cosmix-powerd — battery and power state

cosmix-powerd is the event-driven UPower citizen for battery, charging and power-source state. It turns the local system D-Bus view into a read-only power.* Bus namespace and small lifecycle topics.

What it does

Reads UPower's virtual DisplayDevice and all devices returned by EnumerateDevices.
Exposes presence, percentage, charging state, time-to-empty/full, energy rate and battery health when UPower provides them.
Watches PropertiesChanged, DeviceAdded, DeviceRemoved and org.freedesktop.DBus.NameOwnerChanged; ordinary updates never depend on a polling loop.
Keeps serving the Bus namespace when UPower is absent. The snapshot then has present=false. The daemon requests D-Bus activation once, waits for the owner edge without retry-spinning, and attaches when UPower claims its name.

The signal matches are installed before the first owner query and snapshot so changes during startup remain queued. The tracker task drains the bounded raw signal stream alongside every D-Bus method reply, including activation and the final owner check, so a signal burst cannot back-pressure the shared D-Bus reader ahead of that reply. If a signal lands around a multi-call scan or final owner check, powerd discards that attempt and rescans. After three consecutively raced attempts it publishes nothing, retains the previous complete snapshot, logs the storm once, and uses the consumed signal wake to scan again. A sustained storm can delay freshness but can never publish a mixed state. A five-minute full rescan exists only as a missed-signal backstop.

Running it

/opt/cosmix/bin/cosmix-powerd serve

The supplied cosmix-powerd.service is a systemd user unit because the Bus citizen belongs to the graphical session. The SPEC-10 cosmix-powerd UID/GID row reserves its daemon identity and namespace; it is not the runtime user of this session service.

Bus verbs

The service name is power.

Verb	Purpose
`power.info`	Build identity, current power summary and cumulative publisher loss
`power.ping`	Liveness reply (`{"pong":true}`)
`power.props.get`	Full snapshot, or one path supplied as `args.path`
`power.props.list`	Current leaf paths
`power.props.describe`	Schema for one path supplied as `args.path`
`power.props.watch`	Topic discovery and subscribe-before-read bootstrap instructions

Property namespace

All properties are read-only. Optional numeric leaves are absent when UPower reports that value as unknown.

Path	Meaning
`present`	A system battery or UPS is present
`on_battery`	The system is currently running on battery
`battery.present`	Presence of the aggregate display battery
`battery.kind`	`battery`, `ups`, `line-power`, `peripheral` or `unknown`
`battery.power_supply`	The aggregate device supplies the system
`battery.percentage`	Remaining charge, 0–100
`battery.state`	`unknown`, `charging`, `discharging`, `empty`, `fully-charged`, `pending-charge` or `pending-discharge`
`battery.time_to_empty_s`	Estimated seconds until empty
`battery.time_to_full_s`	Estimated seconds until full
`battery.energy_rate_w`	Watts; positive is discharging, negative is charging
`battery.health_percent`	Full-charge capacity as a percentage of design capacity
`devices.<id>.*`	The same fields for each enumerated UPower device
`lifecycle.props_level`	`L2`
`lifecycle.event_seq`	Daemon-session monotonic event sequence
`lifecycle.publisher_loss`	Cumulative publications lost during this daemon session

<id> is a stable lowercase d_<hex> encoding of the complete UPower object path. This fits the SPEC-07 segment grammar without collisions or leaking D-Bus path syntax. Peripheral batteries are retained in devices.*; the root present flag counts only a present PowerSupply battery or UPS.

Topics

Domain-topic payloads contain event, event_seq and data; power.props.changed uses the standard {path,old,new,ts,cause} shape plus an event_seq header. Consumers subscribe on their Bus connection and then call power.props.get; event_seq reveals an edge between that bootstrap snapshot and later events.

Publication never back-pressures the UPower tracker. State topics coalesce latest-wins by (topic, path/device): property changes by property path, battery changes by device ID, and power-source changes as one state key. The keyed state map holds at most 1024 keys and drops the oldest key at the cap. power.device.added and power.device.removed are events, so they share a 64-item FIFO and are never coalesced or reordered while loss-free.

Any publication loss — latest-wins replacement, keyed-state cap, event FIFO overflow, stale owner epoch, broker send error or broker send timeout — advances the cumulative daemon-session loss counter. The publisher then discards its older backlog and publishes one power.props.changed control frame using the same convention as interactd: gap=true, lost_count, and cause=publisher.loss. On any frame with gap=true, a consumer must discard its projected power state and re-read the complete snapshot via power.props.get before applying later events. lost_count is the number of discarded or failed publications covered by that gap; power.info and lifecycle.publisher_loss expose the cumulative daemon-session total.

Loss diagnostics are episode-limited so a broker outage or state storm cannot turn the synchronous reducer path into a logging bottleneck. The first loss logs its cause; later losses only advance the counters. A successful publish ends the episode and logs its total and duration. During a sustained outage, a continuing summary is emitted no more than once per 60 seconds.

Every outgoing item is tagged with the unique UPower owner epoch. The publisher checks that epoch immediately before its single send await and skips stale items. It does not cancel an accepted send because cancellation cannot retract a frame already accepted by the socket sink; at most one already-sent old-epoch frame may still be in flight, and its event_seq/epoch turnover is covered by the same gap-and-reseed contract.

Topic	Published for
`power.props.changed`	Standard SPEC-07 leaf-level old/new property deltas
`power.battery.changed`	Aggregate or enumerated battery values changed
`power.device.added`	An enumerated UPower device appeared
`power.device.removed`	An enumerated UPower device disappeared
`power.on_battery.changed`	The system switched between mains and battery

Feature boundary

The default cosmix feature builds the UPower adapter and Bus citizen. cargo test -p cosmix-powerd --no-default-features builds and tests only the pure snapshot/diff core, without zbus, Tokio or Cosmix transport dependencies.