fix(http_request): remove conflicting CORS headers (SteamDeckHomebrew…

…#708)

backend/decky_loader/utilities.py

@@ -9,7 +9,7 @@
 from stat import FILE_ATTRIBUTE_HIDDEN # pyright: ignore [reportAttributeAccessIssue, reportUnknownVariableType]
 
 from asyncio import StreamReader, StreamWriter, start_server, gather, open_connection
-from aiohttp import ClientSession
+from aiohttp import ClientSession, hdrs
 from aiohttp.web import Request, StreamResponse, Response, json_response, post
 from typing import TYPE_CHECKING, Callable, Coroutine, Dict, Any, List, TypedDict
 
@@ -153,14 +153,14 @@ async def http_request(self, req: Request) -> StreamResponse:
         headers["User-Agent"] = helpers.user_agent
 
         for excluded_header in excluded_default_headers:
-            self.logger.debug(f"Excluding default header {excluded_header}")
             if excluded_header in headers:
+                self.logger.debug(f"Excluding default header {excluded_header}: {headers[excluded_header]}")
                 del headers[excluded_header]
 
         if "X-Decky-Fetch-Excluded-Headers" in req.headers:
             for excluded_header in req.headers["X-Decky-Fetch-Excluded-Headers"].split(", "):
-                self.logger.debug(f"Excluding header {excluded_header}")
                 if excluded_header in headers:
+                    self.logger.debug(f"Excluding header {excluded_header}: {headers[excluded_header]}")
                     del headers[excluded_header]
 
         for header in req.headers:
@@ -187,7 +187,21 @@ async def http_request(self, req: Request) -> StreamResponse:
         # defeat the point of this proxy.
         async with ClientSession(auto_decompress=False) as web:
             async with web.request(req.method, url, headers=headers, data=body, ssl=helpers.get_ssl_context()) as web_res:
-                res = StreamResponse(headers=web_res.headers, status=web_res.status)
+                # Whenever the aiohttp_cors is used, it expects a near complete control over whatever headers are needed
+                # for `aiohttp_cors.ResourceOptions`. As a server, if you delegate CORS handling to aiohttp_cors,
+                # the headers below must NOT be set. Otherwise they would be overwritten by aiohttp_cors and there would be 
+                # logic bugs, so it was probably a smart choice to assert if the headers are present.
+                #
+                # However, this request handler method does not act like our own local server, it always acts like a proxy 
+                # where we do not have control over the response headers. For responses that do not allow CORS, we add the support
+                # via aiohttp_cors. For responses that allow CORS, we have to remove the conflicting headers to allow
+                # aiohttp_cors handle it for us as if there was no CORS support.
+                aiohttp_cors_compatible_headers = web_res.headers.copy()
+                aiohttp_cors_compatible_headers.popall(hdrs.ACCESS_CONTROL_ALLOW_ORIGIN, default=None)
+                aiohttp_cors_compatible_headers.popall(hdrs.ACCESS_CONTROL_ALLOW_CREDENTIALS, default=None)
+                aiohttp_cors_compatible_headers.popall(hdrs.ACCESS_CONTROL_EXPOSE_HEADERS, default=None)
+
+                res = StreamResponse(headers=aiohttp_cors_compatible_headers, status=web_res.status)
                 if web_res.headers.get('Transfer-Encoding', '').lower() == 'chunked':
                     res.enable_chunked_encoding()