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, 11 insertions, 0 deletions

diff --git a/lib/stdlib/test/gen_event_SUITE.erl b/lib/stdlib/test/gen_event_SUITE.erl
index 5819ef3890..60a1ba8c60 100644
--- a/lib/stdlib/test/gen_event_SUITE.erl
+++ b/lib/stdlib/test/gen_event_SUITE.erl
@@ -974,6 +974,10 @@ get_state(Config) when is_list(Config) ->
     [{dummy1_h,false,State1},{dummy1_h,id,State2}] = lists:sort(Result1),
     Result2 = sys:get_state(Pid, 5000),
     [{dummy1_h,false,State1},{dummy1_h,id,State2}] = lists:sort(Result2),
+    ok = sys:suspend(Pid),
+    Result3 = sys:get_state(Pid),
+    [{dummy1_h,false,State1},{dummy1_h,id,State2}] = lists:sort(Result3),
+    ok = sys:resume(Pid),
     ok = gen_event:stop(Pid),
     ok.
 
@@ -998,4 +1002,11 @@ replace_state(Config) when is_list(Config) ->
     Replace3 = fun(_) -> exit(fail) end,
     [{dummy1_h,false,NState2}] = sys:replace_state(Pid, Replace3),
     [{dummy1_h,false,NState2}] = sys:get_state(Pid),
+    %% verify state replaced if process sys suspended
+    NState3 = "replaced again and again",
+    Replace4 = fun({dummy1_h,false,_}=S) -> setelement(3,S,NState3) end,
+    ok = sys:suspend(Pid),
+    [{dummy1_h,false,NState3}] = sys:replace_state(Pid, Replace4),
+    ok = sys:resume(Pid),
+    [{dummy1_h,false,NState3}] = sys:get_state(Pid),
     ok.