shared: add nmc_client_new_waitsync() and nmc_client_new_async*() helpers

Benefits:

- nmc_client_new_async*() allows to set properties on the NMClient
  instance before calling g_async_initable_init_async().
  It also allows to subscribe to any signals (like NM_CLIENT_DEVICE_ADDED)
  before actually iterating the GMainContext. This is a sensible and
  supported thing to do!

- nmc_client_new_waitsync() iterates the GMainContext until the (async)
  initialization is complete. That is different from synchronous nm_client_new(),
  which does not iterate the caller's GMainContext, and hence needs an
  internal context to ensure the order of events is honored.

- nmc_client_new_waitsync() always returns the NMClient instance, even
  if initialization fails.
  That is useful if you need the nm_client_get_context_busy_watcher() instance
  to ensure all pending messages are completed.

shared/nm-libnm-aux/nm-libnm-aux.c

@@ -3,3 +3,119 @@
 #include "nm-default.h"
 
 #include "nm-libnm-aux.h"
+
+/*****************************************************************************/
+
+NMClient *
+nmc_client_new_async_valist (GCancellable *cancellable,
+                             GAsyncReadyCallback callback,
+                             gpointer user_data,
+                             const char *first_property_name,
+                             va_list ap)
+{
+	NMClient *nmc;
+
+	nmc = NM_CLIENT (g_object_new_valist (NM_TYPE_CLIENT, first_property_name, ap));
+	g_async_initable_init_async (G_ASYNC_INITABLE (nmc),
+	                             G_PRIORITY_DEFAULT,
+	                             cancellable,
+	                             callback,
+	                             user_data);
+	return nmc;
+}
+
+NMClient *
+nmc_client_new_async (GCancellable *cancellable,
+                      GAsyncReadyCallback callback,
+                      gpointer user_data,
+                      const char *first_property_name,
+                      ...)
+{
+	NMClient *nmc;
+	va_list ap;
+
+	va_start (ap, first_property_name);
+	nmc = nmc_client_new_async_valist (cancellable,
+	                                   callback,
+	                                   user_data,
+	                                   first_property_name,
+	                                   ap);
+	va_end (ap);
+	return nmc;
+}
+
+/*****************************************************************************/
+
+typedef struct {
+	GMainLoop *main_loop;
+	NMClient *nmc;
+	GError *error;
+} ClientCreateData;
+
+static void
+_nmc_client_new_waitsync_cb (GObject *source_object,
+                             GAsyncResult *result,
+                             gpointer user_data)
+{
+	ClientCreateData *data = user_data;
+
+	g_async_initable_init_finish (G_ASYNC_INITABLE (source_object),
+	                              result,
+	                              &data->error);
+	g_main_loop_quit (data->main_loop);
+}
+
+/**
+ * nmc_client_new:
+ * @cancellable: the cancellable to abort the creation.
+ * @out_nmc: (out): (transfer full): if give, transfers a reference
+ *   to the NMClient instance. Note that this never fails to create
+ *   the NMClient GObject, but depending on the return value,
+ *   the instance was successfully initialized or not.
+ * @error: the error if creation fails.
+ * @first_property_name: the name of the first property
+ * @...: the value of the first property, followed optionally by more
+ *  name/value pairs, followed by %NULL
+ *
+ * Returns: %TRUE, if the client was successfully initalized.
+ *
+ * This uses nm_client_new_async() to create a NMClient instance,
+ * but it iterates the current GMainContext until the client is
+ * ready. As such, it waits for the client creation to complete
+ * (like sync nm_client_new()) but it iterates the caller's GMainContext
+ * (unlike sync nm_client_new()). This is often preferable, because
+ * sync nm_client_new() needs to create an additional internal GMainContext
+ * that it can iterate instead. That has a performance overhead that
+ * is often unnecessary.
+ */
+gboolean
+nmc_client_new_waitsync (GCancellable *cancellable,
+                         NMClient **out_nmc,
+                         GError **error,
+                         const char *first_property_name,
+                         ...)
+{
+	gs_unref_object NMClient *nmc = NULL;
+	nm_auto_unref_gmainloop GMainLoop *main_loop = g_main_loop_new (g_main_context_get_thread_default (), FALSE);
+	ClientCreateData data = {
+		.main_loop = main_loop,
+	};
+	va_list ap;
+
+	va_start (ap, first_property_name);
+	nmc = nmc_client_new_async_valist (cancellable,
+	                                   _nmc_client_new_waitsync_cb,
+	                                   &data,
+	                                   first_property_name,
+	                                   ap);
+	va_end (ap);
+
+	g_main_loop_run (main_loop);
+
+	NM_SET_OUT (out_nmc, g_steal_pointer (&nmc));
+	if (data.error) {
+		g_propagate_error (error, data.error);
+		return FALSE;
+	}
+	return TRUE;
+}

shared/nm-libnm-aux/nm-libnm-aux.h

@@ -3,4 +3,22 @@
 #ifndef __NM_LIBNM_AUX_H__
 #define __NM_LIBNM_AUX_H__
 
+NMClient *nmc_client_new_async_valist (GCancellable *cancellable,
+                                       GAsyncReadyCallback callback,
+                                       gpointer user_data,
+                                       const char *first_property_name,
+                                       va_list ap);
+
+NMClient *nmc_client_new_async (GCancellable *cancellable,
+                                GAsyncReadyCallback callback,
+                                gpointer user_data,
+                                const char *first_property_name,
+                                ...);
+
+gboolean nmc_client_new_waitsync (GCancellable *cancellable,
+                                  NMClient **out_nmc,
+                                  GError **error,
+                                  const char *first_property_name,
+                                  ...);
+
 #endif /* __NM_LIBNM_AUX_H__ */