[lxsaah]

Description

Stage 2 of #391, into the #392 feature branch. mqtt-connector-demo-common becomes the runtime-agnostic home of everything both MQTT demos share, and both demos move to connect + publish / subscribe.

mqtt-connector-demo-common

• Contracts: Temperature and TemperatureCommand are always Serialize / Deserialize / JSON #[derive(Linkable)], on no_std + alloc too. The data-contracts, serde, alloc and derive features are gone, along with the hand-rolled no_std JSON (to_json_vec, from_json, parse_json_no_std).
• sim.rs (new):
  • SimThermometers { indoor, outdoor, server_room } hands out owned SimChannels that are neither Clone nor Copy.
  • Thermometer::on(channel) is an Input<T> for every T: From<Reading>, with .every_secs(n) and .readings(n).
  • impl From<Reading> for Temperature lives next to the contract.
  • It only uses RuntimeContext, so the same input runs on Tokio and Embassy and replaces the three producer functions in each demo.
• Keys and monitors are unchanged. The README is updated.

tokio-mqtt-connector-demo

• let mqtt = builder.connect(..), .input(Thermometer::on(indoor).readings(5)), publish / subscribe with the keys' #[link_address] topics.
• main.rs went from 215 to 154 lines; most of the rest is the module docs.
• MQTT behaviour is unchanged:
  • same topics and QoS / retain
  • five readings per sensor, two seconds apart
  • the same payload bytes, since the derive uses serde_json::to_vec, as to_json_vec did on std

embassy-mqtt-connector-demo

• The records use the shared thermometers, which keep cycling endlessly as before, plus publish / subscribe.

• The serial remote-access server stays on with_connector, since it takes no links.

• Payloads now come from serde_json instead of the hand-rolled one-decimal formatter, so they're identical to the Tokio demo's.

• Release size for thumbv8m.main-none-eabihf (arm-none-eabi-size -A):

  section	before	after	delta
  .text	557,548	539,948	−17,600
  .rodata	65,568	65,608	+40
  .data	532	532	0
  .bss	124,292	124,308	+16

The board module and the B1 PushButton follow in stage 3.

Checks

CI doesn't run on PRs into the feature branch, so I ran these locally:

• mqtt-connector-demo-common tests with std, and the no_std build
• cargo clippy -D warnings on the common crate (no_std and std), the tokio demo, and the embassy demo for thumbv8m.main-none-eabihf
• the embassy demo in debug and release for thumbv8m.main-none-eabihf
• a tokio demo run without a broker:
  • build() succeeds
  • MQTT subscribes 2 topics
  • all three thermometers produce every 2 s
  • only the connection is refused, as expected

Not done: a run against a real broker, because Docker's daemon wasn't running. The embassy demo was not flashed.

Related Issue

• Part of [FEAT] publish / subscribe for Linkable contracts, migrate the MQTT demos, document input conventions #391 / [FEAT] reg.input / reg.output with connection handles, so a wrong wiring is a compile error #388 (PR feat: reg.input / reg.output, connector handles and publish / subscribe (#388) #392)

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