Enhancements

Enable the AirPlay 2 build to operate an optional classic-AirPlay-only service or to gracefully degrade to classic AirPlay, as follows:
  1. Add a new command-line option '-—service-type=<type>' and an equivalent configuration entry 'service_type = "<type>"' in the 'general' section of the configuration file,
     where <type> can be "auto", "classic" or "airplay2":
      1. "auto" (default) means that the service will be AirPlay 2 if NQPTP is running. If NQPTP is not running, classic AirPlay service will be provided instead, and in that case,
      "(Classic)" will be appended to the default AirPlay service name visible to AirPlay clients like Apple Music, for example "RaspberryPi3B (Classic)".
      2. "classic" means the service will be classic AirPlay (aka AirPlay 1).
      3. "airplay2" means the service will be the modern AirPlay 2. In this case, as distinct from "auto", if NQPTP is not running, Shairport Sync will log an error and terminate.
  2. In the systemd service file, NQPTP is now a "Want" rather than a "Require". If it's present, then it will be launched before Shairport Sync. If it's absent, Shairport Sync will launch anyway.
  3. Improve the delivery of input format changes and emit 'sdsc' metadata when changes occur.
  4. Emit new format information in the log if statistics is enabled.

Docker Changes
  1. Support for linux/arm/v6 has been dropped, as Docker is no longer supported.
  2. NQPTP is not started in the AirPlay 2 Docker image if '--service-type=classic' or '--service-type=airplay1' is in the command line options at the end of the docker run command.
     The purpose is to ensure that ports 319 and 320 are left alone when the AirPlay 2 image is set to provide Classic service only.
     Note that setting the configuration file 'service_type' to 'classic' will not prevent NQPTP from starting up -- you must use the command line option.
  3. A new "dev" target has been added. It is a large image containing the custom-built FFmpeg library, NQPTP, Avahi and D-Bus along with the Shairport Sync source and
     all necessary development tools. When started, Avahi, D-Bus and NQPTP are all installed and running. The bash shell has also been added and is entered.

Stability Improvements
  Reorganise session preemption to fully terminate the existing session before starting a new one.
  Don't delay closing the event port to wait for it to be closed at the client end.
  Add a safe_socket_close() function to ensure sockets are fully closed. Use -1 to designate closed rather than 0, to prevent attempts to reclose sockets, causing mayhem.
  Re-order FFmpeg decommissioning during teardown.

ADVANCED TOPICS/PulseAudioAndPipeWire.md

@@ -1,6 +1,9 @@
 # Working with PulseAudio or PipeWire
 Many Linux systems, especially desktop Linuxes with a GUI, have [PipeWire](https://pipewire.org) or [PulseAudio](https://www.freedesktop.org/wiki/Software/PulseAudio/) installed as [sound servers](https://en.wikipedia.org/wiki/Sound_server).
-PipeWire and PulseAudio are widely used and have the great advantage of being easily able to mix audio from multiple sources. The slight downside is that audio may be further processed (e.g. transcoded) on its way to the output device.
+PipeWire and PulseAudio are widely used and have the great advantage of being easily able to mix audio from multiple sources. 
+
+However, the main thing to remember about PipeWire and PulseAudio sound servers is they only become available when a user logs in -- that is, they are set up as _user services_.
+Shairport Sync relies on them and therefore it must also be set up as a user service. Shairport Sync can not be set up as a system service because the PipeWire or PulseAudio services are not available when system services are launched just after system startup.
 
 To use PipeWire or PulseAudio-based systems, Shairport Sync must be set up as a user service.
 
@@ -10,26 +13,5 @@ To use PipeWire or PulseAudio-based systems, Shairport Sync must be set up as a
 3. To check if PipeWire support is built into Shairport Sync, check that the string `PipeWire` is included in the version string. (Enter `$ shairport-sync -V` to get the version string.)  Similarly, the version string will include `PulseAudio` if the PulseAudio backend is built in.
 4. Remember to specify which backend Shairport Sync should use in the configuration file or on the command line.
 
-## Automatic Startup of Shairport Sync
-
-The main thing to remember about PipeWire and PulseAudio sound servers is that the services they offer only become available when a user logs in -- that is, they are set up as _user services_.
-Shairport Sync relies on them, so it must also be set up as a user service; it can not be set up as a system service because the PipeWire or PulseAudio services needed by Shairport Sync are not available when system services are launched just after system startup. 
-
-### Starting Shairport Sync as a User Service
-To make Shairport Sync start as a user service, (assuming you built and installed Shairport Sync using the BUILD.md guide), ensure you are logged in as the appropriate user and enter:
-```
-$ systemctl --user enable shairport-sync
-```
-Make sure it is _not enabled_ as a system service:
-```
-# systemctl disable shairport-sync
-```
-and then reboot.
-
-#### Problems with User Service
-Shairport Sync will function perfectly well as a user service, but there are a number of things to bear in mind:
-
-1. The AirPlay service will only be available when the user is logged in. When the user logs out, Shairport Sync will terminate and the AirPlay service will disappear.
-2. If the Linux system is a desktop Linux with a GUI, audio will be sent to the default output only when the user is logged in through the GUI.
-
-Automatic user login may help address these problems.
+## Starting Shairport Sync as a User Service
+Please refer to [this](https://github.com/mikebrady/shairport-sync/blob/development/BUILD.md#5-enable-and-start-service) section to discover how to start Shairport Sync as a user service.