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, 19 insertions, 1 deletions

diff --git a/lib/stdlib/test/gen_fsm_SUITE.erl b/lib/stdlib/test/gen_fsm_SUITE.erl
index fd15838b7d..8aeec07ae8 100644
--- a/lib/stdlib/test/gen_fsm_SUITE.erl
+++ b/lib/stdlib/test/gen_fsm_SUITE.erl
@@ -426,6 +426,14 @@ get_state(Config) when is_list(Config) ->
     {idle, State} = sys:get_state(gfsm),
     {idle, State} = sys:get_state(gfsm, 5000),
     stop_it(Pid2),
+
+    %% check that get_state works when pid is sys suspended
+    {ok, Pid3} = gen_fsm:start(gen_fsm_SUITE, {state_data, State}, []),
+    {idle, State} = sys:get_state(Pid3),
+    ok = sys:suspend(Pid3),
+    {idle, State} = sys:get_state(Pid3, 5000),
+    ok = sys:resume(Pid3),
+    stop_it(Pid3),
     ok.
 
 replace_state(Config) when is_list(Config) ->
@@ -442,8 +450,18 @@ replace_state(Config) when is_list(Config) ->
     {state0, NState2} = sys:get_state(Pid),
     %% verify no change in state if replace function crashes
     Replace3 = fun(_) -> error(fail) end,
-    {state0, NState2} = sys:replace_state(Pid, Replace3),
+    {'EXIT',{{callback_failed,
+	      {gen_fsm,system_replace_state},{error,fail}},_}} =
+	(catch sys:replace_state(Pid, Replace3)),
     {state0, NState2} = sys:get_state(Pid),
+    %% verify state replaced if process sys suspended
+    ok = sys:suspend(Pid),
+    Suffix2 = " and again",
+    NState3 = NState2 ++ Suffix2,
+    Replace4 = fun({StateName, _}) -> {StateName, NState3} end,
+    {state0, NState3} = sys:replace_state(Pid, Replace4),
+    ok = sys:resume(Pid),
+    {state0, NState3} = sys:get_state(Pid, 5000),
     stop_it(Pid),
     ok.