-w flag doesn't create parent directories for warning/error file

[GlenNicholls]

Describe the bug

When running sphinx-build with the -w FILE flag, it raises the following exception if the directory where the file exists doesn't exist. Essentially, if the directory build doesn't exist when I run sphinx-build, it doesn't create the directory before the file which I find weird because I'm using that directory for the output of the sphinx build...

sphinx-build -n -T -b html -w build/docs/html.log docs build/docs/html

sphinx-build: error: cannot open warning file 'build/docs/html.log': [Errno 2] No such file or directory: 'build/docs/html.log'
ERROR: InvocationError for command /usr/src/app/fw-tools-pyeda/.tox/docs/bin/sphinx-build -n -T -b html -j auto -d /usr/src/app/fw-tools-pyeda/.tox/docs/tmp/doctrees docs build/docs/html -w build/docs/html.log (exited with code 2)
____________________________________________________________________________________________________ summary ____________________________________________________________________________________________________
ERROR:   docs: commands failed

How to Reproduce

$ git clone https://github.com/.../some_project
$ cd some_project
$ pip install -r requirements.txt
$ cd docs
$ make html SPHINXOPTS="-D language=de"
$ # open _build/html/index and see bla bla

Expected behavior

I would think the -w flag should create all parent directories if they don't exist. sphinx-build creates parents, so it's weird that -w doesn't.

Your project

It is proprietary unfortunately

Screenshots

No response

OS

Ubuntu Focal (docker buildpack-deps:focal)

Python version

3.8

Sphinx version

4.3.2

Sphinx extensions

'sphinx.ext.autodoc',     'sphinx.ext.doctest',     'sphinx.ext.ifconfig',     'sphinx.ext.intersphinx',     'sphinx.ext.napoleon',     'sphinx.ext.viewcode',      'sphinxarg.ext',      'sphinx_toolbox.more_autodoc.regex',

Extra tools

tox

Additional context

The actual tox command is below:

[testenv:docs]
basepython = python3
description = Build the Sphinx documentation.
sitepackages = true
extras = docs
commands =
    sphinx-build -n -T -b html -j auto -d {envtmpdir}/doctrees docs build/docs/html -w build/docs/html.log