mcp/streamable: use event store to fix unbounded memory issues (#335)

This CL utilizes the event store to write outgoing messages and removes
the unbounded outgoing data structure.

It also adds a new interface [EventStore.Open]

For #190

mcp/event.go

@@ -153,6 +153,11 @@ func scanEvents(r io.Reader) iter.Seq2[Event, error] {
 //
 // All of an EventStore's methods must be safe for use by multiple goroutines.
 type EventStore interface {
+	// Open prepares the event store for a given stream. It ensures that the
+	// underlying data structure for the stream is initialized, making it
+	// ready to store event streams.
+	Open(_ context.Context, sessionID string, streamID StreamID) error
+
 	// Append appends data for an outgoing event to given stream, which is part of the
 	// given session.
 	Append(_ context.Context, sessionID string, _ StreamID, data []byte) error
@@ -162,6 +167,7 @@ type EventStore interface {
 	// Once the iterator yields a non-nil error, it will stop.
 	// After's iterator must return an error immediately if any data after index was
 	// dropped; it must not return partial results.
+	// The stream must have been opened previously (see [EventStore.Open]).
 	After(_ context.Context, sessionID string, _ StreamID, index int) iter.Seq2[[]byte, error]
 
 	// SessionClosed informs the store that the given session is finished, along
@@ -256,11 +262,20 @@ func NewMemoryEventStore(opts *MemoryEventStoreOptions) *MemoryEventStore {
 	}
 }
 
-// Append implements [EventStore.Append] by recording data in memory.
-func (s *MemoryEventStore) Append(_ context.Context, sessionID string, streamID StreamID, data []byte) error {
+// Open implements [EventStore.Open]. It ensures that the underlying data
+// structures for the given session are initialized and ready for use.
+func (s *MemoryEventStore) Open(_ context.Context, sessionID string, streamID StreamID) error {
 	s.mu.Lock()
 	defer s.mu.Unlock()
+	s.init(sessionID, streamID)
+	return nil
+}
 
+// init is an internal helper function that ensures the nested map structure for a
+// given sessionID and streamID exists, creating it if necessary. It returns the
+// dataList associated with the specified IDs.
+// Requires s.mu.
+func (s *MemoryEventStore) init(sessionID string, streamID StreamID) *dataList {
 	streamMap, ok := s.store[sessionID]
 	if !ok {
 		streamMap = make(map[StreamID]*dataList)
@@ -271,6 +286,14 @@ func (s *MemoryEventStore) Append(_ context.Context, sessionID string, streamID
 		dl = &dataList{}
 		streamMap[streamID] = dl
 	}
+	return dl
+}
+
+// Append implements [EventStore.Append] by recording data in memory.
+func (s *MemoryEventStore) Append(_ context.Context, sessionID string, streamID StreamID, data []byte) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	dl := s.init(sessionID, streamID)
 	// Purge before adding, so at least the current data item will be present.
 	// (That could result in nBytes > maxBytes, but we'll live with that.)
 	s.purge()