core: Change compose to operate purely in-memory

This is *significantly* faster than checking out each branch into the
real filesystem, then importing it again.

src/libostree/ostree-mutable-tree.c

@@ -95,6 +95,31 @@ ostree_mutable_tree_set_contents_checksum (OstreeMutableTree *self,
 const char *
 ostree_mutable_tree_get_contents_checksum (OstreeMutableTree *self)
 {
+  GHashTableIter iter;
+  gpointer key, value;
+
+  if (!self->contents_checksum)
+    return NULL;
+
+  /* Ensure the cache is valid; this implementation is a bit
+   * lame in that we walk the whole tree every time this
+   * getter is called; a better approach would be to invalidate
+   * all of the parents whenever a child is modified.
+   *
+   * However, we only call this function once right now.
+   */
+  g_hash_table_iter_init (&iter, self->subdirs);
+  while (g_hash_table_iter_next (&iter, &key, &value))
+    {
+      OstreeMutableTree *subdir = value;
+      if (!ostree_mutable_tree_get_contents_checksum (subdir))
+        {
+          g_free (self->contents_checksum);
+          self->contents_checksum = NULL;
+          return NULL;
+        }
+    }
+
   return self->contents_checksum;
 }
 
@@ -122,6 +147,7 @@ ostree_mutable_tree_replace_file (OstreeMutableTree *self,
       goto out;
     }
 
+  ostree_mutable_tree_set_contents_checksum (self, NULL);
   g_hash_table_replace (self->files,
                         g_strdup (name),
                         g_strdup (checksum));
@@ -153,6 +179,7 @@ ostree_mutable_tree_ensure_dir (OstreeMutableTree *self,
   if (!ret_dir)
     {
       ret_dir = ostree_mutable_tree_new ();
+      ostree_mutable_tree_set_contents_checksum (self, NULL);
       g_hash_table_insert (self->subdirs, g_strdup (name), g_object_ref (ret_dir));
     }
   

src/libostree/ostree-repo-file.c

@@ -280,6 +280,12 @@ ostree_repo_file_ensure_resolved (OstreeRepoFile  *self,
     return TRUE;
 }
 
+const char *
+ostree_repo_file_get_commit (OstreeRepoFile  *self)
+{
+  return ostree_repo_file_get_root (self)->commit;
+}
+
 gboolean
 ostree_repo_file_get_xattrs (OstreeRepoFile  *self,
                               GVariant       **out_xattrs,

src/libostree/ostree-repo-file.h

@@ -75,6 +75,8 @@ gboolean ostree_repo_file_is_tree (OstreeRepoFile  *self);
 
 const char * ostree_repo_file_get_checksum (OstreeRepoFile  *self);
 
+const char * ostree_repo_file_get_commit (OstreeRepoFile  *self);
+
 GFile *ostree_repo_file_nontree_get_local (OstreeRepoFile  *self);
 
 int     ostree_repo_file_tree_find_child  (OstreeRepoFile  *self,

src/libostree/ostree-repo.c

@@ -1380,6 +1380,7 @@ ostree_repo_stage_directory_to_mtree (OstreeRepo           *self,
   GChecksum *child_file_checksum = NULL;
   GVariant *xattrs = NULL;
   GInputStream *file_input = NULL;
+  gboolean repo_dir_was_empty = FALSE;
 
   /* We can only reuse checksums directly if there's no modifier */
   if (OSTREE_IS_REPO_FILE (dir) && modifier == NULL)
@@ -1388,7 +1389,9 @@ ostree_repo_stage_directory_to_mtree (OstreeRepo           *self,
   if (repo_dir)
     {
       ostree_mutable_tree_set_metadata_checksum (mtree, ostree_repo_file_get_checksum (repo_dir));
-      ostree_mutable_tree_set_contents_checksum (mtree, ostree_repo_file_tree_get_content_checksum (repo_dir));
+      repo_dir_was_empty = 
+        g_hash_table_size (ostree_mutable_tree_get_files (mtree)) == 0
+        && g_hash_table_size (ostree_mutable_tree_get_subdirs (mtree)) == 0;
     }
   else
     {
@@ -1484,6 +1487,9 @@ ostree_repo_stage_directory_to_mtree (OstreeRepo           *self,
       goto out;
     }
 
+  if (repo_dir && repo_dir_was_empty)
+    ostree_mutable_tree_set_contents_checksum (mtree, ostree_repo_file_tree_get_content_checksum (repo_dir));
+
   ret = TRUE;
  out:
   g_clear_object (&dir_enum);
@@ -1593,10 +1599,12 @@ ostree_repo_stage_mtree (OstreeRepo           *self,
   GVariant *serialized_tree = NULL;
   GHashTableIter hash_iter;
   gpointer key, value;
+  const char *existing_checksum;
 
-  if (ostree_mutable_tree_get_contents_checksum (mtree))
+  existing_checksum = ostree_mutable_tree_get_contents_checksum (mtree);
+  if (existing_checksum)
     {
-      ret_contents_checksum = g_strdup (ostree_mutable_tree_get_contents_checksum (mtree));
+      ret_contents_checksum = g_strdup (existing_checksum);
     }
   else
     {