Implement unsupported API for AgnocastOnlyExecutor

[Koichi98]

The example is spin_until_future_complete. There might be some others.

[eyr1n]

AgnocastOnlyExecutor spin API support plan

Background

AgnocastOnlyExecutor class only exposes a single spin-related API spin(). While this is sufficient for the simplest execution path, it does not provide a more flexible scheduling interface.

In practice, the lack of spin_* family of APIs makes it difficult to implement executor scheduling patterns such as:

• executing only one callback
• spinning until a future completes
• spinning a specific node, etc.

Goal

Implement the missing spin-related executor APIs for AgnocastOnlyExecutor.

At this point, I am planning to create three pull requests. Each PR is based on the implementation in the previous PR.

PR 1: Spin API primitives

• is_spinning
  • returns true if the executor is currently spinning
• spin_once
  • collect work once and execute the next available work with timeout
• spin_until_future_complete
  • spin until the future is complete with timeout

PR 2: Basic spin_* API

• spin_some
  • collect work once and execute all available work with timeout
• spin_all
  • collect and execute work repeatedly until no more work is available

PR 3: Spin API per node

These APIs are based on Step 1 and Step 2. However, each function adds a node to the executor before spinning, and then removes the node.

• spin_node_once
• spin_node_some
• spin_node_all

Non-goals

• full internal and external implementation equals with rclcpp::Executor
• executor refactoring unrelated to spin-related APIs

Changes to the existing code

• get_next_agnocast_executable
  • the current implementation takes the timeout as an integer, but I may change the argument type to std::chrono::duration to align with rclcpp
• may align the shutdown detection code with rclcpp

Test plan

PR 1: Spin API primitives

• is_spinning
  • test procedure: call is_spinning with a node NodeForExecutorTest
  • expected behavoir: returns true if NodeForExecutorTest is spinning
• spin_once
  • test procedure: call spin_once with a node
  • expected behavior:
    • blocking if the executor has no executable
    • returns immediately if the executor has one or more executables
• spin_until_future_complete
  • test procedure: call spin_until_future_complete with a node
  • expected behavior:

References

• rclcpp::Executor declaration (executor.hpp)
• rclcpp::Executor implementation (executor.cpp)
• rclcpp::executors::SingleThreadedExecutor declaration (single_threaded_executor.hpp)
• rclcpp::executors::SingleThreadedExecutor implementation (single_threaded_executor.cpp)