Preserve crlf newlines in blackd

CHANGES.md

@@ -36,7 +36,7 @@
 
 ### _Blackd_
 
-<!-- Changes to blackd -->
+- Windows style (CRLF) newlines will be preserved (#3257).
 
 ### Integrations
 

src/blackd/__init__.py

@@ -133,6 +133,13 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
             executor, partial(black.format_file_contents, req_str, fast=fast, mode=mode)
         )
 
+        # Preserve CRLF newlines
+        if req_str[req_str.find("\n") - 1] == "\r":
+            formatted_str = formatted_str.replace("\n", "\r\n")
+            # If, after swapping newlines, nothing changed, then say so
+            if formatted_str == req_str:
+                raise black.NothingChanged
+
         # Only output the diff in the HTTP response
         only_diff = bool(request.headers.get(DIFF_HEADER, False))
         if only_diff:

tests/test_blackd.py

@@ -212,3 +212,13 @@ async def test_cors_headers_present(self) -> None:
             response = await self.client.post("/", headers={"Origin": "*"})
             self.assertIsNotNone(response.headers.get("Access-Control-Allow-Origin"))
             self.assertIsNotNone(response.headers.get("Access-Control-Expose-Headers"))
+
+        @unittest_run_loop
+        async def test_preserve_newline(self) -> None:
+            for data in (b"c\r\nc\r\n", b"l\nl\n"):
+                # test preserved newlines when reformatted
+                response = await self.client.post("/", data=data + b" ")
+                self.assertEqual(await response.text(), data.decode())
+                # test 204 when no change
+                response = await self.client.post("/", data=data)
+                self.assertEqual(response.status, 204)