fix: prevent JSON middleware from throwing 500s on non-transformed content

Author: alukach

src/stac_auth_proxy/utils/middleware.py

@@ -54,18 +54,19 @@ async def transform_response(message: Message) -> None:
             nonlocal start_message
             nonlocal body
 
-            if message["type"] == "http.response.start":
-                # Delay sending start message until we've processed the body
-                start_message = message
-                return
-            assert start_message is not None
+            start_message = start_message or message
+
             if not self.should_transform_response(
                 request=Request(scope),
                 response_headers=Headers(scope=start_message),
             ):
-                return await send(message)
-            if message["type"] != "http.response.body":
-                return await send(message)
+                # For non-JSON responses, send the start message immediately
+                await send(message)
+                return
+
+            # Delay sending start message until we've processed the body
+            if message["type"] == "http.response.start":
+                return
 
             body += message["body"]
 
@@ -83,7 +84,6 @@ async def transform_response(message: Message) -> None:
 
             # Update content-length header
             headers["content-length"] = str(len(body))
-            assert start_message, "Expected start_message to be set"
             start_message["headers"] = [
                 (key.encode(), value.encode()) for key, value in headers.items()
             ]

tests/test_middleware.py

@@ -0,0 +1,105 @@
+"""Tests for middleware utilities."""
+
+from typing import Any
+
+from fastapi import FastAPI, Response
+from starlette.datastructures import Headers
+from starlette.requests import Request
+from starlette.testclient import TestClient
+from starlette.types import ASGIApp
+
+from stac_auth_proxy.utils.middleware import JsonResponseMiddleware
+
+
+class ExampleJsonResponseMiddleware(JsonResponseMiddleware):
+    """Example implementation of JsonResponseMiddleware."""
+
+    def __init__(self, app: ASGIApp):
+        """Initialize the middleware."""
+        self.app = app
+
+    def should_transform_response(
+        self, request: Request, response_headers: Headers
+    ) -> bool:
+        """Transform JSON responses based on content type."""
+        return response_headers.get("content-type", "") == "application/json"
+
+    def transform_json(self, data: Any) -> Any:
+        """Add a test field to the response."""
+        if isinstance(data, dict):
+            data["transformed"] = True
+        return data
+
+
+def test_json_response_middleware():
+    """Test that JSON responses are properly transformed."""
+    app = FastAPI()
+    app.add_middleware(ExampleJsonResponseMiddleware)
+
+    @app.get("/test")
+    async def test_endpoint():
+        return {"message": "test"}
+
+    client = TestClient(app)
+    response = client.get("/test")
+    assert response.status_code == 200
+    assert response.headers["content-type"] == "application/json"
+    data = response.json()
+    assert data["message"] == "test"
+    assert data["transformed"] is True
+
+
+def test_json_response_middleware_no_transform():
+    """Test that responses are not transformed when should_transform_response returns False."""
+    app = FastAPI()
+    app.add_middleware(ExampleJsonResponseMiddleware)
+
+    @app.get("/test")
+    async def test_endpoint():
+        return Response(
+            content='{"message": "test"}',
+            media_type="application/x-json",  # Different from application/json
+        )
+
+    client = TestClient(app)
+    response = client.get("/test")
+    assert response.status_code == 200
+    assert "application/x-json" in response.headers["content-type"]
+    data = response.json()
+    assert data["message"] == "test"
+    assert "transformed" not in data
+
+
+def test_json_response_middleware_chunked():
+    """Test that chunked JSON responses are properly transformed."""
+    app = FastAPI()
+    app.add_middleware(ExampleJsonResponseMiddleware)
+
+    @app.get("/test")
+    async def test_endpoint():
+        return {"message": "test", "large_field": "x" * 10000}
+
+    client = TestClient(app)
+    response = client.get("/test")
+    assert response.status_code == 200
+    assert response.headers["content-type"] == "application/json"
+    data = response.json()
+    assert data["message"] == "test"
+    assert data["transformed"] is True
+    assert len(data["large_field"]) == 10000
+
+
+def test_json_response_middleware_error_handling():
+    """Test that JSON parsing errors are handled gracefully."""
+    app = FastAPI()
+    app.add_middleware(ExampleJsonResponseMiddleware)
+
+    @app.get("/test")
+    async def test_endpoint():
+        return Response(content="invalid json", media_type="text/plain")
+
+    client = TestClient(app)
+    response = client.get("/test")
+    assert response.status_code == 200
+    assert "text/plain" in response.headers["content-type"]
+    assert response.text == "invalid json"