compose: Raise up the code to read the previous (OSTree) commit

I'm planning to replace the caching code with something that inspects
the previous commit rather than a lookaside cache, so raise this code
up to a higher level.

src/rpmostree-compose-builtin-tree.c

@@ -727,6 +727,7 @@ rpmostree_compose_builtin_tree (int             argc,
                                 GError        **error)
 {
   gboolean ret = FALSE;
+  GError *temp_error = NULL;
   GOptionContext *context = g_option_context_new ("- Run yum and commit the result to an OSTree repository");
   const char *ref;
   RpmOstreeTreeComposeContext selfdata = { NULL, };
@@ -738,6 +739,8 @@ rpmostree_compose_builtin_tree (int             argc,
   gs_free char *cached_compose_checksum = NULL;
   gs_free char *new_compose_checksum = NULL;
   gs_unref_object GFile *cachedir = NULL;
+  gs_unref_object GFile *previous_root = NULL;
+  gs_free char *previous_checksum = NULL;
   gs_unref_object GFile *yumroot = NULL;
   gs_unref_object GFile *targetroot = NULL;
   gs_unref_object GFile *yumroot_varcache = NULL;
@@ -901,17 +904,34 @@ rpmostree_compose_builtin_tree (int             argc,
       goto out;
     }
 
-  yumroot = g_file_get_child (self->workdir, "rootfs.tmp");
-  if (!gs_shutil_rm_rf (yumroot, cancellable, error))
-    goto out;
-  targetroot = g_file_get_child (self->workdir, "rootfs");
-
   ref = _rpmostree_jsonutil_object_require_string_member (treefile, "ref", error);
   if (!ref)
     goto out;
 
   ref_unix = g_strdelimit (g_strdup (ref), "/", '_');
 
+  if (!ostree_repo_read_commit (repo, ref, &previous_root, &previous_checksum,
+                                cancellable, &temp_error))
+    {
+      if (g_error_matches (temp_error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND))
+        { 
+          g_clear_error (&temp_error);
+          g_print ("No previous commit for %s\n", ref);
+        }
+      else
+        {
+          g_propagate_error (error, temp_error);
+          goto out;
+        }
+    }
+  else
+    g_print ("Previous commit: %s\n", previous_checksum);
+
+  yumroot = g_file_get_child (self->workdir, "rootfs.tmp");
+  if (!gs_shutil_rm_rf (yumroot, cancellable, error))
+    goto out;
+  targetroot = g_file_get_child (self->workdir, "rootfs");
+
   bootstrap_packages = g_ptr_array_new ();
   packages = g_ptr_array_new ();
 
@@ -936,6 +956,7 @@ rpmostree_compose_builtin_tree (int             argc,
     self->serialized_treefile = g_bytes_new_take (treefile_buf, len);
   }
 
+  if (previous_root != NULL)
     {
       gboolean generate_from_previous = TRUE;
 
@@ -947,7 +968,7 @@ rpmostree_compose_builtin_tree (int             argc,
 
       if (generate_from_previous)
         {
-        if (!rpmostree_generate_passwd_from_previous (repo, yumroot, ref,
+          if (!rpmostree_generate_passwd_from_previous (repo, yumroot, previous_root,
                                                         cancellable, error))
             goto out;
         }

src/rpmostree-passwd-util.c

@@ -734,30 +734,14 @@ concat_passwd_file (GFile           *yumroot,
 gboolean
 rpmostree_generate_passwd_from_previous (OstreeRepo      *repo,
                                          GFile           *yumroot,
-                                         const char      *ref,
+                                         GFile           *previous_root,
                                          GCancellable    *cancellable,
                                          GError         **error)
 {
   gboolean ret = FALSE;
-  GError *temp_error = NULL;
-  gs_unref_object GFile *previous_root = NULL;
   gs_unref_object GFile *yumroot_etc_group = g_file_resolve_relative_path (yumroot, "etc/group");
   gs_unref_object GFile *out = NULL;
 
-  if (!ostree_repo_read_commit (repo, ref, &previous_root, NULL, NULL, &temp_error))
-    {
-      if (g_error_matches (temp_error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND))
-        { 
-          g_clear_error (&temp_error);
-          ret = TRUE;
-        }
-      else
-        {
-          g_propagate_error (error, temp_error);
-        }
-      goto out;
-    }
-
   if (!concat_passwd_file (yumroot, previous_root, "passwd",
                            cancellable, error))
     goto out;

src/rpmostree-passwd-util.h

@@ -43,6 +43,6 @@ rpmostree_check_groups (OstreeRepo      *repo,
 gboolean
 rpmostree_generate_passwd_from_previous (OstreeRepo      *repo,
                                          GFile           *yumroot,
-                                         const char      *ref,
+                                         GFile           *previous_root,
                                          GCancellable    *cancellable,
                                          GError         **error);