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.

scripts/shairport-sync.conf

@@ -14,11 +14,16 @@ general =
 //				%v for the version number, e.g. 3.0 and
 //				%V for the full version string, e.g. 3.3-OpenSSL-Avahi-ALSA-soxr-metadata-sysconfdir:/etc
 //		Overall length can not exceed 50 characters. Example: "Shairport Sync %v on %H".
+//
 //	password = "secret"; // leave this commented out if you don't want to require a password
+//	About the "service_type" setting below: On an AirPlay-2-capable Shairport Sync, if you wish to offer only the older "classic" AirPlay (sometimes called AirPlay 1), set service_type to "classic".
+//	A potential use of this would be for Apple Music on Windows, which is not compatible with the AirPlay 2 service offered by Shairport Sync, but which is compatible with the classic service.
+//	The default is "auto", which means that the AirPlay 2 service will be provided if NQPTP is present and "classic" AirPlay will be provided otherwise, with "(Classic)" appended to the service name (see above).
+//	service_type = "auto"; // This can be "auto", "airplay2" or "classic".
 //	The interpolation setting below controls how Shairport Sync adds or removes frames of audio to keep in sync.
 //			"auto" (default) measures the processor's floating point speed and chooses "soxr" if available and it is fast enough. Otherwise, "vernier" is selected.
-//			"soxr" uses the SoX library to recode a packet of frames to a new packet containing more or fewer frames. This needs a processor with fast floating point capability.
 //			"vernier" recodes a packet of frames to a new packet containing more or fewer frames. This is recommended for low powered devices.
+//			"soxr" uses the SoX library to recode a packet of frames to a new packet containing more or fewer frames. This needs a processor with fast floating point capability.
 //			"basic" causes the simple removal or insertion of frames in a packet of frames. Not recommended.
 //	interpolation = "auto"; // aka "stuffing". Default is "auto". Alternatives are "vernier", "basic" or "soxr". Choose "soxr" only if you have a reasonably fast processor and Shairport Sync has been built with "soxr" support.
 //	output_backend = "alsa"; // Run "shairport-sync -h" to get a list of all output_backends, e.g. "alsa", "pipe", "stdout". The default is the first one.

scripts/shairport-sync.service.in

@@ -4,6 +4,7 @@ After=sound.target
 Wants=network-online.target
 After=network.target network-online.target
 @SYSTEMD_AFTER_ARGS@
+@SYSTEMD_WANTS_ARGS@
 @SYSTEMD_REQUIRES_ARGS@
 StartLimitIntervalSec=300
 StartLimitBurst=5