GDBusConnection: check for initialization where needed for thread-safety

Also document which fields require such a check in order to have correct
threading semantics.

This usage doesn't matches the GInitable documentation, which suggests
use of a GError - but using an uninitialized GDBusConnection is
programming error, and not usefully recoverable. (The GInitable
documentation may have been a mistake - GNOME#662208.) Also, not all of
the places where we need it can raise a GError.

The check serves a dual purpose: it turns a non-deterministic crash into
a deterministic critical warning, and is also a memory barrier for
thread-safety. All of these functions dereference or return fields that
are meant to be protected by FLAG_INITIALIZED, so they could crash or
return an undefined value to their caller without this, if called from a
thread that isn't the one that called initable_init() (although I can't
think of any way to do that without encountering a memory barrier,
undefined behaviour, or a race condition that leads to undefined
behaviour if the non-initializing thread wins the race).

One exception is that initable_init() itself makes a synchronous call.
We deal with that by passing new internal flags up the call stack, to
reassure g_dbus_connection_send_message_unlocked() that it can go ahead.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=661689
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=661992
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: David Zeuthen <davidz@redhat.com>

gio/gioenums.h

@@ -1053,6 +1053,7 @@ typedef enum {
   G_DBUS_CALL_FLAGS_NONE = 0,
   G_DBUS_CALL_FLAGS_NO_AUTO_START = (1<<0)
 } GDBusCallFlags;
+/* (1<<31) is reserved for internal use by GDBusConnection, do not use it. */
 
 /**
  * GDBusMessageType:
@@ -1208,6 +1209,7 @@ typedef enum
   G_DBUS_SEND_MESSAGE_FLAGS_NONE = 0,
   G_DBUS_SEND_MESSAGE_FLAGS_PRESERVE_SERIAL = (1<<0)
 } GDBusSendMessageFlags;
+/* (1<<31) is reserved for internal use by GDBusConnection, do not use it. */
 
 /**
  * GCredentialsType: