Module transport 

Transport seam: commands, one-shot replies, and persistent watch sinks.

This module is the message-shaped boundary between a daemon’s transport-agnostic channel logic and its transport backends (the abstract-socket daemon today, a Binder backend later). The channel logic sees commands and watchers, never byte streams: each backend’s only job is to turn its transport’s native events into TransportEvents and push structured update values into WatcherSink::push.

The seam is generic over the command and reply payload types (C and R); the channel logic (e.g. FgCtx/FpsCtx) provides the domain types and never knows which backend delivered them.

Two shapes, kept distinct

• ReplySink is one-shot: the single reply to a command, consumed exactly once. The socket backend answers in-line (no hop); a Binder backend unblocks a waiting transaction thread.
• WatcherId / WatcherSink are persistent: a long-lived push target with its own close / death lifecycle, outliving any single transaction.

Backends may implement both with the same underlying handle (a socket backend uses one fd for both roles over its lifetime); the seam does not assume that in general.

Single-owner watcher registry

A watch registration is itself a command, so it flows through the same ingestion path as ping/status. The reactor thread is therefore the only actor that ever allocates a WatcherId; TransportEvent::WatcherClosed and TransportEvent::WatcherDied flow back through the same path and are processed on that same thread.

What ships here, and what ships with the Binder backend

This module ships the message types only. The bounded per-channel ingestion queue + eventfd is Binder-backend machinery: only a Binder backend’s on_transact fires on an arbitrary threadpool thread (a genuine cross-thread boundary), while the socket backend’s thread already is the owning reactor thread and answers in-line with zero hop. The queue is added when the Binder backend lands, not as dead machinery before any consumer exists.

Structs

ReplySink

One-shot reply target for a command.

WatcherId

An opaque long-lived handle identifying one watch subscriber.

WatcherSink

Persistent push target for watch updates.

Enums

TransportEvent

A transport-agnostic command or watcher-lifecycle event.