feat(rumqttd): expose retain flag on local LinkTx publish#1050
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`local::LinkTx::publish` and `try_publish` hardcode `retain: false`, which prevents in-process callers from seeding the retained store with snapshot-class topics. Embedded-broker setups that want to serve a state snapshot to late subscribers on first SUBSCRIBE currently have to spin up a loopback rumqttc client to publish with retain over the wire, or wait for the next live publish. Add `publish_with_retain` and `try_publish_with_retain` that take an explicit retain flag. Existing `publish` / `try_publish` keep the previous behaviour and now delegate, so the change is fully backwards-compatible.
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Type of change
New feature (non-breaking change which adds functionality).
What this fixes
local::LinkTx::publishandtry_publishhardcoderetain: false(rumqttd/src/link/local.rs:223,242), which prevents an in-process publisher from seeding the broker's retained store. Anyone embeddingrumqttdand wanting to serve a snapshot to late subscribers on first SUBSCRIBE has had to either:rumqttcclient and publish-with-retain over the wire (extra dependency, extra connect/CONNACK race window), orWhat this PR does
Add two thin wrappers that take an explicit retain flag:
The existing
publishandtry_publishkeep their previous behaviour (retain: false) and now delegate, so the change is fully backwards-compatible.Use case
ec-ui-light(a Tauri desktop app that ships an embeddedrumqttdto replay captured Victron MQTT fixtures) needs to seed the retained store with a small set of dashboard-critical topics (system/0/Serial,system/0/Dc/Battery/Soc,system/0/Ac/Grid/...) before the WebView's MQTT client subscribes. Doing this through a loopbackrumqttcclient raced the WebView's first SUBSCRIBE; callingpublish_with_retaindirectly on a freshly-builtLinkTxpopulates the retained store synchronously and the race goes away.Checklist
cargo fmt[Unreleased]