repo: Don't try to perform labeling if SELinux is disabled

The default for container execution is that `/sys/fs/selinux`
is not mounted, and the libselinux library function `is_selinux_enabled`
should be used to dynamically check if the system should attempt to perform SELinux labeling.

This is how it's done by rpm, ostree, and systemd for example.

But this code unconditionally tries to label if it finds a policy,
which breaks in an obscure corner case
when executed inside a container that includes policy files (e.g.
fedora/rhel-bootc) but when we're not using overlayfs for the backend
(with BUILDAH_BACKEND=vfs).

libdnf/repo/Repo.cpp

@@ -676,34 +676,36 @@ static int create_temporary_directory(char *name_template) {
     int old_default_context_was_retrieved= 0;
     struct selabel_handle *labeling_handle = NULL;
 
-    /* A purpose of this piece of code is to deal with applications whose
-     * security policy overrides a file context for temporary files but don't
-     * know that libdnf executes GnuPG which expects a default file context. */
-    if (0 == getfscreatecon(&old_default_context)) {
-        old_default_context_was_retrieved = 1;
-    } else {
-        logger->debug(tfm::format("Failed to retrieve a default SELinux context"));
-    }
+    if (is_selinux_enabled()) {
+        /* A purpose of this piece of code is to deal with applications whose
+         * security policy overrides a file context for temporary files but don't
+         * know that libdnf executes GnuPG which expects a default file context. */
+        if (0 == getfscreatecon(&old_default_context)) {
+            old_default_context_was_retrieved = 1;
+        } else {
+            logger->debug(tfm::format("Failed to retrieve a default SELinux context"));
+        }
 
-    labeling_handle = selabel_open(SELABEL_CTX_FILE, NULL, 0);
-    if (NULL == labeling_handle) {
-        logger->debug(tfm::format("Failed to open a SELinux labeling handle: %s",
-                    strerror(errno)));
-    } else {
-        if (selabel_lookup(labeling_handle, &new_default_context, name_template, 0700)) {
-            /* Here we could hard-code "system_u:object_r:user_tmp_t:s0", but
-             * that value should be really defined in default file context
-             * SELinux policy. Only log that the policy is incpomplete. */
-            logger->debug(tfm::format("Failed to look up a default SELinux label for \"%s\"",
-                        name_template));
+        labeling_handle = selabel_open(SELABEL_CTX_FILE, NULL, 0);
+        if (NULL == labeling_handle) {
+            logger->debug(tfm::format("Failed to open a SELinux labeling handle: %s",
+                        strerror(errno)));
         } else {
-            if (setfscreatecon(new_default_context)) {
-                logger->debug(tfm::format("Failed to set default SELinux context to \"%s\"",
-                            new_default_context));
+            if (selabel_lookup(labeling_handle, &new_default_context, name_template, 0700)) {
+                /* Here we could hard-code "system_u:object_r:user_tmp_t:s0", but
+                 * that value should be really defined in default file context
+                 * SELinux policy. Only log that the policy is incpomplete. */
+                logger->debug(tfm::format("Failed to look up a default SELinux label for \"%s\"",
+                            name_template));
+            } else {
+                if (setfscreatecon(new_default_context)) {
+                    logger->debug(tfm::format("Failed to set default SELinux context to \"%s\"",
+                                new_default_context));
+                }
+                freecon(new_default_context);
             }
-            freecon(new_default_context);
+            selabel_close(labeling_handle);
         }
-        selabel_close(labeling_handle);
     }
 #endif