markdown/tabbed_sections: Raise exception for missing tab name.

This fixes the issue  where 'None' would appear in the rendered
html in case of a missing tab display_name. Now,
'test-help-documentation' will fail in case of any tab display_name
being missing.

In case of a tab_section with no tabs, currently a single tab with
the name 'null_tab' gets added. Added the display name 'None' for
'null_tab', to keep in line with the existing behaviour.

Fixes #19822

templates/zerver/tests/markdown/test_tabbed_sections_missing_tabs.md

@@ -0,0 +1,11 @@
+# Heading
+
+{start_tabs}
+{tab|ios}
+iOS instructions
+
+{tab|minix}
+
+Minix instructions. We expect an exception because the minix tab doesn't have a declared label.
+
+{end_tabs}

zerver/lib/markdown/tabbed_sections.py

@@ -27,7 +27,7 @@ NAV_BAR_TEMPLATE = """
 """.strip()
 
 NAV_LIST_ITEM_TEMPLATE = """
-<li data-language="{data_language}" tabindex="0">{name}</li>
+<li data-language="{data_language}" tabindex="0">{label}</li>
 """.strip()
 
 DIV_TAB_CONTENT_TEMPLATE = """
@@ -141,10 +141,16 @@ class TabbedSectionsPreprocessor(Preprocessor):
     def generate_nav_bar(self, tab_section: Dict[str, Any]) -> str:
         li_elements = []
         for tab in tab_section["tabs"]:
-            li = NAV_LIST_ITEM_TEMPLATE.format(
-                data_language=tab.get("tab_name"), name=TAB_SECTION_LABELS.get(tab.get("tab_name"))
-            )
+            tab_name = tab.get("tab_name")
+            tab_label = TAB_SECTION_LABELS.get(tab_name)
+            if tab_label is None:
+                raise ValueError(
+                    f"Tab '{tab_name}' is not present in TAB_SECTION_LABELS in zerver/lib/markdown/tabbed_sections.py"
+                )
+
+            li = NAV_LIST_ITEM_TEMPLATE.format(data_language=tab_name, label=tab_label)
             li_elements.append(li)
+
         return NAV_BAR_TEMPLATE.format(tabs="\n".join(li_elements))
 
     def parse_tabs(self, lines: List[str]) -> Optional[Dict[str, Any]]:

zerver/tests/test_templates.py

@@ -92,6 +92,15 @@ footer
         expected_html_sans_whitespace = expected_html.replace(" ", "").replace("\n", "")
         self.assertEqual(content_sans_whitespace, expected_html_sans_whitespace)
 
+    def test_markdown_tabbed_sections_missing_tabs(self) -> None:
+        template = get_template("tests/test_markdown.html")
+        context = {
+            "markdown_test_file": "zerver/tests/markdown/test_tabbed_sections_missing_tabs.md",
+        }
+        expected_regex = "^Tab 'minix' is not present in TAB_SECTION_LABELS in zerver/lib/markdown/tabbed_sections.py$"
+        with self.assertRaisesRegex(ValueError, expected_regex):
+            template.render(context)
+
     def test_markdown_nested_code_blocks(self) -> None:
         template = get_template("tests/test_markdown.html")
         context = {