“'myst' reference target not found: b.md#anchor” when building in parallel

Describe the problem

I configured myst_heading_anchors = 2 as documented, but anchor links are giving me this warning if I enable parallelism (sphinx-build -W -j N for any N other than 1):

Warning, treated as error:
/tmp/myst-anchor-test/docs/dir/a.md:2:'myst' reference target not found: b.md#anchor

When building without parallelism, there is no warning and the anchor works fine.

Link to your repository or website

https://github.com/andersk/myst-anchor-test

Steps to reproduce

$ pip install Sphinx myst-parser
…
Successfully installed Jinja2-3.0.1 MarkupSafe-2.0.1 Pygments-2.9.0 Sphinx-4.1.2 alabaster-0.7.12 attrs-21.2.0 babel-2.9.1 certifi-2021.5.30 charset-normalizer-2.0.4 docutils-0.17.1 idna-3.2 imagesize-1.2.0 markdown-it-py-1.1.0 mdit-py-plugins-0.2.8 myst-parser-0.15.1 packaging-21.0 pyparsing-2.4.7 pytz-2021.1 pyyaml-5.4.1 requests-2.26.0 snowballstemmer-2.1.0 sphinxcontrib-applehelp-1.0.2 sphinxcontrib-devhelp-1.0.2 sphinxcontrib-htmlhelp-2.0.0 sphinxcontrib-jsmath-1.0.1 sphinxcontrib-qthelp-1.0.3 sphinxcontrib-serializinghtml-1.1.5 urllib3-1.26.6
$ git clone https://github.com/andersk/myst-anchor-test.git
$ cd myst-anchor-test
$ sphinx-build -W -j2 docs build
Running Sphinx v4.1.2
…
Warning, treated as error:
/tmp/myst-anchor-test/docs/dir/a.md:2:'myst' reference target not found: b.md#anchor
$ sphinx-build -W docs build
Running Sphinx v4.1.2
build succeeded.

The HTML pages are in build.

If you want to reproduce again, rm -rf build.

The version of Python you’re using

3.9.6

Your operating system

NixOS 21.11

Versions of your packages

$ pip freeze
alabaster==0.7.12
attrs==21.2.0
Babel==2.9.1
certifi==2021.5.30
charset-normalizer==2.0.4
docutils==0.17.1
idna==3.2
imagesize==1.2.0
Jinja2==3.0.1
markdown-it-py==1.1.0
MarkupSafe==2.0.1
mdit-py-plugins==0.2.8
myst-parser==0.15.1
packaging==21.0
Pygments==2.9.0
pyparsing==2.4.7
pytz==2021.1
PyYAML==5.4.1
requests==2.26.0
snowballstemmer==2.1.0
Sphinx==4.1.2
sphinxcontrib-applehelp==1.0.2
sphinxcontrib-devhelp==1.0.2
sphinxcontrib-htmlhelp==2.0.0
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==1.0.3
sphinxcontrib-serializinghtml==1.1.5
urllib3==1.26.6

Additional context

No response

[chrisjsewell]

Its okay, I know why it is, fixing now

[andersk]

Indeed, removing the myst_anchors test seems to fix the problem completely:

--- a/myst_parser/myst_refs.py
+++ b/myst_parser/myst_refs.py
@@ -203,7 +203,7 @@ class MystReferenceResolver(ReferencesResolver):
     ) -> Optional[Element]:
         """Resolve doc with anchor."""
         target = node["reftarget"]  # type: str
-        if not ("#" in target and hasattr(self.env, "myst_anchors")):
+        if "#" not in target:
             return None
         # the link may be a heading anchor; we need to first get the relative path
         rel_path, anchor = target.rsplit("#", 1)