Preserve crlf line endings in blackd

CHANGES.md

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

docs/the_black_code_style/current_style.md

@@ -406,6 +406,10 @@ file that are not enforced yet but might be in a future version of the formatter
 - use variable annotations instead of type comments, even for stubs that target older
   versions of Python.
 
+### Line endings
+_Black_ will normalize line endings (`\n` or `\r\n`) based on the first line ending of
+the file.
+
 ## Pragmatism
 
 Early versions of _Black_ used to be absolutist in some respects. They took after its

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 line endings
+        if req_str[req_str.find("\n") - 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:
+                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_black.py

@@ -1286,6 +1286,14 @@ def test_preserves_line_endings_via_stdin(self) -> None:
             if nl == "\n":
                 self.assertNotIn(b"\r\n", output)
 
+    def test_normalize_line_endings(self) -> None:
+        with TemporaryDirectory() as workspace:
+            test_file = Path(workspace) / "test.py"
+            for data, expected in ((b"c\r\nc\n ", b"c\r\nc\r\n"), (b"l\nl\r\n ", b"l\nl\n")):
+                test_file.write_bytes(data)
+                ff(test_file, write_back=black.WriteBack.YES)
+                self.assertEqual(test_file.read_bytes(), expected)
+
     def test_assert_equivalent_different_asts(self) -> None:
         with self.assertRaises(AssertionError):
             black.assert_equivalent("{}", "None")

tests/test_blackd.py

@@ -209,3 +209,20 @@ 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_preserves_line_endings(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)
+
+    @unittest_run_loop
+    async def test_normalizes_line_endings(self) -> None:
+        for data, expected in ((b"c\r\nc\n", "c\r\nc\r\n"), (b"l\nl\r\n", "l\nl\n")):
+            response = await self.client.post("/", data=data)
+            self.assertEqual(await response.text(), expected)
+            self.assertEqual(response.status, 200)