Don't proxy the full response headers (PP-1713) (#2055)

This solves an issue we were seeing where the Content-Encoding header is set on the request, but since Python requests automatically decodes the transfer encoding, we were mistakenly forwarding on the Content-Encoding header, without encoded data.

src/palace/manager/api/circulation.py

@@ -271,6 +271,14 @@ def response(self) -> Response:
         )
 
 
+class FetchResponse(Response):
+    """
+    Response object that defaults to no mimetype if none is provided.
+    """
+
+    default_mimetype = None
+
+
 class FetchFulfillment(UrlFulfillment, LoggerMixin):
     """
     Fulfill a loan by fetching a URL and returning the content. This should be
@@ -314,12 +322,16 @@ def response(self) -> Response:
             )
             raise
 
-        headers = dict(response.headers)
+        headers = {"Cache-Control": "private"}
 
         if self.content_type:
             headers["Content-Type"] = self.content_type
+        elif "Content-Type" in response.headers:
+            headers["Content-Type"] = response.headers["Content-Type"]
 
-        return Response(response.content, status=response.status_code, headers=headers)
+        return FetchResponse(
+            response.content, status=response.status_code, headers=headers
+        )
 
 
 class LoanInfo(CirculationInfo):

tests/manager/api/test_circulation.py

@@ -55,7 +55,10 @@ class TestFetchFulfillment:
     def test_fetch_fulfillment(self) -> None:
         http = MockHTTPClient()
         http.queue_response(
-            204, content="This is some content.", media_type="application/xyz"
+            204,
+            content="This is some content.",
+            media_type="application/xyz",
+            other_headers={"X-Test": "test"},
         )
         fulfillment = FetchFulfillment("http://some.location", "foo/bar")
         with http.patch():
@@ -68,6 +71,7 @@ def test_fetch_fulfillment(self) -> None:
         # Any content type set on the fulfillment, overrides the content type from the request.
         assert response.content_type == "foo/bar"
         assert http.requests == ["http://some.location"]
+        assert "X-Test" not in response.headers
 
         # If no content type is set on the fulfillment, the content type from the request is used.
         http = MockHTTPClient()
@@ -83,6 +87,16 @@ def test_fetch_fulfillment(self) -> None:
         [(args, kwargs)] = http.requests_args
         assert kwargs["allow_redirects"] is True
 
+        # If the content type is not set on the fulfillment, and the response does not have a content type,
+        # we fall back to no content type.
+        http = MockHTTPClient()
+        http.queue_response(200, content="Other content.")
+        fulfillment = FetchFulfillment("http://some.other.location")
+        with http.patch():
+            response = fulfillment.response()
+        assert isinstance(response, Response)
+        assert response.content_type is None
+
     def test_fetch_fulfillment_include_headers(self) -> None:
         # If include_headers is set, the headers are set when the fetch is made, but
         # not included in the response.