builder-main: Commit mirrored media as screenshot ref to exported repo

This is needed to store the application icons and the mirrored
screenshots (as done by appstream) to the ostree repo. This can be
useful in many ways, the primary being flat-manager extracts [1] these
screenshot refs and stores them in the server so that application
store frontends can use their own mirrored screenshot and icon URLs.

This has been for the longest time done at various downstream levels
such as in buildbot [2], flatpak-github-actions [3] and vorarbeiter [4]
(the successor to buildbot) but it's more user friendly to have
flatpak-builder do this extra work.

[1]: flatpak/flat-manager@eff4300
[2]: https://github.com/flathub-infra/buildbot/blob/9d8e2b5483b2e446389548446ebf9c3934380682/master/buildbot/flathub_master.py#L1081
[3]: https://github.com/flatpak/flatpak-github-actions/blob/6684584b07d86113b1acfc80b3e4667f16f617c3/flatpak-builder/index.js#L280-L286
[4]: https://github.com/flathub-infra/vorarbeiter/blob/7f2f04a562e73ba4f46044dae7c99f8e9e64a39d/justfile#L185-L189

Author: bbhtt

doc/flatpak-builder.xml

@@ -528,7 +528,9 @@
                   in the appstream xml to the specified URL. The resulting files
                   will be stored in the "screenshots" subdirectory in the app directory
                   for versions earlier than 1.3.4 and "files/share/app-info/media"
-                  subdirectory for newer versions.
+                  subdirectory for newer versions. Since version 1.4.5 this
+                  will also create a screenshot ref in the exported OSTree
+                  repo for each architecture containing the mirrored media.
                 </para></listitem>
             </varlistentry>
 

src/builder-main.c

@@ -27,6 +27,7 @@
 
 #include <glib/gi18n.h>
 #include <gio/gio.h>
+#include <ostree.h>
 #include "libglnx.h"
 
 #include "builder-flatpak-utils.h"
@@ -263,6 +264,80 @@ do_export (BuilderContext *build_context,
                          (const gchar * const *) args->pdata, NULL);
 }
 
+static gboolean
+commit_screenshot_ref (const char *repo_path,
+                       const char *app_dir_path,
+                       const char *arch,
+                       GError **error)
+{
+  g_autofree char *media_dir_path = NULL;
+  g_autoptr(GFile) media_dir = NULL;
+  g_autoptr(GFile) repo_file = NULL;
+  g_autoptr(OstreeRepo) repo = NULL;
+  g_autoptr(OstreeMutableTree) tree = NULL;
+  g_autoptr(GFile) tree_root = NULL;
+  g_autofree char *commit_checksum = NULL;
+  g_autofree char *ref_name = NULL;
+  g_autoptr(OstreeRepoCommitModifier) modifier = NULL;
+  gboolean ret = FALSE;
+
+  media_dir_path = g_build_filename (app_dir_path, "files/share/app-info/media", NULL);
+  media_dir = g_file_new_for_path (media_dir_path);
+
+  if (!g_file_query_exists (media_dir, NULL)) {
+    g_print ("Media directory does not exist, skipping commit\n");
+    return TRUE;
+  }
+
+  ref_name = g_strdup_printf ("screenshots/%s", arch);
+
+  repo_file = g_file_new_for_path (repo_path);
+  repo = ostree_repo_new (repo_file);
+  if (!ostree_repo_open (repo, NULL, error)) {
+    return FALSE;
+  }
+
+  modifier = ostree_repo_commit_modifier_new (OSTREE_REPO_COMMIT_MODIFIER_FLAGS_CANONICAL_PERMISSIONS,
+                                             NULL, NULL, NULL);
+  if (!ostree_repo_prepare_transaction (repo, NULL, NULL, error)) {
+    return FALSE;
+  }
+
+  tree = ostree_mutable_tree_new ();
+
+  if (!ostree_repo_write_directory_to_mtree (repo, media_dir, tree, modifier, NULL, error))
+    goto out;
+
+  if (!ostree_repo_write_mtree (repo, tree, &tree_root, NULL, error))
+    goto out;
+
+  if (!ostree_repo_write_commit (repo,
+                                 NULL,
+                                 NULL,
+                                 NULL,
+                                 NULL,
+                                 OSTREE_REPO_FILE (tree_root),
+                                 &commit_checksum,
+                                 NULL, error))
+    goto out;
+
+  ostree_repo_transaction_set_ref (repo, NULL, ref_name, commit_checksum);
+
+  if (!ostree_repo_commit_transaction (repo, NULL, NULL, error))
+    goto out;
+
+  g_print ("Committed screenshot ref: %s (%s)\n", ref_name, commit_checksum);
+  ret = TRUE;
+
+out:
+  if (!ret && repo) {
+    if (!ostree_repo_abort_transaction (repo, NULL, NULL))
+      g_warning ("Failed to abort ostree transaction");
+  }
+
+  return ret;
+}
+
 static gboolean
 do_install (BuilderContext *build_context,
             const gchar    *repodir,
@@ -974,6 +1049,17 @@ main (int    argc,
           return 1;
         }
 
+        if (opt_mirror_screenshots_url) {
+          if (!commit_screenshot_ref (flatpak_file_get_path_cached (export_repo),
+                                      app_dir_path,
+                                      builder_context_get_arch (build_context),
+                                      &error)) {
+
+              g_printerr ("Failed to commit screenshot ref: %s\n", error->message);
+              return 1;
+          }
+        }
+
       /* Export regular locale extensions */
       dir_enum = g_file_enumerate_children (app_dir, "standard::name,standard::type",
                                             G_FILE_QUERY_INFO_NOFOLLOW_SYMLINKS,