Function pa_playback_session_open 

pub(crate) fn pa_playback_session_open(
    inner: &Inner,
    spec: &Spec,
    device: Option<&str>,
    attr: &BufferAttr,
) -> Result<PaPlaybackSession, SessionOpenError>

Open a playback session: mainloop + context + playback stream driven to Ready on the bounded pump, honoring stop_pending at every turn. The mirror of pa_capture_session_open.

Every failure is Transient: connect_playback resolves a sink name asynchronously, so a wrong device name and a server that is still coming up are the same failed stream state here, and the caller’s retry budget is what bounds either one.