Core client: main loop logic

The main loop of the core client repeatedly calls the following function:

bool CLIENT_STATE::do_something() {
    bool action=false;

    if (check_suspend_activities()) return false;
    action |= net_xfers->poll();
    action |= http_ops->poll();
    action |= file_xfers->poll();
    action |= active_tasks->poll();
    action |= scheduler_rpcs->poll();
    action |= start_apps();
    action |= pers_xfers->poll();
    action |= handle_running_apps();
    action |= handle_pers_file_xfers();
    action |= garbage_collect();
    write_state_file_if_needed();
    return action;
}

This function initiates new activities as needed, and checks for the completion of current activities. It is to be called periodically from either a sleep loop (command-line program) or timer handler of event loop (GUI program). It returns true if any change occurred, in which case it should be called again without sleeping.

The various functions called are as follows:

check_suspend_activities checks for conditions such as recent mouse/keyboard input, or running on batteries, in which user preferences dictate that no work be done.
net_xfers->poll(), http_ops->poll(), file_xfers->poll() and pers_xfers->poll() manage the internal transitions of the various FSM layers.
start_apps() checks whether it's possible to start an application, i.e. a CPU slot is vacant and there's a result with all its input files present. If so it starts the application.
handle_running_apps() checks whether a running application has exited, and if so cleans up after it.
handle_pers_file_xfers() starts new file transfers as needed.
garbage_collect() checks for objects that can be discarded. For example, if a file is non-sticky and is no longer referenced by any work units or results, both the FILE_INFO and the underlying file can be deleted. If a result has been completed and acknowledged, the RESULT object can be deleted.
write_state_file_if_needed(): any of the above functions that changes state in a way that should be written to client_state.xml (e.g. that needs to survive this execution of the core client) sets a flag client_state_dirty. write_state_file_if_needed() writes client_state.xml if this flag is set.

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