Fix infinite loop when text contains multiple unclosed comments

The updatepos() override was resetting the parser position to 0,
causing infinite loops when multiple bogus/unclosed comments
appeared in the input (e.g., `</` or `<!--` followed by non-standard
characters). Now tracks the position of the triggering `<` and
advances past it instead of rewinding to the start of the buffer.

Fixes #1578

Author: btucker

markdown/htmlparser.py

@@ -111,6 +111,7 @@ def __init__(self, md: Markdown, *args, **kwargs):
         self.lineno_start_cache = [0]
 
         self.override_comment_update = False
+        self.override_comment_start = 0
 
         # This calls self.reset
         super().__init__(*args, **kwargs)
@@ -124,6 +125,7 @@ def reset(self):
         self._cache: list[str] = []
         self.cleandoc: list[str] = []
         self.lineno_start_cache = [0]
+        self.override_comment_start = 0
 
         super().reset()
 
@@ -276,15 +278,17 @@ def handle_comment(self, data: str):
         i = self.line_offset + self.offset + len(data) + 4
         if self.rawdata[i:i + 3] != '-->':
             self.handle_data('<')
+            pos = self.line_offset + self.offset
+            self.override_comment_start = pos - 1 if self.rawdata[pos - 1:pos] == '<' else pos
             self.override_comment_update = True
             return
         self.handle_empty_tag('<!--{}-->'.format(data), is_block=True)
 
     def updatepos(self, i: int, j: int) -> int:
         if self.override_comment_update:
             self.override_comment_update = False
-            i = 0
-            j = 1
+            i = self.override_comment_start
+            j = self.override_comment_start + 1
         return super().updatepos(i, j)
 
     def handle_decl(self, data: str):

tests/test_syntax/blocks/test_html_blocks.py

@@ -1684,3 +1684,17 @@ def test_noname_tag(self):
                 """
             )
         )
+
+    def test_multiple_bogus_comments_no_hang(self):
+        """Test that multiple bogus comments (</` patterns) don't cause infinite loop."""
+        self.assertMarkdownRenders(
+            '`</` and `</`',
+            '<p><code>&lt;/</code> and <code>&lt;/</code></p>'
+        )
+
+    def test_multiple_unclosed_comments_no_hang(self):
+        """Test that multiple unclosed comments don't cause infinite loop."""
+        self.assertMarkdownRenders(
+            '<!-- and <!--',
+            '<p>&lt;!-- and &lt;!--</p>'
+        )