init: propagate exit conditions of children

This patch extends the configuration concept of init with an additional
sub node for the <start> node:

<start name="noux">
  <exit propagate="yes"/>
  ...
</start>

If the 'propagate' attribute is set to "yes", the exit of the respective
child will appear to init's parent as the exit of the entire init
subsystem.

Fixes #1686

repos/os/doc/init.txt

@@ -299,6 +299,27 @@ as LOG output. To enable the verbose mode, assign the value "yes" to the
 'verbose' attribute of the '<config>' node.
 
 
+Propagation of exit events
+==========================
+
+A component can notify its parent about its graceful exit via the exit RPC
+function of the parent interface. By default, init responds to such a
+notification from one of its children by merely printing a log message but
+ignores it otherwise. However, there are scenarios where the exit of a
+particular child should result in the exit of the entire init component. To
+propagate the exit of a child to the parent of init, start nodes can host the
+optional sub node '<exit>' with the attribute 'propagate' set to "yes".
+
+! <config>
+!   <start name="noux">
+!     <exit propagate="yes"/>
+!     ...
+!   </start>
+! </config>
+
+The exit value specified by the exiting child is forwarded to init's parent.
+
+
 Executing children in chroot environments on Linux
 ==================================================
 

repos/os/include/init/child.h

@@ -802,6 +802,23 @@ class Init::Child : Genode::Child_policy
 			_child.notify_resource_avail();
 		}
 
+		void exit(int exit_value) override
+		{
+			try {
+				if (_start_node.sub_node("exit").attribute("propagate").has_value("yes")) {
+					Genode::env()->parent()->exit(exit_value);
+					return;
+				}
+			} catch (...) { }
+
+			/*
+			 * Print a message as the exit is not handled otherwise. There are
+			 * a number of automated tests that rely on this message. It is
+			 * printed by the default implementation of 'Child_policy::exit'.
+			 */
+			Child_policy::exit(exit_value);
+		}
+
 		Genode::Native_pd_args const *pd_args() const { return &_pd_args; }
 };