fix: graceful SSE drain on session manager shutdown

wiggzz · wiggzz · commit dd4ca8d21bda · 2026-03-09T12:12:51.000-05:00
Terminate all active transports before cancelling the task group during shutdown, allowing EventSourceResponse to send a final more_body=False chunk for clean HTTP close instead of a connection reset. Upstream PR: modelcontextprotocol#2239
diff --git a/src/mcp/server/streamable_http.py b/src/mcp/server/streamable_http.py
@@ -809,7 +809,7 @@ async def _validate_request_headers(self, request: Request, send: Send) -> bool:
 
     async def _validate_session(self, request: Request, send: Send) -> bool:
         """Validate the session ID in the request."""
-        if not self.mcp_session_id:  # pragma: lax no cover
+        if not self.mcp_session_id:
             # If we're not using session IDs, return True
             return True
 
@@ -842,7 +842,7 @@ async def _validate_protocol_version(self, request: Request, send: Send) -> bool
         protocol_version = request.headers.get(MCP_PROTOCOL_VERSION_HEADER)
 
         # If no protocol version provided, assume default version
-        if protocol_version is None:  # pragma: no cover
+        if protocol_version is None:
             protocol_version = DEFAULT_NEGOTIATED_VERSION
 
         # Check if the protocol version is supported
diff --git a/src/mcp/server/streamable_http_manager.py b/src/mcp/server/streamable_http_manager.py
@@ -90,6 +90,9 @@ def __init__(
         self._session_creation_lock = anyio.Lock()
         self._server_instances: dict[str, StreamableHTTPServerTransport] = {}
 
+        # Track in-flight stateless transports for graceful shutdown
+        self._stateless_transports: set[StreamableHTTPServerTransport] = set()
+
         # The task group will be set during lifespan
         self._task_group = None
         # Thread-safe tracking of run() calls
@@ -130,11 +133,28 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
                 yield  # Let the application run
             finally:
                 logger.info("StreamableHTTP session manager shutting down")
+
+                # Terminate all active transports before cancelling the task
+                # group.  This closes their in-memory streams, which lets
+                # EventSourceResponse send a final ``more_body=False`` chunk
+                # — a clean HTTP close instead of a connection reset.
+                for transport in list(self._server_instances.values()):
+                    try:
+                        await transport.terminate()
+                    except Exception:  # pragma: no cover
+                        logger.debug("Error terminating transport during shutdown", exc_info=True)
+                for transport in list(self._stateless_transports):
+                    try:
+                        await transport.terminate()
+                    except Exception:  # pragma: no cover
+                        logger.debug("Error terminating stateless transport during shutdown", exc_info=True)
+
                 # Cancel task group to stop all spawned tasks
                 tg.cancel_scope.cancel()
                 self._task_group = None
                 # Clear any remaining server instances
                 self._server_instances.clear()
+                self._stateless_transports.clear()
 
     async def handle_request(self, scope: Scope, receive: Receive, send: Send) -> None:
         """Process ASGI request with proper session handling and transport setup.
@@ -166,6 +186,9 @@ async def _handle_stateless_request(self, scope: Scope, receive: Receive, send:
             security_settings=self.security_settings,
         )
 
+        # Track for graceful shutdown
+        self._stateless_transports.add(http_transport)
+
         # Start server in a new task
         async def run_stateless_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORED):
             async with http_transport.connect() as streams:
@@ -185,13 +208,16 @@ async def run_stateless_server(*, task_status: TaskStatus[None] = anyio.TASK_STA
         # This ensures the server task is cancelled when the request
         # finishes, preventing zombie tasks from accumulating.
         # See: https://github.com/modelcontextprotocol/python-sdk/issues/1764
-        async with anyio.create_task_group() as request_tg:
-            await request_tg.start(run_stateless_server)
-            # Handle the HTTP request directly in the caller's context
-            # (not as a child task) so execution flows back naturally.
-            await http_transport.handle_request(scope, receive, send)
-            # Cancel the request-scoped task group to stop the server task.
-            request_tg.cancel_scope.cancel()
+        try:
+            async with anyio.create_task_group() as request_tg:
+                await request_tg.start(run_stateless_server)
+                # Handle the HTTP request directly in the caller's context
+                # (not as a child task) so execution flows back naturally.
+                await http_transport.handle_request(scope, receive, send)
+                # Cancel the request-scoped task group to stop the server task.
+                request_tg.cancel_scope.cancel()
+        finally:
+            self._stateless_transports.discard(http_transport)
 
         # Terminate after the task group exits — the server task is already
         # cancelled at this point, so this is just cleanup (sets _terminated
diff --git a/tests/server/test_streamable_http_manager.py b/tests/server/test_streamable_http_manager.py
@@ -10,11 +10,11 @@
 import pytest
 from starlette.types import Message
 
-from mcp import Client
+from mcp import Client, types
 from mcp.client.streamable_http import streamable_http_client
 from mcp.server import Server, ServerRequestContext, streamable_http_manager
 from mcp.server.streamable_http import MCP_SESSION_ID_HEADER, StreamableHTTPServerTransport
-from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
+from mcp.server.streamable_http_manager import StreamableHTTPASGIApp, StreamableHTTPSessionManager
 from mcp.types import INVALID_REQUEST, ListToolsResult, PaginatedRequestParams
 
 
@@ -490,3 +490,165 @@ def test_session_idle_timeout_rejects_non_positive():
 def test_session_idle_timeout_rejects_stateless():
     with pytest.raises(RuntimeError, match="not supported in stateless"):
         StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=30, stateless=True)
+
+
+MCP_HEADERS = {
+    "Accept": "application/json, text/event-stream",
+    "Content-Type": "application/json",
+}
+
+_INITIALIZE_REQUEST = {
+    "jsonrpc": "2.0",
+    "id": 1,
+    "method": "initialize",
+    "params": {
+        "protocolVersion": "2025-03-26",
+        "capabilities": {},
+        "clientInfo": {"name": "test", "version": "0.1"},
+    },
+}
+
+_INITIALIZED_NOTIFICATION = {
+    "jsonrpc": "2.0",
+    "method": "notifications/initialized",
+}
+
+_TOOL_CALL_REQUEST = {
+    "jsonrpc": "2.0",
+    "id": 2,
+    "method": "tools/call",
+    "params": {"name": "slow_tool", "arguments": {"message": "hello"}},
+}
+
+
+def _make_slow_tool_server() -> tuple[Server, anyio.Event]:
+    """Create an MCP server with a tool that blocks forever, returning
+    the server and an event that fires when the tool starts executing."""
+    tool_started = anyio.Event()
+
+    async def handle_call_tool(ctx: ServerRequestContext, params: types.CallToolRequestParams) -> types.CallToolResult:
+        tool_started.set()
+        await anyio.sleep_forever()
+        return types.CallToolResult(  # pragma: no cover
+            content=[types.TextContent(type="text", text="never reached")]
+        )
+
+    async def handle_list_tools(
+        ctx: ServerRequestContext, params: PaginatedRequestParams | None
+    ) -> ListToolsResult:  # pragma: no cover
+        return ListToolsResult(
+            tools=[
+                types.Tool(
+                    name="slow_tool",
+                    description="A tool that blocks forever",
+                    input_schema={"type": "object", "properties": {"message": {"type": "string"}}},
+                )
+            ]
+        )
+
+    app = Server("test-graceful-shutdown", on_call_tool=handle_call_tool, on_list_tools=handle_list_tools)
+    return app, tool_started
+
+
+class SSECloseTracker:
+    """ASGI middleware that tracks whether SSE responses close cleanly.
+
+    In HTTP, a clean close means sending a final empty chunk (``0\\r\\n\\r\\n``).
+    At the ASGI protocol level this corresponds to a
+    ``{"type": "http.response.body", "more_body": False}`` message.
+
+    Without graceful drain, the server task is cancelled but nothing closes
+    the stateless transport's streams — the SSE response hangs indefinitely
+    and never sends the final body.  A reverse proxy (e.g. nginx) would log
+    "upstream prematurely closed connection while reading upstream".
+    """
+
+    def __init__(self, app: StreamableHTTPASGIApp) -> None:
+        self.app = app
+        self.sse_streams_opened = 0
+        self.sse_streams_closed_cleanly = 0
+
+    async def __call__(self, scope: dict[str, Any], receive: Any, send: Any) -> None:
+        is_sse = False
+
+        async def tracking_send(message: dict[str, Any]) -> None:
+            nonlocal is_sse
+            if message["type"] == "http.response.start":
+                for name, value in message.get("headers", []):
+                    if name == b"content-type" and b"text/event-stream" in value:
+                        is_sse = True
+                        self.sse_streams_opened += 1
+                        break
+            elif message["type"] == "http.response.body" and is_sse:
+                if not message.get("more_body", False):
+                    self.sse_streams_closed_cleanly += 1
+            await send(message)
+
+        await self.app(scope, receive, tracking_send)
+
+
+@pytest.mark.anyio
+async def test_graceful_shutdown_closes_sse_streams_cleanly():
+    """Verify that shutting down the session manager closes in-flight SSE
+    streams with a proper ``more_body=False`` ASGI message.
+
+    This is the ASGI equivalent of sending the final HTTP chunk — the signal
+    that reverse proxies like nginx use to distinguish a clean close from a
+    connection reset ("upstream prematurely closed connection").
+
+    Without the graceful-drain fix, stateless transports are not tracked by
+    the session manager.  On shutdown nothing calls ``terminate()`` on them,
+    so SSE responses hang indefinitely and never send the final body.  With
+    the fix, ``run()``'s finally block iterates ``_stateless_transports`` and
+    terminates each one, closing the underlying memory streams and letting
+    ``EventSourceResponse`` complete normally.
+    """
+    app, tool_started = _make_slow_tool_server()
+    manager = StreamableHTTPSessionManager(app=app, stateless=True)
+
+    tracker = SSECloseTracker(StreamableHTTPASGIApp(manager))
+
+    manager_ready = anyio.Event()
+
+    with anyio.fail_after(10):
+        async with anyio.create_task_group() as tg:
+
+            async def run_lifespan_and_shutdown() -> None:
+                async with manager.run():
+                    manager_ready.set()
+                    with anyio.fail_after(5):
+                        await tool_started.wait()
+                # manager.run() exits — graceful shutdown runs here
+
+            async def make_requests() -> None:
+                with anyio.fail_after(5):
+                    await manager_ready.wait()
+                async with (
+                    httpx.ASGITransport(tracker, raise_app_exceptions=False) as transport,
+                    httpx.AsyncClient(transport=transport, base_url="http://testserver") as client,
+                ):
+                    # Initialize
+                    resp = await client.post("/mcp/", json=_INITIALIZE_REQUEST, headers=MCP_HEADERS)
+                    resp.raise_for_status()
+
+                    # Send initialized notification
+                    resp = await client.post("/mcp/", json=_INITIALIZED_NOTIFICATION, headers=MCP_HEADERS)
+                    assert resp.status_code == 202
+
+                    # Send slow tool call — returns an SSE stream that blocks
+                    # until shutdown terminates it
+                    await client.post(
+                        "/mcp/",
+                        json=_TOOL_CALL_REQUEST,
+                        headers=MCP_HEADERS,
+                        timeout=httpx.Timeout(10, connect=5),
+                    )
+
+            tg.start_soon(run_lifespan_and_shutdown)
+            tg.start_soon(make_requests)
+
+    assert tracker.sse_streams_opened > 0, "Test should have opened at least one SSE stream"
+    assert tracker.sse_streams_closed_cleanly == tracker.sse_streams_opened, (
+        f"All {tracker.sse_streams_opened} SSE stream(s) should have closed with "
+        f"more_body=False, but only {tracker.sse_streams_closed_cleanly} did"
+    )
