-- A GUI publication binds paused to its display epoch. Agent barrier changes
-- must therefore invalidate that agent's displays in the same transaction,
-- even when the writer does not use the API helper. Independent agents keep
-- their epochs. This is allocation, not a Runner barrier acknowledgement.
CREATE FUNCTION fence_agent_barrier_displays() RETURNS TRIGGER LANGUAGE plpgsql AS $$
DECLARE
    target_bot TEXT;
    was_paused BOOLEAN := FALSE;
    is_paused BOOLEAN := FALSE;
BEGIN
    IF TG_OP <> 'INSERT' THEN
        target_bot := OLD.bot_id;
        was_paused := OLD.paused;
    END IF;
    IF TG_OP <> 'DELETE' THEN
        target_bot := NEW.bot_id;
        is_paused := NEW.paused;
    END IF;
    -- A primary key move also removes the old bot's barrier.
    IF TG_OP='UPDATE' AND NEW.bot_id IS DISTINCT FROM OLD.bot_id THEN
        IF OLD.paused THEN
            UPDATE computer_screens SET display_epoch=display_epoch+1
            WHERE bot_id=OLD.bot_id;
        END IF;
        was_paused := FALSE;
    END IF;
    IF was_paused IS DISTINCT FROM is_paused THEN
        UPDATE computer_screens SET display_epoch=display_epoch+1
        WHERE bot_id=target_bot;
    END IF;
    RETURN NULL;
END
$$;

CREATE TRIGGER agent_barrier_display_epochs
AFTER INSERT OR UPDATE OR DELETE ON agent_mutation_barriers
FOR EACH ROW EXECUTE FUNCTION fence_agent_barrier_displays();

-- Existing paused rows may already have a published unpaused GUI epoch.
UPDATE computer_screens s SET display_epoch=s.display_epoch+1
WHERE EXISTS(SELECT 1 FROM agent_mutation_barriers b WHERE b.bot_id=s.bot_id AND b.paused);