Limit depth of blockquotes using Python's recursion limit.

docs/change_log/release-3.3.md

@@ -53,6 +53,7 @@ The following new features have been included in the 3.3 release:
 
 The following bug fixes are included in the 3.3 release:
 
+* Avoid a `RecursionError` from deeply nested blockquotes (#799).
 * Fix issues with complex emphasis (#979).
 * Limitations of `attr_list` extension are Documented (#965).
 

markdown/blockprocessors.py

@@ -276,7 +276,7 @@ class BlockQuoteProcessor(BlockProcessor):
     RE = re.compile(r'(^|\n)[ ]{0,3}>[ ]?(.*)')
 
     def test(self, parent, block):
-        return bool(self.RE.search(block))
+        return bool(self.RE.search(block)) and not util.nearing_recursion_limit()
 
     def run(self, parent, blocks):
         block = blocks.pop(0)

markdown/test_tools.py

@@ -20,9 +20,10 @@
 """
 
 import os
+import sys
 import unittest
 import textwrap
-from . import markdown
+from . import markdown, util
 
 try:
     import tidylib
@@ -73,6 +74,32 @@ def dedent(self, text):
         return textwrap.dedent(text).strip()
 
 
+class recursionlimit:
+    """
+    A context manager which temporarily modifies the Python recursion limit.
+
+    The testing framework, coverage, etc. may add an arbitrary number of levels to the depth. To maintain consistency
+    in the tests, the current stack depth is determined when called, then added to the provided limit.
+
+    Example usage:
+
+        with recursionlimit(20):
+            # test code here
+
+    See https://stackoverflow.com/a/50120316/866026
+    """
+
+    def __init__(self, limit):
+        self.limit = util._get_stack_depth() + limit
+        self.old_limit = sys.getrecursionlimit()
+
+    def __enter__(self):
+        sys.setrecursionlimit(self.limit)
+
+    def __exit__(self, type, value, tb):
+        sys.setrecursionlimit(self.old_limit)
+
+
 #########################
 # Legacy Test Framework #
 #########################

markdown/util.py

@@ -26,6 +26,7 @@
 import warnings
 import xml.etree.ElementTree
 from .pep562 import Pep562
+from itertools import count
 
 try:
     from importlib import metadata
@@ -156,6 +157,23 @@ def code_escape(text):
     return text
 
 
+def _get_stack_depth(size=2):
+    """Get stack size for caller's frame.
+    See https://stackoverflow.com/a/47956089/866026
+    """
+    frame = sys._getframe(size)
+
+    for size in count(size):
+        frame = frame.f_back
+        if not frame:
+            return size
+
+
+def nearing_recursion_limit():
+    """Return true if current stack depth is withing 100 of maximum limit."""
+    return sys.getrecursionlimit() - _get_stack_depth() < 100
+
+
 """
 MISC AUXILIARY CLASSES
 =============================================================================

tests/test_syntax/blocks/test_blockquotes.py

@@ -0,0 +1,51 @@
+"""
+Python Markdown
+
+A Python implementation of John Gruber's Markdown.
+
+Documentation: https://python-markdown.github.io/
+GitHub: https://github.com/Python-Markdown/markdown/
+PyPI: https://pypi.org/project/Markdown/
+
+Started by Manfred Stienstra (http://www.dwerg.net/).
+Maintained for a few years by Yuri Takhteyev (http://www.freewisdom.org).
+Currently maintained by Waylan Limberg (https://github.com/waylan),
+Dmitry Shachnev (https://github.com/mitya57) and Isaac Muse (https://github.com/facelessuser).
+
+Copyright 2007-2020 The Python Markdown Project (v. 1.7 and later)
+Copyright 2004, 2005, 2006 Yuri Takhteyev (v. 0.2-1.6b)
+Copyright 2004 Manfred Stienstra (the original version)
+
+License: BSD (see LICENSE.md for details).
+"""
+
+from markdown.test_tools import TestCase, recursionlimit
+
+
+class TestBlockquoteBlocks(TestCase):
+
+    # TODO: Move legacy tests here
+
+    def test_nesting_limit(self):
+        # Test that the nesting limit is within 100 levels of recursion limit. Future code changes could cause the
+        # recursion limit to need adjusted here. We need to acocunt for all of Markdown's internal calls. Finally, we
+        # need to account for the 100 level cushion which we are testing.
+        with recursionlimit(120):
+            self.assertMarkdownRenders(
+                '>>>>>>>>>>',
+                self.dedent(
+                    """
+                    <blockquote>
+                    <blockquote>
+                    <blockquote>
+                    <blockquote>
+                    <blockquote>
+                    <p>&gt;&gt;&gt;&gt;&gt;</p>
+                    </blockquote>
+                    </blockquote>
+                    </blockquote>
+                    </blockquote>
+                    </blockquote>
+                    """
+                )
+            )