osdep: initialize glib threads in all QEMU tools

glib versions prior to 2.31.0 require an explicit g_thread_init() call
to enable multi-threading.

Failure to initialize threading causes glib to take single-threaded code
paths without synchronization.  For example, the g_slice allocator will
crash due to race conditions.

Fix this for all QEMU tool programs (qemu-nbd, qemu-io, qemu-img) by
moving the g_thread_init() call from vl.c:main() into a new
osdep.c:thread_init() constructor function.

thread_init() has __attribute__((constructor)) and is automatically
invoked by the runtime during startup.

We can now drop the "simple" trace backend's g_thread_init() call since
thread_init() already called it.

Note that we must keep coroutine-gthread.c's g_thread_init() call which
is located in a constructor function.  There is no guarantee for
constructor function ordering so thread_init() may only be called later.

Reported-by: Mario de Chenno <mario.dechenno@unina2.it>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

util/osdep.c

@@ -436,6 +436,24 @@ int socket_init(void)
     return 0;
 }
 
+/* Ensure that glib is running in multi-threaded mode */
+static void __attribute__((constructor)) thread_init(void)
+{
+    if (!g_thread_supported()) {
+#if !GLIB_CHECK_VERSION(2, 31, 0)
+        /* Old versions of glib require explicit initialization.  Failure to do
+         * this results in the single-threaded code paths being taken inside
+         * glib.  For example, the g_slice allocator will not be thread-safe
+         * and cause crashes.
+         */
+        g_thread_init(NULL);
+#else
+        fprintf(stderr, "glib threading failed to initialize.\n");
+        exit(1);
+#endif
+    }
+}
+
 #ifndef CONFIG_IOVEC
 /* helper function for iov_send_recv() */
 static ssize_t