Add a new exit handler to enable Shairport Sync to exit cleanly — that is, on exit, to clean up everything, e.g. open files, ports, sockets, devices, threads, it has been using.

If cleanup isn’t finished after one second, exit will occur unconditionally.
Use the new exit handler for normal exit, for SIGINT and SIGTERM handling, for abnormal exit after fatal errors, and for handling quit requests from the D-Bus and MPRIS interfaces.

When an unfixable error occurs without an installed unfixable error handler, unlock the mutex protecting Shairport Sync’s alsa backend to permit a clean exit.

Fix a very low-level mutex-locking error that interfered with debugging.

Simplify much mutex lock handling.
Remove lots of old and unused code, variables and definitions.

Update audio_sndio.c to remove some harmless compiler warnings.

Undo adding two seconds for messages to be output before _Exit() on die() to allow messages to be output...
Undo making die() exit unconditionally.

utilities/debug.c

@@ -42,6 +42,8 @@ int debugger_show_file_and_line = 1;
 static uint64_t ns_time_at_startup = 0;
 static uint64_t ns_time_at_last_debug_message;
 
+void (*local_exit_requester)(const int exit_status) = NULL;
+
 // always lock use this when accessing the ns_time_at_last_debug_message
 static pthread_mutex_t debug_timing_lock = PTHREAD_MUTEX_INITIALIZER;
 
@@ -57,13 +59,14 @@ uint64_t debug_get_absolute_time_in_ns() {
   return time_now_ns;
 }
 
-void debug_init(int level, int show_elapsed_time, int show_relative_time, int show_file_and_line) {
+void debug_init(int level, int show_elapsed_time, int show_relative_time, int show_file_and_line, void (*exit_requester)(const int exit_status)) {
   ns_time_at_startup = debug_get_absolute_time_in_ns();
   ns_time_at_last_debug_message = ns_time_at_startup;
   debuglev = level;
   debugger_show_elapsed_time = show_elapsed_time;
   debugger_show_relative_time = show_relative_time;
   debugger_show_file_and_line = show_file_and_line;
+  local_exit_requester = exit_requester;
 }
 
 int debug_level() { return debuglev; };
@@ -144,8 +147,14 @@ void _die(const char *filename, const int linenumber, const char *format, ...) {
   // syslog(LOG_ERR, "%s", b);
   fprintf(stderr, "%s\n", b);
   pthread_setcancelstate(oldState, NULL);
-  usleep(2000000); //allow some time for printing of message
-  _Exit(EXIT_FAILURE);
+  if (local_exit_requester != NULL) {
+    local_exit_requester(EXIT_FAILURE);
+    // wait for the exit request to be honoured
+    usleep(1000000);
+    // if not, head for the hills
+    fprintf(stderr, "fatal error: exit cleanup could not be completed.\n");
+    _Exit(EXIT_FAILURE);
+  }
 }
 
 void _warn(const char *filename, const int linenumber, const char *format, ...) {