✅ Add tests for streaming cancellation

tests/test_stream_cancellation.py

@@ -0,0 +1,89 @@
+"""
+Test that async streaming endpoints can be cancelled without hanging.
+
+Ref: https://github.com/fastapi/fastapi/issues/14680
+
+"""
+
+from collections.abc import AsyncIterable
+
+import anyio
+import pytest
+from fastapi import FastAPI
+from fastapi.responses import StreamingResponse
+
+pytestmark = [
+    pytest.mark.anyio,
+    pytest.mark.filterwarnings("ignore::pytest.PytestUnraisableExceptionWarning"),
+]
+
+
+app = FastAPI()
+
+
+@app.get("/stream-raw", response_class=StreamingResponse)
+async def stream_raw() -> AsyncIterable[str]:
+    """Async generator with no internal await - would hang without checkpoint."""
+    i = 0
+    while True:
+        yield f"item {i}\n"
+        i += 1
+
+
+@app.get("/stream-jsonl")
+async def stream_jsonl() -> AsyncIterable[int]:
+    """JSONL async generator with no internal await."""
+    i = 0
+    while True:
+        yield i
+        i += 1
+
+
+async def _run_asgi_and_cancel(app: FastAPI, path: str, timeout: float) -> bool:
+    """Call the ASGI app for *path* and cancel after *timeout* seconds.
+
+    Returns `True` if the cancellation was delivered (i.e. it did not hang).
+    """
+    chunks: list[bytes] = []
+
+    async def receive():  # type: ignore[no-untyped-def]
+        # Simulate a client that never disconnects, rely on cancellation
+        await anyio.sleep(float("inf"))
+        return {"type": "http.disconnect"}
+
+    async def send(message: dict) -> None:  # type: ignore[type-arg]
+        if message["type"] == "http.response.body":
+            chunks.append(message.get("body", b""))
+
+    scope = {
+        "type": "http",
+        "asgi": {"version": "3.0", "spec_version": "2.0"},
+        "http_version": "1.1",
+        "method": "GET",
+        "path": path,
+        "query_string": b"",
+        "root_path": "",
+        "headers": [],
+        "server": ("test", 80),
+    }
+
+    with anyio.move_on_after(timeout) as cancel_scope:
+        await app(scope, receive, send)  # type: ignore[arg-type]
+
+    # If we got here within the timeout the generator was cancellable.
+    # cancel_scope.cancelled_caught is True when move_on_after fired.
+    return cancel_scope.cancelled_caught or len(chunks) > 0
+
+
+async def test_raw_stream_cancellation() -> None:
+    """Raw streaming endpoint should be cancellable within a reasonable time."""
+    cancelled = await _run_asgi_and_cancel(app, "/stream-raw", timeout=3.0)
+    # The key assertion: we reached this line at all (didn't hang).
+    # cancelled will be True because the infinite generator was interrupted.
+    assert cancelled
+
+
+async def test_jsonl_stream_cancellation() -> None:
+    """JSONL streaming endpoint should be cancellable within a reasonable time."""
+    cancelled = await _run_asgi_and_cancel(app, "/stream-jsonl", timeout=3.0)
+    assert cancelled