lib/repo-finder: Add basic support for finding remote URIs by ref name

Add an initial OstreeRepoFinder interface (but no implementations),
which will find remote URIs by ref names and collection IDs, the
combination of which is globally unique.

The new API is used in a new ostree_repo_find_updates() function, which
resolves a list of ref names to update into a set of remote URIs to pull
them from, which can be treated as mirrors. It is an attempt to
generalise resolution of the URIs to pull from, and to generalise
determination of the order and parallelisation which they should be
downloaded from in.

Includes fixes by Krzesimir Nowak <krzesimir@kinvolk.io>.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #924
Approved by: cgwalters

src/libostree/ostree-repo.h

@@ -27,6 +27,7 @@
 #include "ostree-async-progress.h"
 #ifdef OSTREE_ENABLE_EXPERIMENTAL_API
 #include "ostree-ref.h"
+#include "ostree-repo-finder.h"
 #endif
 #include "ostree-sepolicy.h"
 #include "ostree-gpg-verify-result.h"
@@ -1080,6 +1081,33 @@ ostree_repo_pull_one_dir (OstreeRepo               *self,
                           GCancellable             *cancellable,
                           GError                  **error);
 
+
+
+#if 0
+FIXME
+Called with: remote_name, refs, override-commit-ids
+or: URL, refs, override-commit-ids
+=> we only need refs; could use the remote_name or URL as additional results
+
+Summary file is downloaded first, so this would result in multiple downloads of
+the summary, but we don’t care because of caching.
+
+Big problem preventing this from being the overall API: presenting the download
+sizes in the gnome-software UI before the user chooses to download.
+
+_OSTREE_PUBLIC
+gboolean ostree_repo_find_remotes_squashed (OstreeRepo           *self,
+                                                   const gchar * const  *refs, -> options
+                                                   GVariant             *options,
+                                                   OstreeRepoFinder    **finders, -> options
+                                                   GMainContext         *context, -> nope
+                                                   OstreeAsyncProgress  *progress,
+                                                   GCancellable         *cancellable,
+                                                   GError              **error);
+#endif
+
+
+
 _OSTREE_PUBLIC
 gboolean ostree_repo_pull_with_options (OstreeRepo             *self,
                                         const char             *remote_name_or_baseurl,
@@ -1090,6 +1118,39 @@ gboolean ostree_repo_pull_with_options (OstreeRepo             *self,
 
 #ifdef OSTREE_ENABLE_EXPERIMENTAL_API
 
+_OSTREE_PUBLIC
+void ostree_repo_find_remotes_async (OstreeRepo                         *self,
+                                     const OstreeCollectionRef * const  *refs,
+                                     GVariant                           *options,
+                                     OstreeRepoFinder                  **finders,
+                                     OstreeAsyncProgress                *progress,
+                                     GCancellable                       *cancellable,
+                                     GAsyncReadyCallback                 callback,
+                                     gpointer                            user_data);
+_OSTREE_PUBLIC
+OstreeRepoFinderResult **ostree_repo_find_remotes_finish (OstreeRepo    *self,
+                                                          GAsyncResult  *result,
+                                                          GError       **error);
+
+_OSTREE_PUBLIC
+void ostree_repo_pull_from_remotes_async (OstreeRepo                           *self,
+                                          const OstreeRepoFinderResult * const *results,
+                                          GVariant                             *options,
+                                          OstreeAsyncProgress                  *progress,
+                                          GCancellable                         *cancellable,
+                                          GAsyncReadyCallback                   callback,
+                                          gpointer                              user_data);
+_OSTREE_PUBLIC
+gboolean ostree_repo_pull_from_remotes_finish (OstreeRepo    *self,
+                                               GAsyncResult  *result,
+                                               GError       **error);
+
+_OSTREE_PUBLIC
+gchar *ostree_repo_resolve_keyring_for_collection (OstreeRepo    *self,
+                                                   const gchar   *collection_id,
+                                                   GCancellable  *cancellable,
+                                                   GError       **error);
+
 _OSTREE_PUBLIC
 gboolean ostree_repo_list_collection_refs (OstreeRepo    *self,
                                            const char    *match_collection_id,