chore: import upstream snapshot with attribution

examples/stories/sse_polling/event_store.py

@@ -0,0 +1,34 @@
+"""Minimal in-memory `EventStore` for the SSE-resumability example.
+
+Sequential integer IDs so the wire is readable; a production server would back
+this interface with persistent storage so replay survives a process restart.
+"""
+
+from mcp_types import JSONRPCMessage
+
+from mcp.server.streamable_http import EventCallback, EventId, EventMessage, EventStore, StreamId
+
+
+class InMemoryEventStore(EventStore):
+    """Stores every event in arrival order and replays the same-stream tail after a given ID."""
+
+    def __init__(self) -> None:
+        self._events: list[tuple[StreamId, JSONRPCMessage | None]] = []
+
+    async def store_event(self, stream_id: StreamId, message: JSONRPCMessage | None) -> EventId:
+        self._events.append((stream_id, message))
+        return str(len(self._events))
+
+    async def replay_events_after(self, last_event_id: EventId, send_callback: EventCallback) -> StreamId | None:
+        try:
+            cursor = int(last_event_id)
+        except ValueError:
+            return None
+        if not 0 < cursor <= len(self._events):
+            return None
+        stream_id, _ = self._events[cursor - 1]
+        for index in range(cursor, len(self._events)):
+            event_stream_id, message = self._events[index]
+            if event_stream_id == stream_id and message is not None:
+                await send_callback(EventMessage(message, str(index + 1)))
+        return stream_id