Incorrect initial callback event count with KEEP_ALL history policy

[bmartin427]

Bug report

When using the callback API (EventsExecutor) on a subscription with KEEP_ALL history policy, the event count passed to the initial callback is inappropriately limited to the QoS history depth value, which is zero for this QoS type. This causes the callback not to take any of the pending subscription data. The data taken by the subscription callback following this point is stale indefinitely; i.e. the original data is only taken upon a subsequent new event, and that data is only taken on yet another subsequent event.

Required Info:

• Operating System:
  • Ubuntu 22.04
• Installation type:
  • binaries
• Version or commit hash:
  • 1.6.0-2jammy.20240711.232935
• DDS implementation:
  • rmw_cyclonedds_cpp
• Client library (if applicable):
  • rclcpp

Steps to reproduce issue

• From an external node, advertise a topic with TRANSIENT_LOCAL durability and KEEP_ALL history policy.
• Publish two messages from that node
• Bring up a node using the callback API to subscribe to that topic
• Publish one more message from the other node

Expected behavior

• Subscribing node receives two messages upon startup
• Subscribing node sees third message immediately upon publication

Actual behavior

• Subscribing node gets notified of 'zero' subscription events when the callback is registered
• Subscribing node receives first message only upon publication of third

Additional information

An identical subscriber node works as expected when RMW_IMPLEMENTATION=rmw_fastrtps_cpp