🐛 Set anti-buffering headers on JSONL streaming responses

Server-Sent Events responses set `Cache-Control: no-cache` and
`X-Accel-Buffering: no` so proxies (e.g. Nginx) deliver events
incrementally instead of buffering the whole response.

JSONL streaming responses are incremental in the same way, but were
missing these headers, so a buffering proxy would hold back the lines
and defeat the streaming. Set the same headers on the JSONL response
for consistency with SSE.

fastapi/routing.py

@@ -658,6 +658,9 @@ def get_request_handler(
                     media_type="application/jsonl",
                     background=solved_result.background_tasks,
                 )
+                response.headers["Cache-Control"] = "no-cache"
+                # For Nginx proxies to not buffer the streamed response
+                response.headers["X-Accel-Buffering"] = "no"
                 response.headers.raw.extend(solved_result.response.headers.raw)
             elif dependant.is_async_gen_callable or dependant.is_gen_callable:
                 # Raw streaming with explicit response_class (e.g. StreamingResponse)

tests/test_tutorial/test_stream_json_lines/test_tutorial001.py

@@ -43,6 +43,25 @@ def test_stream_items(client: TestClient, path: str):
     assert lines == expected_items
 
 
+@pytest.mark.parametrize(
+    "path",
+    [
+        "/items/stream",
+        "/items/stream-no-async",
+        "/items/stream-no-annotation",
+        "/items/stream-no-async-no-annotation",
+    ],
+)
+def test_stream_sets_streaming_headers(client: TestClient, path: str):
+    """JSONL streaming responses should disable proxy buffering and caching,
+    so intermediaries deliver items incrementally (matching SSE responses)."""
+    response = client.get(path)
+    assert response.status_code == 200, response.text
+    # Tells proxies (e.g. Nginx) not to buffer the streamed response
+    assert response.headers["x-accel-buffering"] == "no"
+    assert response.headers["cache-control"] == "no-cache"
+
+
 def test_openapi_schema(client: TestClient):
     response = client.get("/openapi.json")
     assert response.status_code == 200, response.text