compose: Write a preset file to enable ostree-remount.service

I've lost count now of how many times people have hit variants
of https://github.com/projectatomic/centos-release-atomic-host-devel/pull/6
Let's just bake it in.

Closes: #1482
Approved by: jlebon

src/libpriv/rpmostree-postprocess.c

@@ -992,6 +992,22 @@ rpmostree_postprocess_final (int            rootfs_dfd,
         return glnx_prefix_error (error, "SELinux postprocess");
     }
 
+  /* We keep hitting issues with the ostree-remount preset not being
+   * enabled; let's just do this rather than trying to propagate the
+   * preset everywhere.
+   */
+  static const char preset_dir[] = "usr/lib/systemd/system-preset";
+  if (!glnx_shutil_mkdir_p_at (rootfs_dfd, preset_dir, 0755, cancellable, error))
+    return FALSE;
+  { g_autofree char *preset_path = g_build_filename (preset_dir, "40-rpm-ostree-auto.preset", NULL);
+    static const char remount_preset[] = "# Written by rpm-ostree compose tree\nenable ostree-remount.service\n";
+    if (!glnx_file_replace_contents_at (rootfs_dfd, preset_path, (guint8*)remount_preset,
+                                        strlen (remount_preset),
+                                        GLNX_FILE_REPLACE_NODATASYNC,
+                                        cancellable, error))
+      return FALSE;
+  }
+
   if (!convert_var_to_tmpfiles_d (rootfs_dfd, cancellable, error))
     return FALSE;
 

tests/compose-tests/test-basic.sh

@@ -30,6 +30,9 @@ assert_file_has_content_literal autovar.txt 'd /var/cache 0755 root root - -'
 assert_file_has_content_literal autovar.txt 'd /var/log/chrony 0755 chrony chrony - -'
 echo "ok autovar"
 
+ostree --repo=${repobuild} cat ${treeref} /usr/lib/systemd/system-preset/40-rpm-ostree-auto.preset > preset.txt
+assert_file_has_content preset.txt '^enable ostree-remount.service$'
+
 if ! rpm-ostree --version | grep -q rust; then
   echo "ok yaml (SKIP)"
 else