fix: Skip content whose empty slug is used in paths

A title made only of punctuation, such as "???", slugifies to an empty
string. When URL patterns use `{slug}`, the content was written to
".html", and two such articles crashed the build. Skip content with such
titles with an error that suggests setting a slug. Patterns that don't
use the slug, such as date-based `ARTICLE_SAVE_AS` patterns, are
unaffected.

pelican/contents.py

@@ -179,6 +179,29 @@ class Content:
                 return False
         return True
 
+    def _has_valid_slug(self) -> bool:
+        """Return false if the slug is empty (e.g., for a title made only of
+        punctuation) but used in the URL or save_as, which would produce file
+        names such as ".html"."""
+        # An empty title is already reported as a missing mandatory property.
+        if getattr(self, "slug", None) != "" or not getattr(self, "title", None):
+            return True
+
+        # Detect whether the slug is used by expanding with a placeholder.
+        paths = (self.url, self.save_as)
+        self.slug = "slug"
+        try:
+            uses_slug = (self.url, self.save_as) != paths
+        finally:
+            self.slug = ""
+        if not uses_slug:
+            return True
+
+        logger.error(
+            "Skipping %s: its slug is empty; set a 'slug' in its metadata", self
+        )
+        return False
+
     def _has_valid_save_as(self) -> bool:
         """Return true if save_as doesn't write outside output path, false
         otherwise."""
@@ -220,6 +243,7 @@ class Content:
         return all(
             [
                 self._has_valid_mandatory_properties(),
+                self._has_valid_slug(),
                 self._has_valid_save_as(),
                 self._has_valid_status(),
             ]

pelican/tests/test_contents.py

@@ -97,6 +97,54 @@ class TestPage(TestBase):
         page = Page("content", metadata={"title": "foobar"})
         self.assertTrue(page._has_valid_mandatory_properties())
 
+    def test_empty_slug_invalid(self):
+        # A title made only of punctuation produces an empty slug, which would
+        # otherwise result in a file named ".html".
+        page = Page("content", metadata={"title": "???"})
+        self.assertEqual(page.slug, "")
+        self.assertFalse(page.is_valid())
+        self.assertLogCountEqual(
+            count=1,
+            msg="Skipping .*: its slug is empty; set a 'slug' in its metadata",
+            level=logging.ERROR,
+        )
+
+        # An explicit slug, or a URL and save_as that don't use the slug, avoid
+        # the empty file name.
+        for metadata in (
+            {"slug": "question-marks"},
+            {"save_as": "q/index.html", "url": "q/"},
+        ):
+            with self.subTest(metadata=metadata):
+                page = Page("content", metadata={"title": "???", **metadata})
+                self.assertTrue(page.is_valid())
+        settings = get_settings(
+            ARTICLE_URL="{date:%Y}/{date:%m}/{date:%d}/",
+            ARTICLE_SAVE_AS="{date:%Y}/{date:%m}/{date:%d}/index.html",
+        )
+        article = Article(
+            "content",
+            metadata={"title": "???", "date": datetime.datetime(2026, 2, 9)},
+            settings=settings,
+        )
+        self.assertTrue(article.is_valid())
+
+        # Overriding save_as alone still leaves the URL as ".html".
+        page = Page("content", metadata={"title": "???", "save_as": "q.html"})
+        self.assertFalse(page.is_valid())
+        self.assertLogCountEqual(
+            count=2,
+            msg="Skipping .*: its slug is empty; set a 'slug' in its metadata",
+            level=logging.ERROR,
+        )
+
+        # An empty title is only reported as a missing title.
+        page = Page("content", metadata={"title": ""})
+        self.assertFalse(page.is_valid())
+        self.assertLogCountEqual(
+            count=2, msg="Skipping .*: its slug is empty.*", level=logging.ERROR
+        )
+
     def test_summary_from_metadata(self):
         # If a :summary: metadata is given, it should be used
         page = Page(**self.page_kwargs)