cpython/commit/b4fc44bb2d209182390b4f9fdf074a46b0165a2f

The bug is fixed, Thanks Chris!  There was a refactoring noted as being nice in my comments on the primary main branch PR that would be nice to have.  But isn't critical.  If you want to make a PR for that, just reuse this bpo-43423 issue number on the PR and assign it to me.

The backports to 3.9 an 3.8 are approved and should automerge after CI finishes.

New changeset 1a5001c606b55226c03fa1046aa8f5e1db2fa67d by Miss Islington (bot) in branch '3.8':
bpo-43423 Fix IndexError in subprocess _communicate function (GH-24777)
https://github.com/python/cpython/commit/1a5001c606b55226c03fa1046aa8f5e1db2fa67d

New changeset ad83fde75463dad2df878ff264f52436eb48bc6b by Miss Islington (bot) in branch '3.9':
bpo-43423 Fix IndexError in subprocess _communicate function (GH-24777)
https://github.com/python/cpython/commit/ad83fde75463dad2df878ff264f52436eb48bc6b