lib/repo: Add ostree_repo_remote_get_gpg_keys()

This function enumerates the trusted GPG keys for a remote and returns
an array of `GVariant`s describing them. This is useful to see which
keys are collected by ostree for a particular remote. The same
information can be gathered with `gpg`. However, since ostree allows
multiple keyring locations, that's only really useful if you have
knowledge of how ostree collects GPG keyrings.

The format of the variants is documented in
`OSTREE_GPG_KEY_GVARIANT_FORMAT`. This format is primarily a copy of
selected fields within `gpgme_key_t` and its subtypes. The fields are
placed within vardicts rather than using a more efficient tuple of
concrete types. This will allow flexibility if more components of
`gpgme_key_t` are desired in the future.

src/libostree/ostree-gpg-verifier.c

@@ -185,6 +185,91 @@ _ostree_gpg_verifier_import_keys (OstreeGpgVerifier  *self,
   return ret;
 }
 
+gboolean
+_ostree_gpg_verifier_list_keys (OstreeGpgVerifier   *self,
+                                const char * const  *key_ids,
+                                GPtrArray          **out_keys,
+                                GCancellable        *cancellable,
+                                GError             **error)
+{
+  GLNX_AUTO_PREFIX_ERROR("GPG", error);
+  g_auto(gpgme_ctx_t) context = NULL;
+  g_autoptr(GOutputStream) pubring_stream = NULL;
+  g_autofree char *tmp_dir = NULL;
+  g_autoptr(GPtrArray) keys = NULL;
+  gpgme_error_t gpg_error = 0;
+  gboolean ret = FALSE;
+
+  if (g_cancellable_set_error_if_cancelled (cancellable, error))
+    goto out;
+
+  context = ot_gpgme_new_ctx (NULL, error);
+  if (context == NULL)
+    goto out;
+
+  if (!ot_gpgme_ctx_tmp_home_dir (context, &tmp_dir, &pubring_stream,
+                                  cancellable, error))
+    goto out;
+
+  if (!_ostree_gpg_verifier_import_keys (self, context, pubring_stream,
+                                         cancellable, error))
+    goto out;
+
+  keys = g_ptr_array_new_with_free_func ((GDestroyNotify) gpgme_key_unref);
+  if (key_ids != NULL)
+    {
+      for (guint i = 0; key_ids[i] != NULL; i++)
+        {
+          gpgme_key_t key = NULL;
+
+          gpg_error = gpgme_get_key (context, key_ids[i], &key, 0);
+          if (gpg_error != GPG_ERR_NO_ERROR)
+            {
+              ot_gpgme_throw (gpg_error, error, "Unable to find key \"%s\"",
+                              key_ids[i]);
+              goto out;
+            }
+
+          /* Transfer ownership. */
+          g_ptr_array_add (keys, key);
+        }
+    }
+  else
+    {
+      gpg_error = gpgme_op_keylist_start (context, NULL, 0);
+      while (gpg_error == GPG_ERR_NO_ERROR)
+        {
+          gpgme_key_t key = NULL;
+
+          gpg_error = gpgme_op_keylist_next (context, &key);
+          if (gpg_error != GPG_ERR_NO_ERROR)
+            break;
+
+          /* Transfer ownership. */
+          g_ptr_array_add (keys, key);
+        }
+
+      if (gpgme_err_code (gpg_error) != GPG_ERR_EOF)
+        {
+          ot_gpgme_throw (gpg_error, error, "Unable to list keys");
+          goto out;
+        }
+    }
+
+  if (out_keys != NULL)
+    *out_keys = g_steal_pointer (&keys);
+
+  ret = TRUE;
+
+ out:
+  if (tmp_dir != NULL) {
+    ot_gpgme_kill_agent (tmp_dir);
+    (void) glnx_shutil_rm_rf_at (AT_FDCWD, tmp_dir, NULL, NULL);
+  }
+
+  return ret;
+}
+
 OstreeGpgVerifyResult *
 _ostree_gpg_verifier_check_signature (OstreeGpgVerifier  *self,
                                       GBytes             *signed_data,