Teach sys:get_status/1,2 to call Mod:format_status/2

Restore the ability for gen_server and gen_fsm callback
modules to format their own state for display under the
sys:get_status/1,2 calls.

This ability is extremely useful for new behavior modules
based on gen_server or gen_fsm, so that they can display
their status in a more meaningful way than just dumping
the state record. It is also generally useful for applications
wanting to display their gen_server or gen_fsm callback module
state in something other than the default manner.

Also document the previously undocumented the
gen_server:format_status/2 and gen_fsm:format_status/2 optional
callback functions that, if exported by the callback module, are
invoked when sys:get_status/1,2 are called.

Add unit tests to ensure that format_status/2 functions exported
from a gen_fsm callback module and a gen_server callback module
are called when sys:get_status/1,2 are called.

6 files changed, 117 insertions, 9 deletions

diff --git a/lib/stdlib/doc/src/gen_fsm.xml b/lib/stdlib/doc/src/gen_fsm.xml
index f5d8b9bb48..739cd0bffd 100644
--- a/lib/stdlib/doc/src/gen_fsm.xml
+++ b/lib/stdlib/doc/src/gen_fsm.xml
@@ -729,6 +729,36 @@ gen_fsm:sync_send_all_state_event -----> Module:handle_sync_event/4
           updated internal data.</p>
       </desc>
     </func>
+    <func>
+      <name>Module:format_status(normal, [PDict, StateData]) -> Status</name>
+      <fsummary>Optional function for providing a term describing the
+        current gen_fsm status.</fsummary>
+      <type>
+        <v>PDict = [{Key, Value}]</v>
+        <v>StateData = term()</v>
+        <v>Status = [term()]</v>
+      </type>
+      <desc>
+        <p><em>This callback is optional, so callback modules need not
+          export it. The gen_fsm module provides a default
+          implementation of this function that returns the callback
+          module state data.</em></p>
+        <p>This function is called by a gen_fsm process when one
+          of <seealso marker="sys#get_status/1">sys:get_status/1,2</seealso>
+          is invoked to get the gen_fsm status. A callback module
+          wishing to customise the <c>sys:get_status/1,2</c> return
+          value exports an instance of <c>format_status/2</c> that
+          returns a term describing the current status of the
+          gen_fsm.</p>
+        <p><c>PDict</c> is the current value of the gen_fsm's
+          process dictionary.</p>
+        <p><c>StateData</c> is the internal state data of the
+          gen_fsm.</p>
+        <p>The function should return <c>Status</c>, a list of one or
+          more terms that customise the details of the current state
+          and status of the gen_fsm.</p>
+      </desc>
+    </func>
   </funcs>
 
   <section>
diff --git a/lib/stdlib/doc/src/gen_server.xml b/lib/stdlib/doc/src/gen_server.xml
index 8496802259..30c04d1d52 100644
--- a/lib/stdlib/doc/src/gen_server.xml
+++ b/lib/stdlib/doc/src/gen_server.xml
@@ -598,6 +598,35 @@ gen_server:abcast     -----> Module:handle_cast/2
         <p>The function should return the updated internal state.</p>
       </desc>
     </func>
+    <func>
+      <name>Module:format_status(normal, [PDict, State]) -> Status</name>
+      <fsummary>Optional function for providing a term describing the
+        current gen_server status.</fsummary>
+      <type>
+        <v>PDict = [{Key, Value}]</v>
+        <v>State = term()</v>
+        <v>Status = [term()]</v>
+      </type>
+      <desc>
+        <p><em>This callback is optional, so callback modules need not
+          export it. The gen_server module provides a default
+          implementation of this function that returns the callback
+          module state.</em></p>
+        <p>This function is called by a gen_server process when one
+          of <seealso marker="sys#get_status/1">sys:get_status/1,2</seealso>
+          is invoked to get the gen_server status. A callback module
+          wishing to customise the <c>sys:get_status/1,2</c> return
+          value exports an instance of <c>format_status/2</c> that
+          returns a term describing the current status of the
+          gen_server.</p>
+        <p><c>PDict</c> is the current value of the gen_server's
+          process dictionary.</p>
+        <p><c>State</c> is the internal state of the gen_server.</p>
+        <p>The function should return <c>Status</c>, a list of one or
+          more terms that customise the details of the current state
+          and status of the gen_server.</p>
+      </desc>
+    </func>
   </funcs>
 
   <section>
diff --git a/lib/stdlib/doc/src/sys.xml b/lib/stdlib/doc/src/sys.xml
index a395a8a415..51fd5ee8d4 100644
--- a/lib/stdlib/doc/src/sys.xml
+++ b/lib/stdlib/doc/src/sys.xml
@@ -237,6 +237,17 @@
       </type>
       <desc>
         <p>Gets the status of the process.</p>
+        <p>The value of <c>Misc</c> varies for different types of
+          processes. For example, a <c>gen_server</c> process returns
+          the callback module's state, and a <c>gen_fsm</c> process
+          returns information such as its current state name. Callback
+          modules for <c>gen_server</c> and <c>gen_fsm</c> can also
+          customise the value of <c>Misc</c> by exporting
+          a <c>format_status/2</c> function that contributes
+          module-specific information;
+          see <seealso marker="gen_server#format_status/2">gen_server:format_status/2</seealso>
+          and <seealso marker="gen_fsm#format_status/2">gen_fsm:format_status/2</seealso>
+          for more details.</p>
       </desc>
     </func>
     <func>
diff --git a/lib/stdlib/src/sys.erl b/lib/stdlib/src/sys.erl
index e0f2dbcd3c..12209c16d7 100644
--- a/lib/stdlib/src/sys.erl
+++ b/lib/stdlib/src/sys.erl
@@ -245,8 +245,17 @@ do_cmd(SysState, Other, _Parent, _Mod, Debug, Misc) ->
     {SysState, {error, {unknown_system_msg, Other}}, Debug, Misc}.
 
 get_status(SysState, Parent, Mod, Debug, Misc) ->
+    PDict = get(),
+    FmtMisc =
+        case erlang:function_exported(Mod, format_status, 2) of
+            true ->
+                FmtArgs = [PDict, SysState, Parent, Debug, Misc],
+                Mod:format_status(normal, FmtArgs);
+            _ ->
+                Misc
+        end,
     {status, self(), {module, Mod},
-     [get(), SysState, Parent, Debug, Misc]}.
+     [PDict, SysState, Parent, Debug, FmtMisc]}.
 
 %%-----------------------------------------------------------------
 %% These are the system debug commands.
diff --git a/lib/stdlib/test/gen_fsm_SUITE.erl b/lib/stdlib/test/gen_fsm_SUITE.erl
index 62f8b2f9dd..23c1d9a193 100644
--- a/lib/stdlib/test/gen_fsm_SUITE.erl
+++ b/lib/stdlib/test/gen_fsm_SUITE.erl
@@ -30,7 +30,7 @@
 
 -export([shutdown/1]).
 
--export([sys/1, sys1/1]).
+-export([sys/1, sys1/1, call_format_status/1]).
 
 -export([hibernate/1,hiber_idle/3,hiber_wakeup/3,hiber_idle/2,hiber_wakeup/2]).
 
@@ -42,7 +42,7 @@
 
 % The gen_fsm behaviour
 -export([init/1, handle_event/3, handle_sync_event/4, terminate/3,
-	 handle_info/3]).
+	 handle_info/3, format_status/2]).
 -export([idle/2,	idle/3,
 	 timeout/2,
 	 wfor_conf/2,	wfor_conf/3,
@@ -305,7 +305,7 @@ shutdown(Config) when is_list(Config) ->
     ok.
 
 
-sys(suite) -> [sys1].
+sys(suite) -> [sys1, call_format_status].
 
 sys1(Config) when is_list(Config) ->
     ?line {ok, Pid} = 
@@ -317,6 +317,13 @@ sys1(Config) when is_list(Config) ->
     ?line sys:resume(Pid),
     ?line stop_it(Pid).
 
+call_format_status(Config) when is_list(Config) ->
+    ?line {ok, Pid} = gen_fsm:start(gen_fsm_SUITE, [], []),
+    ?line Status = sys:get_status(Pid),
+    ?line {status, Pid, _Mod, [_PDict, running, _Parent, _, Data]} = Status,
+    ?line [format_status_called | _] = lists:reverse(Data),
+    ?line stop_it(Pid).
+
 
 %% Hibernation
 hibernate(suite) -> [];
@@ -836,3 +843,6 @@ handle_sync_event(stop_shutdown_reason, _From, _State, Data) ->
     {stop, {shutdown,reason}, {shutdown,reason}, Data};
 handle_sync_event({get, _Pid}, _From, State, Data) ->
     {reply, {state, State, Data}, State, Data}.
+
+format_status(_Opt, [_Pdict, _StateData]) ->
+    [format_status_called].
diff --git a/lib/stdlib/test/gen_server_SUITE.erl b/lib/stdlib/test/gen_server_SUITE.erl
index 86a5a65ba3..6efdce78a1 100644
--- a/lib/stdlib/test/gen_server_SUITE.erl
+++ b/lib/stdlib/test/gen_server_SUITE.erl
@@ -30,7 +30,7 @@
 	 call_remote_n1/1, call_remote_n2/1, call_remote_n3/1, spec_init/1,
 	 spec_init_local_registered_parent/1, 
 	 spec_init_global_registered_parent/1,
-	 otp_5854/1, hibernate/1, otp_7669/1
+	 otp_5854/1, hibernate/1, otp_7669/1, call_format_status/1
 	]).
 
 % spawn export
@@ -42,7 +42,7 @@
 
 % The gen_server behaviour
 -export([init/1, handle_call/3, handle_cast/2,
-	 handle_info/2, terminate/2]).
+	 handle_info/2, terminate/2, format_status/2]).
 
 all(suite) ->
     [start, crash, call, cast, cast_fast, info,
@@ -51,7 +51,7 @@ all(suite) ->
      call_remote_n2, call_remote_n3, spec_init,
      spec_init_local_registered_parent,
      spec_init_global_registered_parent,
-     otp_5854,hibernate,otp_7669].
+     otp_5854, hibernate, otp_7669, call_format_status].
 
 -define(default_timeout, ?t:minutes(1)).
  
@@ -851,7 +851,7 @@ otp_5854(Config) when is_list(Config) ->
     ok.
 
 %% If initialization fails (with ignore or {stop,Reason}),
-%% make sure that the process is not registered when gen_sever:start()
+%% make sure that the process is not registered when gen_server:start()
 %% returns.
 
 otp_7669(Config) when is_list(Config) ->
@@ -887,6 +887,24 @@ do_otp_7669_stop() ->
 					     ?MODULE, stop, []),
     ?line undefined = global:whereis_name(?MODULE).
 
+%% Verify that sys:get_status correctly calls our format_status/2 fun
+%%
+call_format_status(suite) ->
+    [];
+call_format_status(doc) ->
+    ["Test that sys:get_status/1,2 calls format_status/2"];
+call_format_status(Config) when is_list(Config) ->
+    ?line {ok, Pid} = gen_server:start_link({local, call_format_status},
+                                            gen_server_SUITE, [], []),
+    ?line Status1 = sys:get_status(call_format_status),
+    ?line {status, Pid, _Mod, [_PDict, running, _Parent, _, Data1]} = Status1,
+    ?line [format_status_called | _] = lists:reverse(Data1),
+    ?line Status2 = sys:get_status(call_format_status, 5000),
+    ?line {status, Pid, _Mod, [_PDict, running, _Parent, _, Data2]} = Status2,
+    ?line [format_status_called | _] = lists:reverse(Data2),
+    ok.
+
+
 %%--------------------------------------------------------------
 %% Help functions to spec_init_*
 start_link(Init, Options) ->
@@ -1046,4 +1064,5 @@ terminate({From, stopped_info}, _State) ->
 terminate(_Reason, _State) ->
     ok.
 
-
+format_status(_Opt, [_PDict, _State]) ->
+    [format_status_called].