bin/compose: Move /etc/{passwd,group} injection later

There's no reason to touch things in the target root *before*
we've even downloaded the repodata.  Further, doing so has
the potential trigger the "existing root" logic in libdnf if
it detects `/usr`.  This doesn't matter now since we're writing
to `/etc/`, but it will for later work, so let's put these bits
in the right place.

Prep for <https://github.com/projectatomic/rpm-ostree/pull/927>

Closes: #931
Approved by: jlebon

src/app/rpmostree-compose-builtin-tree.c

@@ -96,6 +96,7 @@ typedef struct {
 
   RpmOstreeContext *corectx;
   GFile *treefile;
+  GFile *previous_root;
   GFile *workdir;
   gboolean workdir_is_tmp;
   int workdir_dfd;
@@ -113,6 +114,7 @@ rpm_ostree_tree_compose_context_free (RpmOstreeTreeComposeContext *ctx)
   g_clear_pointer (&ctx->treefile_context_dirs, (GDestroyNotify)g_ptr_array_unref);
   g_clear_object (&ctx->corectx);
   g_clear_object (&ctx->treefile);
+  g_clear_object (&ctx->previous_root);
   if (ctx->workdir_is_tmp)
     (void) glnx_shutil_rm_rf_at (AT_FDCWD, gs_file_get_path_cached (ctx->workdir), NULL, NULL);
   g_clear_object (&ctx->workdir);
@@ -459,6 +461,28 @@ install_packages_in_root (RpmOstreeTreeComposeContext  *self,
   if (!rpmostree_context_download (self->corectx, cancellable, error))
     return FALSE;
 
+  /* Before we install packages, inject /etc/{passwd,group} if configured. */
+  g_autoptr(GFile) treefile_dirpath = g_file_get_parent (self->treefile);
+  gboolean generate_from_previous = TRUE;
+  if (!_rpmostree_jsonutil_object_get_optional_boolean_member (treedata,
+                                                               "preserve-passwd",
+                                                               &generate_from_previous,
+                                                               error))
+    return FALSE;
+
+  if (generate_from_previous)
+    {
+      if (!rpmostree_generate_passwd_from_previous (self->repo, rootfs_dfd,
+                                                    treefile_dirpath,
+                                                    self->previous_root, treedata,
+                                                    cancellable, error))
+        return FALSE;
+    }
+
+  /* Now actually run through librpm to install the packages.  Note this bit
+   * will be replaced in the future with a unified core:
+   * https://github.com/projectatomic/rpm-ostree/issues/729
+   */
   { g_auto(GLnxConsoleRef) console = { 0, };
     g_autoptr(DnfState) hifstate = dnf_state_new ();
 
@@ -750,8 +774,7 @@ impl_compose_tree (const char      *treefile_pathstr,
       return FALSE;
   }
 
-  g_autoptr(GFile) previous_root = NULL;
+  if (!ostree_repo_read_commit (self->repo, self->ref, &self->previous_root, &self->previous_checksum,
-  if (!ostree_repo_read_commit (self->repo, self->ref, &previous_root, &self->previous_checksum,
                                 cancellable, &temp_error))
     {
       if (g_error_matches (temp_error, G_IO_ERROR, G_IO_ERROR_NOT_FOUND))
@@ -846,27 +869,7 @@ impl_compose_tree (const char      *treefile_pathstr,
     self->serialized_treefile = g_bytes_new_take (treefile_buf, len);
   }
 
-  g_autoptr(GFile) treefile_dirpath = g_file_get_parent (self->treefile);
+  /* Download rpm-md repos, packages, do install */
-  if (TRUE)
-    {
-      gboolean generate_from_previous = TRUE;
-
-      if (!_rpmostree_jsonutil_object_get_optional_boolean_member (treefile,
-                                                                   "preserve-passwd",
-                                                                   &generate_from_previous,
-                                                                   error))
-        return FALSE;
-
-      if (generate_from_previous)
-        {
-          if (!rpmostree_generate_passwd_from_previous (self->repo, rootfs_fd,
-                                                        treefile_dirpath,
-                                                        previous_root, treefile,
-                                                        cancellable, error))
-            return FALSE;
-        }
-    }
-
   g_autofree char *new_inputhash = NULL;
   { gboolean unmodified = FALSE;
 
@@ -899,6 +902,7 @@ impl_compose_tree (const char      *treefile_pathstr,
   if (g_strcmp0 (g_getenv ("RPM_OSTREE_BREAK"), "post-yum") == 0)
     return FALSE;
 
+  /* Start postprocessing */
   if (!rpmostree_treefile_postprocessing (rootfs_fd, self->treefile_context_dirs->pdata[0],
                                           self->serialized_treefile, treefile,
                                           next_version, cancellable, error))
@@ -912,6 +916,7 @@ impl_compose_tree (const char      *treefile_pathstr,
   if (!rpmostree_copy_additional_files (yumroot, self->treefile_context_dirs->pdata[0], treefile, cancellable, error))
     return FALSE;
 
+  g_autoptr(GFile) treefile_dirpath = g_file_get_parent (self->treefile);
   if (!rpmostree_check_passwd (self->repo, yumroot, treefile_dirpath, treefile,
                                self->previous_checksum,
                                cancellable, error))