Case 109: flow::graph Policy Tag Renames¶

Field	Value
Verdict	🔴 BREAKING
Category	Breaking
Platforms	Linux, macOS, Windows
Flags	API break
Detected `ChangeKind`s	`typedef_removed`, `type_removed`
Source files	browse on GitHub

Category: Source API / oneTBB regression suite | Verdict: 🔴 BREAKING

What breaks¶

A header-only set of policy-tag types (queueing, rejecting) used as template parameters for function_node is renamed wholesale (buffering_policy, backpressure_policy). The instantiation anchor typedef (queue_node) is also dropped and replaced. Consumer source that wrote any of the v1 names fails to compile against v2 headers.

Because the offending types are header-only template parameter tags, no exported library symbol changes: the mylib_flow_run ABI is identical between v1 and v2. The .so will continue to link with old consumers, but any rebuild against the new headers breaks.

Why this is in the oneTBB regression suite¶

Mirrors oneTBB 2021's reshuffle of tbb::flow::* policy types — a documented historical break from the upstream maintainers.

How abicheck catches it¶

The diff exposes:

TYPEDEF_REMOVED: queue_node
TYPE_REMOVED: queueing, rejecting
TYPE_ADDED: buffering_policy, backpressure_policy
TYPEDEF_ADDED (informational): buffer_node

That set is enough to classify the diff as API_BREAK without any new detector. Use member_name / type_pattern suppressions if a downstream consumer wants to silence informational additions.

Code diff¶

v1	v2
`struct queueing {};`	`struct buffering_policy {};`
`struct rejecting {};`	`struct backpressure_policy {};`
`typedef function_node<queueing> queue_node;`	`typedef function_node<buffering_policy> buffer_node;`

How to fix (as a library maintainer)¶

Stage renames across a deprecation cycle: in release N–1, declare the new names and add using queueing = buffering_policy; aliases marked [[deprecated]]. In release N, remove the old tags.
Audit downstream consumers (build farms, sample repos) before removing legacy aliases — header-only renames look invisible to binary-only ABI checkers, so a "compiles for me" test sweep on representative consumers is the only reliable signal.

References¶

oneTBB 2021 migration guide: flow::graph node policy updates.