Expose resource limit QoS

[Ivan Santiago Paunovic]

Feature request

Feature description

Expose resource limit QoS. If not, KEEP_ALL history policy is not configurable at all.
See comment ros2/rclcpp#727 (comment).

Also, it will be needed if we want to implement KEEP_ALL policy with intraprocess communication (see ros2/rclcpp#727).

[Colin Leitner]

👍

This feature is needed to properly support back pressure (slow receiver, fast producer) and reliable communication. The current implementation hides theses problems with excessive buffering but that's its own major issue (buffer bloat) that could be solved with RESOURCE_LIMITS.

Implementing this should be coupled with either

• an option to rmw_publish with a maximum timeout
• or setting the blocking timeout to 0 in the RMW implementation and returning ASAP. This would be my preferred solution to avoid having a parameter being passed around that most users will never need

In either case we need a return code to distinguish the back pressure case (resource limits exceeded) from real RMW errors or add an argument akin to taken from rmw_take.

Without this, rmw_publish can't be used in a cyclic realtime task without hitting some unknown upper execution bound, although the rest of the API can be used in a polling fashion - and errors must be silently ignored.

I have a simple test case for this that showcases the problem and how FastRTPS and CycloneDDS work with their default settings:

• FastRTPS uses 5000 messages as RESOURCE_LIMITS by default, which causes 10000 message to be in flight between the publisher and subscriber. When the limit hits, you get a random blocking time (~50-140ms) in rmw_publish or much more likely an unspecific RMW_RET_ERROR
• CycloneDDS has unlimited buffering (at least) on the subscriber side, which will cause the subscriber to crash sooner or later when memory runs out. Coupled with a FastRTPS subscriber, the publisher will start blocking without errors, but with seemingly random wait times up to 1000ms in my simple testcase.

Simple testcase:
https://gist.github.com/cleitner/93decfa79a99a8a3b59a795df02b99e7
https://gist.github.com/cleitner/e5eb35f6bcf6639425c06c03dd13fbff