blackd: fix mishandling of single character input (#3558)

CHANGES.md

@@ -45,6 +45,8 @@
 
 <!-- Changes to blackd -->
 
+- Fix an issue in `blackd` with single character input (#3558)
+
 ### Integrations
 
 <!-- For example, Docker, GitHub Actions, pre-commit, editors -->

src/blackd/__init__.py

@@ -152,7 +152,8 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
         )
 
         # Preserve CRLF line endings
-        if req_str[req_str.find("\n") - 1] == "\r":
+        nl = req_str.find("\n")
+        if nl > 0 and req_str[nl - 1] == "\r":
             formatted_str = formatted_str.replace("\n", "\r\n")
             # If, after swapping line endings, nothing changed, then say so
             if formatted_str == req_str:

tests/test_blackd.py

@@ -240,3 +240,9 @@ async def test_normalizes_line_endings(self) -> None:
             response = await self.client.post("/", data=data)
             self.assertEqual(await response.text(), expected)
             self.assertEqual(response.status, 200)
+
+    @unittest_run_loop
+    async def test_single_character(self) -> None:
+        response = await self.client.post("/", data="1")
+        self.assertEqual(await response.text(), "1\n")
+        self.assertEqual(response.status, 200)