Ablog is NOT safe for parallel read

roots/test-parallel/conf.py

@@ -0,0 +1 @@
+extensions = ["ablog"]

roots/test-parallel/index.rst

@@ -0,0 +1,7 @@
+test-postlist
+===============
+
+.. toctree::
+   :maxdepth: 1
+
+   postlist

roots/test-parallel/post1.rst

@@ -0,0 +1,4 @@
+.. post:: 2020-12-01
+
+post 1
+=======

roots/test-parallel/post2.rst

@@ -0,0 +1,4 @@
+.. post:: 2020-12-01
+
+post 2
+=======

roots/test-parallel/post3.rst

@@ -0,0 +1,4 @@
+.. post:: 2020-12-01
+
+post 3
+=======

roots/test-parallel/post4.rst

@@ -0,0 +1,4 @@
+.. post:: 2020-12-01
+
+post 4
+=======

roots/test-parallel/postlist.rst

@@ -0,0 +1,4 @@
+postlist
+==========
+
+.. postlist::

src/ablog/__init__.py

@@ -167,6 +167,6 @@ def setup(app):
     app.add_message_catalog(MESSAGE_CATALOG_NAME, locale_dir)
     return {
         "version": __version__,
-        "parallel_read_safe": True,
+        "parallel_read_safe": False,
         "parallel_write_safe": True,
     }

src/ablog/tests/test_parallel.py

@@ -0,0 +1,27 @@
+from pathlib import Path
+from subprocess import run
+
+
+def test_not_safe_for_parallel_read(rootdir: Path, tmp_path: Path):
+    """Ablog is NOT safe for parallel read. In such case, it doesn't collect any posts."""
+    # https://github.com/sunpy/ablog/issues/297
+    # Very ugly hack to change the parallel_read_safe value to True
+    good_read_safe = '"parallel_read_safe": False'
+    bad_read_safe = '"parallel_read_safe": True'
+    init_py_path = Path(__file__).parent.parent / "__init__.py"
+    assert good_read_safe in init_py_path.read_text(encoding="utf-8")
+    bad_init_py = init_py_path.read_text().replace(good_read_safe, bad_read_safe)
+    init_py_path.write_text(bad_init_py, encoding="utf-8")
+
+    # liborjelinek: I wasn't able to demonstrate the issue with the `parallel` argument to the `sphinx` fixture
+    # @pytest.mark.sphinx("html", testroot="parallel", parallel=2)
+    # therefore running sphinx-build externally
+    indir = rootdir / "test-parallel"
+    run(["sphinx-build", "-b", "html", indir.as_posix(), tmp_path.as_posix(), "-j", "auto"], check=True)
+
+    # And posts are not collected by Ablog...
+    html = (tmp_path / "postlist.html").read_text(encoding="utf-8")
+    assert "post 1" not in html
+    assert "post 2" not in html
+    assert "post 3" not in html
+    assert "post 4" not in html