Introduce supervisor2 prep_stop callback

Allows to stop dependencies on supervisor shutdown when it reaches
the max restart intensity

Author: dcorbacho

src/supervisor2.erl

@@ -142,6 +142,8 @@
            [ChildSpec :: child_spec()]}}
     | ignore.
 
+-callback prep_stop() -> ok.
+
 -define(restarting(_Pid_), {restarting,_Pid_}).
 
 %%% ---------------------------------------------------
@@ -629,6 +631,7 @@ handle_info({'EXIT', Pid, Reason}, State) ->
 	{ok, State1} ->
 	    {noreply, State1};
 	{shutdown, State1} ->
+            prep_stop(State1),
 	    {stop, shutdown, State1}
     end;
 
@@ -801,7 +804,8 @@ restart_child(Pid, Reason, State) ->
 try_restart(RestartType, Reason, Child, State) ->
     case handle_restart(RestartType, Reason, Child, State) of
         {ok, NState}       -> {noreply, NState};
-        {shutdown, State2} -> {stop, shutdown, State2}
+        {shutdown, State2} -> prep_stop(State2),
+                              {stop, shutdown, State2}
     end.
 
 do_restart(RestartType, Reason, Child, State) ->
@@ -1504,3 +1508,16 @@ report_progress(Child, SupName) ->
     Progress = [{supervisor, SupName},
 		{started, extract_child(Child)}],
     error_logger:info_report(progress, Progress).
+
+prep_stop(#state{module = Mod}) ->
+    %% Catch any exception - including non-existing prep_stop -
+    %% and continue stopping the supervision tree.
+    %% This is only executed when children are terminating,
+    %% because any other call from a top supervisor or application
+    %% will cause a deadlock stopping applications within prep_stop.
+    try
+        Mod:prep_stop()
+    catch
+        _:_ ->
+            ok
+    end.