[YuanYuYuan]

Problem

The Python create_subscriber cannot stay out of the CPU buffer group of rmw_zenoh_cpp. A subscriber that joins declares a second subscription and announces a buffer backend. On a topic that is known to carry no buffer, the extra subscription has no use. Python has no way to opt out. Under the default Auto of #384 a Python subscriber joins regardless of message type, except with transient-local QoS. The Python bindings build a subscriber with no message schema, so the message type cannot decide.

A joined subscriber subscribes to <topic key> (the base key) and to <topic key>/_buf_cpu (the group key), and its liveliness token ends with backends:cpu:. See ros2/rmw_zenoh#930. The interop guide gives the rules and the cases that never join.

Part of #370.

What changes

• New keyword buffer_group='auto' | 'cpu' | 'off' on create_subscriber, in the queue form and the callback form. The default is 'auto'. A Python subscriber has no message schema, so 'auto' and 'cpu' do the same.
• An unknown string raises ValueError. A non-string raises TypeError. Both name buffer_group.
• The status subscriber of an action client now stays out (Off), because its message has no buffer. The feedback subscriber still joins and has no switch.

Before and after

A transient-local subscriber never joins, because its history comes from the base key, as in rmw_zenoh_cpp; the interop guide gives the details. The rows below assume volatile QoS.

case	today	with this PR
create_subscriber, default	joins	joins, same as today
buffer_group='cpu'	no such argument	joins
buffer_group='off'	no such argument	base key only. The token stays and omits backends:cpu:
buffer_group='other'	no such argument	raises ValueError
action client status subscriber	joins	stays out
action client feedback subscriber	joins	joins, same as today

Compatibility

change	who is affected	action
the status subscriber of Rust and Python action clients stops declaring the group subscription	Rust and Python action clients	none: status delivery is unchanged because the message has no buffer field

A default-deny router that allows only the base key starves a joined Python subscriber of a buffer-aware publisher, with no error. Allow both keys. buffer_group='off' needs no group key. A subscriber with no backends: field gets no buffer message from a buffer-aware publisher. This holds until a release carries ros2/rmw_zenoh#1078, so 'off' does not bring those messages back.

Tests

area	test	checks
validation	test_create_subscriber_rejects_unknown_group, test_create_subscriber_rejects_a_non_string_group	five wrong strings raise ValueError. A non-string raises TypeError
policy	test_policy_takes_effect	four values ('off', 'cpu', 'auto', omitted) in two forms (queue, callback): the token and the group-key delivery match the value. Positive controls show that a cpu twin joins and that a put on the base key arrives
status	action_client_join_policy (buffer_cpu_group.rs)	the status subscriber stays out, and the feedback subscriber of the default client joins

The Python tests are in crates/hiroz-py/tests.

Merge order: this PR depends on no open PR and merges on its own. #386 merges last, because it bumps the release.

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-10-24 03:06 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
crates/hiroz/src/action/client.rs	74.81% <100.00%> (+0.09%)	⬆️
crates/hiroz/src/buffer_group.rs	98.07% <ø> (ø)

... and 1 file with indirect coverage changes

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