Merge pull request #1 from secondlife/signal/quirks

Add quirks mode for legacy clients

README.md

@@ -64,6 +64,12 @@ flowchart TD
 Your ASGI application is wrapped around the `LLSDMiddleware`, which performs
 content negotiation based on `Content-Type` and `Accept` HTTP headers.
 
+## Quirks mode
+
+Passing `quirks=True` to the middleware enables 🤪 **quirks mode**. The behavior
+of this mode matches that of poorly behaved Linden Lab services, where the
+server returns LLSD even if the client has not requested it.
+
 [FastAPI]: https://fastapi.tiangolo.com/
 [LLSD]: https://wiki.secondlife.com/wiki/LLSD
 [MessagePack]: https://msgpack.org/index.html

llsd_asgi/middleware.py

@@ -6,15 +6,13 @@
 
 
 class LLSDMiddleware:
-    def __init__(
-        self,
-        app: ASGIApp,
-    ) -> None:
+    def __init__(self, app: ASGIApp, quirks: bool = False) -> None:
         self.app = app
+        self.quirks = quirks
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         if scope["type"] == "http":
-            responder = _LLSDResponder(self.app)
+            responder = _LLSDResponder(self.app, self.quirks)
             await responder(scope, receive, send)
             return
         await self.app(scope, receive, send)
@@ -35,11 +33,9 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
 
 class _LLSDResponder:
 
-    def __init__(
-        self,
-        app: ASGIApp,
-    ) -> None:
+    def __init__(self, app: ASGIApp, quirks: bool = False) -> None:
         self.app = app
+        self.quirks = quirks
         self.should_decode_from_llsd_to_json = False
         self.should_encode_from_json_to_llsd = False
         self.receive: Receive = unattached_receive
@@ -61,7 +57,17 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
             self.format = _CONTENT_TYPE_TO_FORMAT[self.accept_header]
             self.should_encode_from_json_to_llsd = True
         except KeyError:
-            self.should_encode_from_json_to_llsd = False
+            # Quirks mode matches the behavior of some poorly behaved clients,
+            # which expect llsd responses even if they don't send an Accept header.
+            if self.quirks:
+                if not self.accept_header or (
+                    "*/*" in self.accept_header and "application/json" not in self.accept_header
+                ):
+                    self.format = llsd.format_xml
+                    self.should_encode_from_json_to_llsd = True
+                    self.accept_header = None
+            else:
+                self.should_encode_from_json_to_llsd = False
 
         self.receive = receive
         self.send = send
@@ -104,7 +110,7 @@ async def send_with_llsd(self, message: Message) -> None:
 
         if message["type"] == "http.response.start":
             headers = Headers(raw=message["headers"])
-            if headers["content-type"] != "application/json":
+            if headers.get("content-type") != "application/json":
                 # Client accepts llsd, but the app did not send JSON data.
                 # (Note that it may have sent llsd-encoded data.)
                 self.should_encode_from_json_to_llsd = False
@@ -126,7 +132,11 @@ async def send_with_llsd(self, message: Message) -> None:
             body = self.format(json.loads(body))
 
             headers = MutableHeaders(raw=self.initial_message["headers"])
-            headers["Content-Type"] = self.accept_header
+            if self.accept_header:
+                headers["Content-Type"] = self.accept_header
+            else:
+                # quirks mode allows a response without a Content-Type response header
+                del headers["Content-Type"]
             headers["Content-Length"] = str(len(body))
             message["body"] = body
 

tests/test_middleware.py

@@ -143,3 +143,40 @@ async def lifespan_only_app(scope: Scope, receive: Receive, send: Send) -> None:
     app = LLSDMiddleware(lifespan_only_app)
     scope = {"type": "lifespan"}
     await app(scope, mock_receive, mock_send)
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "accept",
+    [(None), ("*/*"), ("text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8")],
+)
+async def test_quirks(accept: str) -> None:
+    app = LLSDMiddleware(JSONResponse({"message": "Hello, world!"}), quirks=True)
+
+    async with httpx.AsyncClient(app=app, base_url="http://testserver") as client:
+        headers = {}
+        if accept is None:
+            # Emulate a client that doesn't send an Accept header
+            del client._headers["accept"]
+        else:
+            headers["accept"] = accept
+        r = await client.get("/")
+        assert r.status_code == 200
+        assert "content-type" not in r.headers
+        assert llsd.parse_xml(r.content) == {"message": "Hello, world!"}
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "accept",
+    [("application/json,*/*")],
+)
+async def test_quirks_exceptions(accept: str) -> None:
+    app = LLSDMiddleware(JSONResponse({"message": "Hello, world!"}), quirks=True)
+
+    async with httpx.AsyncClient(app=app, base_url="http://testserver") as client:
+        client.headers["accept"] = accept
+        r = await client.get("/")
+        assert r.status_code == 200
+        assert r.headers["content-type"] == "application/json"
+        assert r.json() == {"message": "Hello, world!"}