iceoryx WaitSet integration

[elfenpiff]

Brief feature description

In iceoryx2 we require an event multiplexing mechanism like the iceoryx waitset. It is connected to Node::wait() and maybe integrated into the Node or a separate mechanism.

ToDo List

• [ ] Implement WaitSet
• [ ] C and C++ bindings
• [ ] Add example how event based communication can look like

Terms

• tick - periodic event to produce something
• timeout - periodic deadline event to consume something

API

impl WaitSetBuilder {
  fn new() -> Self;
  fn allocator(); // sets custom allocator to now allocate management stuff on the heap
  fn tick(CyclicTimer); // sets tick timeout, whenever wait/run shall emit WaitEvent::Tick
  fn create();
}

pub enum Attachment<'a, T: SynchronousMultiplexing> {
  Notification(&'a T),
  Deadline((&'a T, Duration)),
  Tick(Duration),
};

impl WaitSet {
  fn attach<T>(attachment: Attachment);
  fn run(callback); // runs in a timed loop defined by tick
  fn try_run(callback); // no ticks? tries to gather all events.
}

Preconsiderations

Details

• SynchronousMultiplexing is a trait implemented by every file descriptor based construct where the file descriptor emits events and can be attached to select, poll or epoll
• Every struct that implements SynchronousMultiplexing shall be attachable
  • Enables the user to implement the trait for sockets and attach them
• A Listener port shall be attachable.
• Listener ports from different Nodes shall be attachable.
  • When the app acts as a gateway between 2 isolated iceoryx2 domains the gateway shall wake up whenever it receives a message from one of the domains without requiring multiple threads
• Node knows all ports, the first idea was to handle background tasks like Publisher::update_connections() in Node::wait() to deliver the history to newly connected Subscribers
• Publisher::update_connections() problem, how is the Subscribers history delivered?
  • can be handled with cyclic Node::wait(CYCLE_TIME) when the node calls it explicitly for every port - can cause a latency up to CYCLE_TIME
  • can be handled explicitly by the user - can be error prone when the user has to call this for every publisher
  • Combine pubsub service with event service with the same name.
    • specify and reserve an IOX2_SERVICE_UPDATE id and emit this whenever a Subscriber did connect to a service
    • Node::wait() fetches event internally and calls update_connections() explicitly
    • requires the user to create the event, attach it and to emit the IOX2_SERVICE_UPDATE event
  • The WaitSet shall emit signals to handle CTRL+c, SIGINT, SIGTERM events in the same call as additional events
  • WaitSet shall provide 3 variants: try_wait, timed_wait, blocking_wait

Results

1. Node::wait() is obsolete?
2. Create WaitsetBuilder and provide optionally allocator for custom dynamic memory, otherwise use heap
3. Waitset::attach_fn() must, Waitset::attach()
4. Aim for

   while event = waitset::wait() {
   event.call(); // call callback set with `attach_fn()`.
   }

   5. Is independent of the node!