Re-write request Content-Type when decoding

README.md

@@ -123,7 +123,7 @@ That's all there is to it! You can now go reduce the size of your payloads.
 
 An ASGI application wrapped around `MessagePackMiddleware` will perform automatic content negotiation based on the client's capabilities. More precisely:
 
-- If the client sends MessagePack-encoded data with the `application/x-msgpack` content type, `msgpack-asgi` will automatically re-encode it to JSON for your application to consume.
+- If the client sends MessagePack-encoded data with the `application/x-msgpack` content type, `msgpack-asgi` will automatically re-encode the body to JSON and re-write the request `Content-Type` to `application/json` for your application to consume. (Note: this means applications will not be able to distinguish between MessagePack and JSON client requests.)
 - If the client sent the `Accept: application/x-msgpack` header, `msgpack-asgi` will automatically re-encode any JSON response data to MessagePack for the client to consume.
 
 (In other cases, `msgpack-asgi` won't intervene at all.)

src/msgpack_asgi/_middleware.py

@@ -50,7 +50,7 @@ def __init__(
         self.started = False
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
-        headers = Headers(scope=scope)
+        headers = MutableHeaders(scope=scope)
         self.should_decode_from_msgpack_to_json = (
             "application/x-msgpack" in headers.get("content-type", "")
         )
@@ -61,6 +61,14 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         )
         self.receive = receive
         self.send = send
+
+        if self.should_decode_from_msgpack_to_json:
+            # We're going to present JSON content to the application,
+            # so rewrite `Content-Type` for consistency and compliance
+            # with possible downstream security checks in some frameworks.
+            # See: https://github.com/florimondmanca/msgpack-asgi/issues/23
+            headers["content-type"] = "application/json"
+
         await self.app(scope, self.receive_with_msgpack, self.send_with_msgpack)
 
     async def receive_with_msgpack(self) -> Message:

tests/test_middleware.py

@@ -30,16 +30,15 @@ async def app(scope: Scope, receive: Receive, send: Send) -> None:
             "/", content=body, headers={"content-type": "application/x-msgpack"}
         )
         assert r.status_code == 200
-        assert r.text == "content_type='application/x-msgpack' message='Hello, world!'"
+        assert r.text == "content_type='application/json' message='Hello, world!'"
 
 
 @pytest.mark.asyncio
 async def test_non_msgpack_request() -> None:
     async def app(scope: Scope, receive: Receive, send: Send) -> None:
         request = Request(scope, receive=receive)
         content_type = request.headers["content-type"]
-        data = await request.json()
-        message = data["message"]
+        message = (await request.body()).decode()
         text = f"content_type={content_type!r} message={message!r}"
 
         response = PlainTextResponse(text)
@@ -48,9 +47,13 @@ async def app(scope: Scope, receive: Receive, send: Send) -> None:
     app = MessagePackMiddleware(app)
 
     async with httpx.AsyncClient(app=app, base_url="http://testserver") as client:
-        r = await client.post("/", json={"message": "Hello, world!"})
+        r = await client.post(
+            "/",
+            content="Hello, world!",
+            headers={"content-type": "text/plain"},
+        )
         assert r.status_code == 200
-        assert r.text == "content_type='application/json' message='Hello, world!'"
+        assert r.text == "content_type='text/plain' message='Hello, world!'"
 
 
 @pytest.mark.asyncio