Don't prune build stages from cache with --{export,finish}-only

A commonly used CI workflow [1] is to chain the following steps:

    > flatpak-builder --stop-at=<module>
    > flatpak build <commands to build module from local checkout>
    > flatpak-builder --finish-only

Unfortunately, the last step always purge all the compilation cache
created by the first one. All build steps are marked unused by default
and since they are skipped due to the option --finish-only, all of them
are always pruned by the gc function.

As a result, flatpak-builder cache becomes useless, a full compilation
is always performed.

Improve this by not cleaning unused stages when no compilation is done
by flatpak-builder (i.e: when flatpak-builder is used with --export-only
or --finish-only option).

[1]: https://gitlab.gnome.org/GNOME/Initiatives/wikis/DevOps-with-Flatpak

src/builder-cache.c

@@ -1210,6 +1210,7 @@ builder_cache_disable_lookups (BuilderCache *self)
 
 gboolean
 builder_gc (BuilderCache *self,
+            gboolean      prune_unused_stages,
             GError      **error)
 {
   gint objects_total;
@@ -1218,20 +1219,23 @@ builder_gc (BuilderCache *self,
   GHashTableIter iter;
   gpointer key, value;
 
-  g_hash_table_iter_init (&iter, self->unused_stages);
-  while (g_hash_table_iter_next (&iter, &key, &value))
+  if (prune_unused_stages)
     {
-      const char *unused_stage = (const char *) key;
-      g_autofree char *unused_ref = get_ref (self, unused_stage);
+      g_hash_table_iter_init (&iter, self->unused_stages);
+      while (g_hash_table_iter_next (&iter, &key, &value))
+        {
+          const char *unused_stage = (const char *) key;
+          g_autofree char *unused_ref = get_ref (self, unused_stage);
 
-      g_debug ("Removing unused ref %s", unused_ref);
+          g_debug ("Removing unused ref %s", unused_ref);
 
-      if (!ostree_repo_set_ref_immediate (self->repo,
-                                          NULL,
-                                          unused_ref,
-                                          NULL,
-                                          NULL, error))
-        return FALSE;
+          if (!ostree_repo_set_ref_immediate (self->repo,
+                                              NULL,
+                                              unused_ref,
+                                              NULL,
+                                              NULL, error))
+            return FALSE;
+        }
     }
 
   g_print ("Pruning cache\n");

src/builder-cache.h

@@ -59,6 +59,7 @@ GPtrArray   *builder_cache_get_changes (BuilderCache *self,
 GPtrArray   *builder_cache_get_all_changes (BuilderCache *self,
                                             GError      **error);
 gboolean      builder_gc (BuilderCache *self,
+                          gboolean      prune_unused_stages,
                           GError      **error);
 
 void builder_cache_checksum_str (BuilderCache *self,

src/builder-main.c

@@ -332,6 +332,7 @@ main (int    argc,
   gboolean is_run = FALSE;
   gboolean is_show_deps = FALSE;
   gboolean app_dir_is_empty = FALSE;
+  gboolean prune_unused_stages = FALSE;
   g_autoptr(FlatpakContext) arg_context = NULL;
   g_autoptr(FlatpakTempDir) cleanup_manifest_dir = NULL;
   g_autofree char *manifest_basename = NULL;
@@ -1064,7 +1065,10 @@ main (int    argc,
         }
     }
 
-  if (!builder_gc (cache, &error))
+  if (!opt_finish_only && !opt_export_only)
+    prune_unused_stages = TRUE;
+
+  if (!builder_gc (cache, prune_unused_stages, &error))
     {
       g_warning ("Failed to GC build cache: %s", error->message);
       g_clear_error (&error);