daemon: Make actually initiating reboot asynchronous

Alternative to https://github.com/coreos/rpm-ostree/pull/2845
which moved the `reboot` invocation into the client (which I think
still makes sense in some cases).

Previously we were starting the reboot before we're returned
a success reply to the client, so it could see the daemon killed
by `SIGTERM` and hence emit a spurious error.

(Really in this case any 3 of the calling process, the client
 binary or the daemon could be killed in any order, so it's messy
 but this just closes one race)

For cleanliness we reject starting any new transactions after the daemon has
started a reboot.

Closes: https://github.com/coreos/rpm-ostree/issues/1348

src/daemon/rpmostreed-daemon.cxx

@@ -61,6 +61,7 @@ struct _RpmostreedDaemon {
   GHashTable *bus_clients; /* <utf8 busname, struct RpmOstreeClient> */
 
   gboolean running;
+  gboolean rebooting;
   GDBusProxy *bus_proxy;
   GSource *idle_exit_source;
   guint rerender_status_id;
@@ -774,6 +775,47 @@ rpmostreed_daemon_exit_now (RpmostreedDaemon *self)
   self->running = FALSE;
 }
 
+static gboolean
+idle_initiate_reboot (void *_unused)
+{
+  sd_journal_print (LOG_INFO, "Initiating reboot requested from transaction");
+
+  /* Note that we synchronously spawn this command, but the command just queues the request and returns.
+   */
+  const char *child_argv[] = { "systemctl", "reboot", NULL };
+  g_autoptr(GError) local_error = NULL;
+  if (!g_spawn_sync (NULL, (char**)child_argv, NULL, (GSpawnFlags)(G_SPAWN_CHILD_INHERITS_STDIN | G_SPAWN_SEARCH_PATH),
+                     NULL, NULL, NULL, NULL, NULL, &local_error))
+    {
+      sd_journal_print (LOG_WARNING, "Failed to initate reboot: %s", local_error->message);
+      // And now...not a lot of great choices.  We could loop and retry, but exiting will surface the error
+      // in an obvious way.
+      exit (1);
+    }
+  
+  return FALSE;
+}
+
+void
+rpmostreed_daemon_reboot (RpmostreedDaemon *self)
+{
+  g_assert (!self->rebooting);
+  self->rebooting = TRUE;
+  /* Queue actually starting the reboot until we return to the client, so
+   * that they get a success message for the transaction.  Otherwise
+   * if the daemon gets killed via SIGTERM they just see the bus connection
+   * broken and may spuriously error out.
+   */
+  g_idle_add_full (G_PRIORITY_LOW, idle_initiate_reboot, NULL, NULL);
+}
+
+gboolean
+rpmostreed_daemon_is_rebooting (RpmostreedDaemon *self)
+{
+  return self->rebooting;
+}
+
+
 void
 rpmostreed_daemon_run_until_idle_exit (RpmostreedDaemon *self)
 {

src/daemon/rpmostreed-daemon.h

@@ -53,6 +53,8 @@ char *             rpmostreed_daemon_client_get_agent_id (RpmostreedDaemon *self
 char *             rpmostreed_daemon_client_get_sd_unit (RpmostreedDaemon *self,
                                                          const char *client);
 void               rpmostreed_daemon_exit_now       (RpmostreedDaemon *self);
+void               rpmostreed_daemon_reboot         (RpmostreedDaemon *self);
+gboolean           rpmostreed_daemon_is_rebooting   (RpmostreedDaemon *self);
 void               rpmostreed_daemon_run_until_idle_exit (RpmostreedDaemon *self);
 void               rpmostreed_daemon_publish        (RpmostreedDaemon *self,
                                                      const gchar *path,

src/daemon/rpmostreed-sysroot.cxx

@@ -818,6 +818,8 @@ rpmostreed_sysroot_prep_for_txn (RpmostreedSysroot     *self,
                                  RpmostreedTransaction **out_compat_txn,
                                  GError               **error)
 {
+  if (rpmostreed_daemon_is_rebooting (rpmostreed_daemon_get()))
+    return glnx_throw (error, "Reboot initiated, cannot start new transaction");
   if (self->transaction)
     {
       if (rpmostreed_transaction_is_compatible (self->transaction, invocation))

src/daemon/rpmostreed-transaction-types.cxx

@@ -546,7 +546,7 @@ rollback_transaction_execute (RpmostreedTransaction *transaction,
     }
 
   if (self->reboot)
-    rpmostreed_reboot (cancellable, error);
+    rpmostreed_daemon_reboot (rpmostreed_daemon_get ());
 
   return TRUE;
 }
@@ -1526,7 +1526,7 @@ deploy_transaction_execute (RpmostreedTransaction *transaction,
         }
 
       if (deploy_has_bool_option (self, "reboot"))
-        rpmostreed_reboot (cancellable, error);
+        rpmostreed_daemon_reboot (rpmostreed_daemon_get ());
     }
   else
     {
@@ -1928,7 +1928,7 @@ initramfs_etc_transaction_execute (RpmostreedTransaction *transaction,
     return FALSE;
 
   if (vardict_lookup_bool (self->options, "reboot", FALSE))
-    rpmostreed_reboot (cancellable, error);
+    rpmostreed_daemon_reboot (rpmostreed_daemon_get ());
 
   return TRUE;
 }
@@ -2066,7 +2066,7 @@ initramfs_state_transaction_execute (RpmostreedTransaction *transaction,
     return FALSE;
 
   if (vardict_lookup_bool (self->options, "reboot", FALSE))
-    rpmostreed_reboot (cancellable, error);
+    rpmostreed_daemon_reboot (rpmostreed_daemon_get ());
 
   return TRUE;
 }
@@ -2625,7 +2625,7 @@ finalize_deployment_transaction_execute (RpmostreedTransaction *transaction,
   (void) rpmostree_syscore_bump_mtime (sysroot, NULL);
 
   sd_journal_print (LOG_INFO, "Finalized deployment; rebooting into %s", checksum);
-  rpmostreed_reboot (cancellable, error);
+  rpmostreed_daemon_reboot (rpmostreed_daemon_get ());
   return TRUE;
 }
 
@@ -2802,7 +2802,7 @@ kernel_arg_transaction_execute (RpmostreedTransaction *transaction,
     return FALSE;
 
   if (vardict_lookup_bool (self->options, "reboot", FALSE))
-    rpmostreed_reboot (cancellable, error);
+    rpmostreed_daemon_reboot (rpmostreed_daemon_get ());
 
   return TRUE;
 }

src/daemon/rpmostreed-utils.cxx

@@ -211,14 +211,6 @@ rpmostreed_refspec_parse_partial (const gchar *new_provided_refspec,
   return TRUE;
 }
 
-void
-rpmostreed_reboot (GCancellable *cancellable, GError **error)
-{
-  const char *child_argv[] = { "systemctl", "reboot", NULL };
-  (void) g_spawn_sync (NULL, (char**)child_argv, NULL, (GSpawnFlags)(G_SPAWN_CHILD_INHERITS_STDIN | G_SPAWN_SEARCH_PATH),
-                       NULL, NULL, NULL, NULL, NULL, NULL);
-}
-
 /**
  * rpmostreed_repo_pull_ancestry:
  * @repo: Repo

src/daemon/rpmostreed-utils.h

@@ -36,8 +36,6 @@ gboolean   rpmostreed_refspec_parse_partial (const gchar *new_provided_refspec,
                                              const gchar *base_refspec,
                                              gchar **out_refspec,
                                              GError **error);
-void
-rpmostreed_reboot (GCancellable *cancellable, GError **error);
 
 /* XXX These pull-ancestry and lookup-version functions should eventually
  *     be integrated into libostree, but it's still a bit premature to do