🧪 Fix fuzzing test failures (#254)

From https://github.com/google/oss-fuzz/tree/master/projects/markdown-it-py,
fixes issues 55363 and 55367

Author: chrisjsewell

markdown_it/rules_block/blockquote.py

@@ -23,7 +23,10 @@ def blockquote(state: StateBlock, startLine: int, endLine: int, silent: bool):
         return False
 
     # check the block quote marker
-    if state.srcCharCode[pos] != 0x3E:  # /* > */
+    try:
+        if state.srcCharCode[pos] != 0x3E:  # /* > */
+            return False
+    except IndexError:
         return False
     pos += 1
 

markdown_it/rules_block/fence.py

@@ -66,8 +66,11 @@ def fence(state: StateBlock, startLine: int, endLine: int, silent: bool):
             #  test
             break
 
-        if state.srcCharCode[pos] != marker:
-            continue
+        try:
+            if state.srcCharCode[pos] != marker:
+                continue
+        except IndexError:
+            break
 
         if state.sCount[nextLine] - state.blkIndent >= 4:
             # closing fence should be indented less than 4 spaces

markdown_it/rules_block/hr.py

@@ -20,7 +20,10 @@ def hr(state: StateBlock, startLine: int, endLine: int, silent: bool):
     if state.sCount[startLine] - state.blkIndent >= 4:
         return False
 
-    marker = state.srcCharCode[pos]
+    try:
+        marker = state.srcCharCode[pos]
+    except IndexError:
+        return False
     pos += 1
 
     # Check hr marker: /* * */ /* - */ /* _ */

markdown_it/rules_block/list.py

@@ -13,7 +13,10 @@ def skipBulletListMarker(state: StateBlock, startLine: int):
     pos = state.bMarks[startLine] + state.tShift[startLine]
     maximum = state.eMarks[startLine]
 
-    marker = state.srcCharCode[pos]
+    try:
+        marker = state.srcCharCode[pos]
+    except IndexError:
+        return -1
     pos += 1
     # Check bullet /* * */ /* - */ /* + */
     if marker != 0x2A and marker != 0x2D and marker != 0x2B:

tests/test_fuzzer.py

@@ -0,0 +1,24 @@
+"""
+These tests are in response to reports from:
+https://github.com/google/oss-fuzz/tree/master/projects/markdown-it-py
+
+In the future, perhaps atheris could be directly used here,
+but it was not directly apparent how to integrate it into pytest.
+"""
+import pytest
+
+from markdown_it import MarkdownIt
+
+TESTS = {
+    55363: ">```\n>",
+    55367: ">-\n>\n>",
+    # 55371: "[](so&#4»0;!"  TODO this did not fail
+    # 55401: "?c_" * 100_000  TODO this did not fail
+}
+
+
+@pytest.mark.parametrize("raw_input", TESTS.values(), ids=TESTS.keys())
+def test_fuzzing(raw_input):
+    md = MarkdownIt()
+    md.parse(raw_input)
+    print(md.render(raw_input))