+typedef struct IOWatchPoll
+{
+ GSource parent;
+
+ GIOChannel *channel;
+ GSource *src;
+
+ IOCanReadHandler *fd_can_read;
+ GSourceFunc fd_read;
+ void *opaque;
+} IOWatchPoll;
+
+static IOWatchPoll *io_watch_poll_from_source(GSource *source)
+{
+ return container_of(source, IOWatchPoll, parent);
+}
+
+static gboolean io_watch_poll_prepare(GSource *source, gint *timeout_)
+{
+ IOWatchPoll *iwp = io_watch_poll_from_source(source);
+ bool now_active = iwp->fd_can_read(iwp->opaque) > 0;
+ bool was_active = iwp->src != NULL;
+ if (was_active == now_active) {
+ return FALSE;
+ }
+
+ if (now_active) {
+ iwp->src = g_io_create_watch(iwp->channel, G_IO_IN | G_IO_ERR | G_IO_HUP);
+ g_source_set_callback(iwp->src, iwp->fd_read, iwp->opaque, NULL);
+ g_source_attach(iwp->src, NULL);
+ } else {
+ g_source_destroy(iwp->src);
+ g_source_unref(iwp->src);
+ iwp->src = NULL;
+ }
+ return FALSE;
+}
+
+static gboolean io_watch_poll_check(GSource *source)
+{
+ return FALSE;
+}
+
+static gboolean io_watch_poll_dispatch(GSource *source, GSourceFunc callback,
+ gpointer user_data)
+{
+ abort();
+}
+
+static void io_watch_poll_finalize(GSource *source)
+{
+ /* Due to a glib bug, removing the last reference to a source
+ * inside a finalize callback causes recursive locking (and a
+ * deadlock). This is not a problem inside other callbacks,
+ * including dispatch callbacks, so we call io_remove_watch_poll
+ * to remove this source. At this point, iwp->src must
+ * be NULL, or we would leak it.
+ *
+ * This would be solved much more elegantly by child sources,
+ * but we support older glib versions that do not have them.
+ */
+ IOWatchPoll *iwp = io_watch_poll_from_source(source);
+ assert(iwp->src == NULL);
+}
+
+static GSourceFuncs io_watch_poll_funcs = {
+ .prepare = io_watch_poll_prepare,
+ .check = io_watch_poll_check,
+ .dispatch = io_watch_poll_dispatch,
+ .finalize = io_watch_poll_finalize,
+};
+
+/* Can only be used for read */
+static guint io_add_watch_poll(GIOChannel *channel,
+ IOCanReadHandler *fd_can_read,
+ GIOFunc fd_read,
+ gpointer user_data)
+{
+ IOWatchPoll *iwp;
+ int tag;
+
+ iwp = (IOWatchPoll *) g_source_new(&io_watch_poll_funcs, sizeof(IOWatchPoll));
+ iwp->fd_can_read = fd_can_read;
+ iwp->opaque = user_data;
+ iwp->channel = channel;
+ iwp->fd_read = (GSourceFunc) fd_read;
+ iwp->src = NULL;
+
+ tag = g_source_attach(&iwp->parent, NULL);
+ g_source_unref(&iwp->parent);
+ return tag;
+}
+
+static void io_remove_watch_poll(guint tag)
+{
+ GSource *source;
+ IOWatchPoll *iwp;
+
+ g_return_if_fail (tag > 0);
+
+ source = g_main_context_find_source_by_id(NULL, tag);
+ g_return_if_fail (source != NULL);
+
+ iwp = io_watch_poll_from_source(source);
+ if (iwp->src) {
+ g_source_destroy(iwp->src);
+ g_source_unref(iwp->src);
+ iwp->src = NULL;
+ }
+ g_source_destroy(&iwp->parent);
+}