Add repository "summary" file and metalink support

For Fedora and potentially other distributions which use globally
distributed mirrors, metalink is a popular solution to redirect
clients to a dynamic set of mirrors.

In order to make metalink work though, it needs *one* file which can
be checksummed.  (Well, potentially we could explode all refs into the
metalink.xml, but that would be a lot more invasive, and a bit weird
as we'd end up checksumming the checksum file).

This commit adds a new command:

$ ostree summary -u

To regenerate the summary file.  Can only be run by one process at a
time.

After that's done, the metalink can be generated based on it, and the
client fetch code will parse and load it.

https://bugzilla.gnome.org/show_bug.cgi?id=729585

src/libostree/ostree-metalink.h

@@ -0,0 +1,66 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2014 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.
+ */
+
+#pragma once
+
+#ifndef __GI_SCANNER__
+
+#include "ostree-fetcher.h"
+
+G_BEGIN_DECLS
+
+#define OSTREE_TYPE_METALINK         (_ostree_metalink_get_type ())
+#define OSTREE_METALINK(o)           (G_TYPE_CHECK_INSTANCE_CAST ((o), OSTREE_TYPE_METALINK, OstreeMetalink))
+#define OSTREE_METALINK_CLASS(k)     (G_TYPE_CHECK_CLASS_CAST((k), OSTREE_TYPE_METALINK, OstreeMetalinkClass))
+#define OSTREE_IS_METALINK(o)        (G_TYPE_CHECK_INSTANCE_TYPE ((o), OSTREE_TYPE_METALINK))
+#define OSTREE_IS_METALINK_CLASS(k)  (G_TYPE_CHECK_CLASS_TYPE ((k), OSTREE_TYPE_METALINK))
+#define OSTREE_METALINK_GET_CLASS(o) (G_TYPE_INSTANCE_GET_CLASS ((o), OSTREE_TYPE_METALINK, OstreeMetalinkClass))
+
+typedef struct OstreeMetalinkClass   OstreeMetalinkClass;
+typedef struct OstreeMetalink   OstreeMetalink;
+
+struct OstreeMetalinkClass
+{
+  GObjectClass parent_class;
+};
+
+GType   _ostree_metalink_get_type (void) G_GNUC_CONST;
+
+OstreeMetalink *_ostree_metalink_new (OstreeFetcher  *fetcher,
+                                      const char     *requested_file,
+                                      guint64         max_size,
+                                      SoupURI        *uri);
+
+SoupURI *_ostree_metalink_get_uri (OstreeMetalink         *self);
+
+void _ostree_metalink_request_async (OstreeMetalink         *self,
+                                     GCancellable          *cancellable,
+                                     GAsyncReadyCallback    callback,
+                                     gpointer               user_data);
+
+gboolean _ostree_metalink_request_finish (OstreeMetalink         *self,
+                                          GAsyncResult           *result,
+                                          SoupURI               **out_target_uri,
+                                          GFile                 **out_data,
+                                          GError                **error);
+
+G_END_DECLS
+
+#endif