Fix MarkdownHeaderTextSplitter not recognizing tilde-fenced code blocks (#13511)

- **Description:** Previously `MarkdownHeaderTextSplitter` did not
consider tilde-fenced code blocks
(https://spec.commonmark.org/0.30/#fenced-code-blocks). This PR fixes
that.
   ````md
   # Bug caused by previous implementation:
   ~~~py
   foo()
   # This is a comment that would be considered header
   bar()
   ~~~
   ````
 - **Tag maintainer:** @baskaryan

libs/langchain/langchain/text_splitter.py

@@ -389,16 +389,23 @@ class MarkdownHeaderTextSplitter:
         initial_metadata: Dict[str, str] = {}
 
         in_code_block = False
+        opening_fence = ""
 
         for line in lines:
             stripped_line = line.strip()
 
-            if stripped_line.startswith("```"):
-                # code block in one row
-                if stripped_line.count("```") >= 2:
+            if not in_code_block:
+                # Exclude inline code spans
+                if stripped_line.startswith("```") and stripped_line.count("```") == 1:
+                    in_code_block = True
+                    opening_fence = "```"
+                elif stripped_line.startswith("~~~"):
+                    in_code_block = True
+                    opening_fence = "~~~"
+            else:
+                if stripped_line.startswith(opening_fence):
                     in_code_block = False
-                else:
-                    in_code_block = not in_code_block
+                    opening_fence = ""
 
             if in_code_block:
                 current_content.append(stripped_line)

libs/langchain/tests/unit_tests/test_text_splitter.py

@@ -1031,6 +1031,77 @@ def test_md_header_text_splitter_3() -> None:
     assert output == expected_output
 
 
+@pytest.mark.parametrize("fence", [("```"), ("~~~")])
+def test_md_header_text_splitter_fenced_code_block(fence: str) -> None:
+    """Test markdown splitter by header: Fenced code block."""
+
+    markdown_document = (
+        "# This is a Header\n\n"
+        f"{fence}\n"
+        "foo()\n"
+        "# Not a header\n"
+        "bar()\n"
+        f"{fence}"
+    )
+
+    headers_to_split_on = [
+        ("#", "Header 1"),
+        ("##", "Header 2"),
+    ]
+
+    markdown_splitter = MarkdownHeaderTextSplitter(
+        headers_to_split_on=headers_to_split_on,
+    )
+    output = markdown_splitter.split_text(markdown_document)
+
+    expected_output = [
+        Document(
+            page_content=f"{fence}\nfoo()\n# Not a header\nbar()\n{fence}",
+            metadata={"Header 1": "This is a Header"},
+        ),
+    ]
+
+    assert output == expected_output
+
+
+@pytest.mark.parametrize(["fence", "other_fence"], [("```", "~~~"), ("~~~", "```")])
+def test_md_header_text_splitter_fenced_code_block_interleaved(
+    fence: str, other_fence: str
+) -> None:
+    """Test markdown splitter by header: Interleaved fenced code block."""
+
+    markdown_document = (
+        "# This is a Header\n\n"
+        f"{fence}\n"
+        "foo\n"
+        "# Not a header\n"
+        f"{other_fence}\n"
+        "# Not a header\n"
+        f"{fence}"
+    )
+
+    headers_to_split_on = [
+        ("#", "Header 1"),
+        ("##", "Header 2"),
+    ]
+
+    markdown_splitter = MarkdownHeaderTextSplitter(
+        headers_to_split_on=headers_to_split_on,
+    )
+    output = markdown_splitter.split_text(markdown_document)
+
+    expected_output = [
+        Document(
+            page_content=(
+                f"{fence}\nfoo\n# Not a header\n{other_fence}\n# Not a header\n{fence}"
+            ),
+            metadata={"Header 1": "This is a Header"},
+        ),
+    ]
+
+    assert output == expected_output
+
+
 def test_solidity_code_splitter() -> None:
     splitter = RecursiveCharacterTextSplitter.from_language(
         Language.SOL, chunk_size=CHUNK_SIZE, chunk_overlap=0