fix sys:get_state/1,2 and sys:replace_state/2,3 when sys suspended

Add two new system callbacks Module:system_get_state/1 and
Module:system_replace_state/2 to allow sys:get_state/1,2 and
sys:replace_state/2,3 to operate correctly even if a process is sys
suspended. Modify gen_server, gen_fsm, and gen_event to support the new
callbacks.

If a callback module does not export these functions, then by default the
Misc value (the same as that passed as the final argument to
sys:handle_system_msg/6, and returned as part of the return value of
sys:get_status/1,2) is treated as the callback module's state.

The previous behaviour of intercepting the system message and passing a
tuple of size 2 as the last argument to sys:handle_system_msg/6 is no
longer supported.

Add tests to verify the correctness of sys:get_state/1,2 and
sys:replace_state/2,3 when processes are sys suspended. Add two tests for
modules that implement special processes, one that exports
system_get_state/1 and system_replace_state/2 and one that doesn't.

Much of the credit for this patch goes to James Fish, who reported the
initial problem and implemented much of the fix.

1 files changed, 80 insertions, 4 deletions

diff --git a/lib/stdlib/test/sys_SUITE.erl b/lib/stdlib/test/sys_SUITE.erl
index c06ba545e7..f38bc87ae5 100644
--- a/lib/stdlib/test/sys_SUITE.erl
+++ b/lib/stdlib/test/sys_SUITE.erl
@@ -19,7 +19,7 @@
 -module(sys_SUITE).
 -export([all/0, suite/0,groups/0,init_per_suite/1, end_per_suite/1, 
 	 init_per_group/2,end_per_group/2,log/1,log_to_file/1,
-	 stats/1,trace/1,suspend/1,install/1]).
+	 stats/1,trace/1,suspend/1,install/1,special_process/1]).
 -export([handle_call/3,terminate/2,init/1]).
 -include_lib("test_server/include/test_server.hrl").
 
@@ -27,14 +27,12 @@
 
 
 %% Doesn't look into change_code at all
-%% Doesn't address writing your own process that understands
-%% system messages at all.
 
 
 suite() -> [{ct_hooks,[ts_install_cth]}].
 
 all() -> 
-    [log, log_to_file, stats, trace, suspend, install].
+    [log, log_to_file, stats, trace, suspend, install, special_process].
 
 groups() -> 
     [].
@@ -157,6 +155,84 @@ install(Config) when is_list(Config) ->
     ?line stop(),
     ok.
 
+special_process(suite) -> [];
+special_process(Config) when is_list(Config) ->
+    ok = spec_proc(sys_sp1),
+    ok = spec_proc(sys_sp2).
+
+spec_proc(Mod) ->
+    {ok,_} = Mod:start_link(100),
+    ok = sys:statistics(Mod,true),
+    ok = sys:trace(Mod,true),
+    1 = Ch = Mod:alloc(),
+    Free = lists:seq(2,100),
+    Replace = case sys:get_state(Mod) of
+		  {[Ch],Free} ->
+		      fun({A,F}) ->
+			      Free = F,
+			      {A,[2,3,4]}
+		      end;
+		  {state,[Ch],Free} ->
+		      fun({state,A,F}) ->
+			      Free = F,
+			      {state,A,[2,3,4]}
+		      end
+	      end,
+    case sys:replace_state(Mod, Replace) of
+	{[Ch],[2,3,4]} -> ok;
+	{state,[Ch],[2,3,4]} -> ok
+    end,
+    ok = Mod:free(Ch),
+    case sys:get_state(Mod) of
+	{[],[1,2,3,4]} -> ok;
+	{state,[],[1,2,3,4]} -> ok
+    end,
+    {ok,[{start_time,_},
+	 {current_time,_},
+	 {reductions,_},
+	 {messages_in,2},
+	 {messages_out,1}]} = sys:statistics(Mod,get),
+    ok = sys:statistics(Mod,false),
+    [] = sys:replace_state(Mod, fun(_) -> [] end),
+    process_flag(trap_exit,true),
+    ok = case catch sys:get_state(Mod) of
+	     [] ->
+		 ok;
+	     {'EXIT',{{callback_failed,
+		       {Mod,system_get_state},{throw,fail}},_}} ->
+		 ok
+	 end,
+    Mod:stop(),
+    WaitForUnregister = fun W() ->
+		       case whereis(Mod) of
+			   undefined -> ok;
+			   _ -> timer:sleep(10), W()
+		       end
+	       end,
+    WaitForUnregister(),
+    {ok,_} = Mod:start_link(4),
+    ok = case catch sys:replace_state(Mod, fun(_) -> {} end) of
+	     {} ->
+		 ok;
+	     {'EXIT',{{callback_failed,
+		       {Mod,system_replace_state},{throw,fail}},_}} ->
+		 ok
+	 end,
+    Mod:stop(),
+    WaitForUnregister(),
+    {ok,_} = Mod:start_link(4),
+    StateFun = fun(_) -> error(fail) end,
+    ok = case catch sys:replace_state(Mod, StateFun) of
+	     {} ->
+		 ok;
+	     {'EXIT',{{callback_failed,
+		       {Mod,system_replace_state},{error,fail}},_}} ->
+		 ok;
+	     {'EXIT',{{callback_failed,StateFun,{error,fail}},_}} ->
+		 ok
+	 end,
+    Mod:stop().
+
 %%%%%%%%%%%%%%%%%%%%
 %% Dummy server