pull: Rework threading communication model

Previously, I've observed bugs where we either:
1) Exit too early, leaving undownloaded objects
2) Hang while downloading

This rewrite hopefully fixes both.

src/libotutil/ot-waitable-queue.h

@@ -0,0 +1,47 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2012 Colin Walters <walters@verbum.org>.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Author: Colin Walters <walters@verbum.org>
+ */
+
+#ifndef __OSTREE_WAITABLE_QUEUE_H__
+#define __OSTREE_WAITABLE_QUEUE_H__
+
+#include <gio/gio.h>
+
+G_BEGIN_DECLS
+
+typedef struct OtWaitableQueue OtWaitableQueue;
+
+OtWaitableQueue *ot_waitable_queue_new (void);
+
+void ot_waitable_queue_push (OtWaitableQueue      *queue,
+                             gpointer              data);
+
+GSource *ot_waitable_queue_create_source (OtWaitableQueue   *queue);
+
+gboolean ot_waitable_queue_pop (OtWaitableQueue *queue,
+                                gpointer        *out_val);
+
+void ot_waitable_queue_ref (OtWaitableQueue *queue);
+void ot_waitable_queue_unref (OtWaitableQueue *queue);
+
+G_END_DECLS
+
+#endif