BugFix: fix DAG doc display (especially for TaskFlow DAGs) (#14564)

Because of how TaskFlow DAGs are constructed, their __doc__ lines
may start with spaces. This fails markdown.markdown(), and the
doc in Markdown format cannot be transformed into HTML properly,
and further fails the doc display in the UI.

This commit fixes this by always doing left strip for each line for the doc md.

airflow/www/utils.py

@@ -337,6 +337,8 @@ def wrapped_markdown(s, css_class=None):
     if s is None:
         return None
 
+    s = '\n'.join(line.lstrip() for line in s.split('\n'))
+
     return Markup(f'<div class="{css_class}" >' + markdown.markdown(s, extensions=['tables']) + "</div>")
 
 

tests/www/test_utils.py

@@ -245,3 +245,14 @@ def test_wrapped_markdown_with_table(self):
             '</td>\n<td>14m</td>\n</tr>\n</tbody>\n'
             '</table></div>'
         ) == rendered
+
+    def test_wrapped_markdown_with_indented_lines(self):
+        rendered = wrapped_markdown(
+            """
+                # header
+                1st line
+                2nd line
+            """
+        )
+
+        assert '<div class="None" ><h1>header</h1>\n<p>1st line\n2nd line</p></div>' == rendered