sepolicy: Add better private API for setfscreatecon

Use `g_auto()` more sanely with a struct implmenting the "is initialized"
pattern.  This is way less ugly for callers, and fixes bugs like
us calling `setfscreatecon()` even if an error occurred beforehand.

Also fold in the logic for "NULL or not loaded" sepolicy into the setup rather
than requiring callers to inline it.

Prep for more users of this function.

Closes: #746
Approved by: jlebon

src/libostree/ostree-sysroot-deploy.c

@@ -30,6 +30,7 @@
 #endif
 
 #include "ostree-sysroot-private.h"
+#include "ostree-sepolicy-private.h"
 #include "ostree-deployment-private.h"
 #include "ostree-core-private.h"
 #include "ostree-linuxfsutil.h"
@@ -733,21 +734,13 @@ selinux_relabel_var_if_needed (OstreeSysroot                 *sysroot,
           return FALSE;
         }
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunused-variable"
-      { ostree_cleanup_sepolicy_fscreatecon gpointer dummy = NULL;
-#pragma GCC diagnostic pop
+      { g_auto(OstreeSepolicyFsCreatecon) con = { 0, };
+        const char *selabeled_abspath = glnx_strjoina ("/", selabeled);
 
-        if (sysroot->sepolicy != NULL
-            && ostree_sepolicy_get_name (sysroot->sepolicy) != NULL)
-          {
-            const char *selabeled_abspath = glnx_strjoina ("/", selabeled);
-            if (!ostree_sepolicy_setfscreatecon (sysroot->sepolicy,
-                                                 selabeled_abspath,
-                                                 0644,
-                                                 error))
-              return FALSE;
-          }
+        if (!_ostree_sepolicy_preparefscreatecon (&con, sysroot->sepolicy,
+                                                  selabeled_abspath,
+                                                  0644, error))
+          return FALSE;
 
         if (!glnx_file_replace_contents_at (os_deploy_dfd, selabeled, (guint8*)"", 0,
                                             GLNX_FILE_REPLACE_DATASYNC_NEW,
@@ -2112,23 +2105,12 @@ ostree_sysroot_deploy_tree (OstreeSysroot     *self,
         return FALSE;
     }
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunused-variable"
-  { ostree_cleanup_sepolicy_fscreatecon gpointer dummy = NULL;
-#pragma GCC diagnostic pop
+  { g_auto(OstreeSepolicyFsCreatecon) con = { 0, };
 
-    /* Explicitly override the label for the origin file to ensure
-     * it's system_conf_t.
-     */
-    if (self->sepolicy != NULL
-        && ostree_sepolicy_get_name (self->sepolicy) != NULL)
-      {
-        if (!ostree_sepolicy_setfscreatecon (self->sepolicy,
-                                             "/etc/ostree/remotes.d/dummy.conf",
-                                             0644,
-                                             error))
-          return FALSE;
-      }
+    if (!_ostree_sepolicy_preparefscreatecon (&con, self->sepolicy,
+                                              "/etc/ostree/remotes.d/dummy.conf",
+                                              0644, error))
+      return FALSE;
 
     /* Don't fsync here, as we assume that's all done in
      * ostree_sysroot_write_deployments().