app: Call RegisterClient with well-known name, use retry loop

This closes a race condition for having the daemon idle exit.  After
the daemon has released its bus name, the dbus-daemon will no longer
allow messages through that targeted its unique name.

Since the intention of the `RegisterClient` method is to be the "knock on the
door", fix this by directly sending a message to the well-known name.

Second, we need to handle the case where the daemon exits without
replying; @jlebon added a `sleep(10)` invocation after the daemon
mainloop quit but before we `ReleaseName`, and I verified these two
things combine to fix that case.

Closes: #606
Approved by: jlebon

src/app/rpmostree-dbus-helpers.c

@@ -140,9 +140,6 @@ rpmostree_load_sysroot (gchar *sysroot,
 {
   const char *bus_name = NULL;
   glnx_unref_object GDBusConnection *connection = NULL;
-  glnx_unref_object RPMOSTreeSysroot *sysroot_proxy = NULL;
-  g_autoptr(GVariantBuilder) options_builder =
-    g_variant_builder_new (G_VARIANT_TYPE ("a{sv}"));
   _cleanup_peer_ GPid peer_pid = 0;
 
   connection = get_connection_for_path (sysroot, force_peer, &peer_pid,
@@ -153,15 +150,6 @@ rpmostree_load_sysroot (gchar *sysroot,
   if (g_dbus_connection_get_unique_name (connection) != NULL)
     bus_name = BUS_NAME;
 
-  sysroot_proxy = rpmostree_sysroot_proxy_new_sync (connection,
-                                                    G_DBUS_PROXY_FLAGS_NONE,
-                                                    bus_name,
-                                                    "/org/projectatomic/rpmostree1/Sysroot",
-                                                    NULL,
-                                                    error);
-  if (sysroot_proxy == NULL)
-    return FALSE;
-
   /* Try to register if we can; it doesn't matter much now since the daemon doesn't
    * auto-exit, though that might change in the future. But only register if we're active or
    * root; the daemon won't allow it otherwise. */
@@ -178,14 +166,52 @@ rpmostree_load_sysroot (gchar *sysroot,
         should_register = FALSE;
     }
 
-  if (should_register)
-    {
-      if (!rpmostree_sysroot_call_register_client_sync (sysroot_proxy,
-                                                        g_variant_builder_end (options_builder),
-                                                        cancellable, error))
-        return FALSE;
+  /* First, call RegisterClient directly for the well-known name, to
+   * cause bus activation and allow race-free idle exit.
+   * https://github.com/projectatomic/rpm-ostree/pull/606
+   * If we get unlucky and try to talk to the daemon in FLUSHING
+   * state, then it won't reply, and we should try again.
+   */
+  static const char sysroot_objpath[] = "/org/projectatomic/rpmostree1/Sysroot";
+  while (should_register)
+    {
+      g_autoptr(GError) local_error = NULL;
+      g_autoptr(GVariantBuilder) options_builder =
+        g_variant_builder_new (G_VARIANT_TYPE ("a{sv}"));
+      g_autoptr(GVariant) res =
+        g_dbus_connection_call_sync (connection, bus_name, sysroot_objpath,
+                                     "org.projectatomic.rpmostree1.Sysroot",
+                                     "RegisterClient",
+                                     g_variant_new ("(@a{sv})", g_variant_builder_end (options_builder)),
+                                     (GVariantType*)"()",
+                                     G_DBUS_CALL_FLAGS_NONE, -1,
+                                     cancellable, &local_error);
+      if (res)
+        break;  /* Success! */
+
+      if (g_dbus_error_is_remote_error (local_error))
+        {
+          g_autofree char *remote_err = g_dbus_error_get_remote_error (local_error);
+          /* If this is true, we caught the daemon after it was doing an
+           * idle exit, but while it still owned the name. Retry.
+           */
+          if (g_str_equal (remote_err, "org.freedesktop.DBus.Error.NoReply"))
+            continue;
+          /* Otherwise, fall through */
+        }
+
+      /* Something else went wrong */
+      g_propagate_error (error, g_steal_pointer (&local_error));
+      return FALSE;
     }
 
+  glnx_unref_object RPMOSTreeSysroot *sysroot_proxy =
+    rpmostree_sysroot_proxy_new_sync (connection, G_DBUS_PROXY_FLAGS_NONE,
+                                      bus_name, "/org/projectatomic/rpmostree1/Sysroot",
+                                      NULL, error);
+  if (sysroot_proxy == NULL)
+    return FALSE;
+
   *out_sysroot_proxy = g_steal_pointer (&sysroot_proxy);
   *out_peer_pid = peer_pid; peer_pid = 0;
   return TRUE;