Method ForkSession

pkg/server/session_manager.go:403–438

ForkSession creates a new session whose history is a deep copy of the parent session up to (but excluding) the Nth user message, with a fork-numbered title (" (fork N)"). userMessageOrdinal counts user-role messages in the flat list returned by Session.GetAllMessages. The read-then-write of

(ctx context.Context, sessionID string, userMessageOrdinal int)

// to keep two concurrent forks on the same parent from racing on the
// auto-numbered title.
func (sm *SessionManager) ForkSession(ctx context.Context, sessionID string, userMessageOrdinal int) (*session.Session, error) {
	sm.mux.Lock()
	defer sm.mux.Unlock()

	parent, err := sm.sessionStore.GetSession(ctx, sessionID)
	if err != nil {
		return nil, err
	}

	itemIndex, err := userMessageOrdinalToItemIndex(parent, userMessageOrdinal)
	if err != nil {
		return nil, err
	}

	forked, err := session.ForkSession(parent, itemIndex)
	if err != nil {
		return nil, err
	}

	// Sibling-aware title so repeated forks of the same parent get
	// (fork 1), (fork 2), … instead of colliding on (fork 1).
	siblings, err := sm.sessionStore.GetSessions(ctx)
	if err != nil {
		return nil, err
	}
	siblingTitles := make([]string, 0, len(siblings))
	for _, s := range siblings {
		siblingTitles = append(siblingTitles, s.Title)
	}
	forked.Title = session.NextForkTitle(parent.Title, siblingTitles)

	if err := sm.sessionStore.AddSession(ctx, forked); err != nil {
		return nil, err
	}
	return forked, nil
}

// userMessageOrdinalToItemIndex maps a 0-based user-message ordinal
// into an index in the parent's Session.Messages Item slice. Returns