Modify code_change/4 to return CallbackMode

Also move check of non-atom states in callback mode
state_functions to where the state function is called.
This gives homogenous diagnostics for state functions,
code_change/4 and system_replace_state StateFun.

Irregularities pointed out by James Fish.

3 files changed, 79 insertions, 60 deletions

diff --git a/lib/stdlib/doc/src/gen_statem.xml b/lib/stdlib/doc/src/gen_statem.xml
index c21398d64d..adca3673be 100644
--- a/lib/stdlib/doc/src/gen_statem.xml
+++ b/lib/stdlib/doc/src/gen_statem.xml
@@ -32,8 +32,9 @@
   <modulesummary>Generic State Machine Behaviour</modulesummary>
   <description>
     <p>
-      A behaviour module for implementing a state machine.
-      Two callback modes are supported.  One for a finite state
+      A behaviour module for implementing a state machine. Two
+      <seealso marker="#type-callback_mode"><em>callback modes</em></seealso>
+      are supported.  One for a finite state
       machine like <seealso marker="gen_fsm">gen_fsm</seealso>
       that requires the state to be an atom and use that state as
       the name of the callback function for a particular state,
@@ -89,13 +90,13 @@ erlang:'!'            -----> Module:StateName/3
       The "<em>state function</em>" for a specific
       <seealso marker="#type-state">state</seealso>
       in a <c>gen_statem</c> is the callback function that is called
-      for all events in this state, and is selected depending on
-      <seealso marker="#type-callback_mode">callback_mode</seealso>
+      for all events in this state, and is selected depending on which
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
       that the implementation specifies when the the server starts.
     </p>
     <p>
-      When
-      <seealso marker="#type-callback_mode">callback_mode</seealso>
+      When the
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
       is <c>state_functions</c>, the state has to be an atom and
       is used as the state function name.  See
       <seealso marker="#Module:StateName/3">
@@ -110,7 +111,8 @@ erlang:'!'            -----> Module:StateName/3
       </seealso> makes the state name <c>terminate</c> unusable.
     </p>
     <p>
-      When <seealso marker="#type-callback_mode">callback_mode</seealso>
+      When the
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
       is <c>handle_event_function</c> the state can be any term
       and the state function name is
       <seealso marker="#Module:handle_event/4">
@@ -208,9 +210,7 @@ erlang:'!'            -----> Module:StateName/3
     <p>
       This example shows a simple pushbutton model
       for a toggling pushbutton implemented with
-      <seealso marker="#type-callback_mode">
-	callback_mode()
-      </seealso>
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
       <c>state_functions</c>.
       You can push the button and it replies if it went on or off,
       and you can ask for a count of how many times it has been
@@ -226,6 +226,7 @@ erlang:'!'            -----> Module:StateName/3
 -export([on/3,off/3]).
 
 name() -> pushbutton_statem. % The registered server name
+callback_mode() -> state_functions.
 
 %% API.  This example uses a registered name name()
 %% and does not link to the caller.
@@ -242,12 +243,12 @@ stop() ->
 terminate(_Reason, _State, _Data) ->
     void.
 code_change(_Vsn, State, Data, _Extra) ->
-    {ok,State,Data}.
+    {callback_mode(),State,Data}.
 init([]) ->
     %% Set the callback mode and initial state + data.
     %% Data is used only as a counter.
     State = off, Data = 0,
-    {state_functions,State,Data}.
+    {callback_mode(),State,Data}.
 
 
 %%% State functions
@@ -297,9 +298,7 @@ ok
 
     <p>
       And just to compare styles here is the same example using
-      <seealso marker="#type-callback_mode">
-	<c>callback_mode</c>
-      </seealso>
+      <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
       <c>state_functions</c>, or rather here is code to replace
       from the <c>init/1</c> function of the <c>pushbutton.erl</c>
       example file above:
@@ -453,10 +452,8 @@ handle_event(_, _, State, Data) ->
       <name name="state_name" />
       <desc>
 	<p>
-	  If
-	  <seealso marker="#type-callback_mode">
-	    callback_mode
-	  </seealso>
+	  If the
+	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
 	  is <c>state_functions</c>,
 	  the state has to be of this type.
 	</p>
@@ -499,7 +496,7 @@ handle_event(_, _, State, Data) ->
       <name name="callback_mode" />
       <desc>
 	<p>
-	  The <c>callback_mode</c> is selected when starting the
+	  The <em>callback mode</em> is selected when starting the
 	  <c>gen_statem</c> using the return value from
 	  <seealso marker="#Module:init/1">Module:init/1</seealso>
 	  or when calling
@@ -1277,7 +1274,7 @@ handle_event(_, _, State, Data) ->
           return <c>{CallbackMode,State,Data}</c> or
 	  <c>{CallbackMode,State,Data,Actions}</c>.
 	  <c>CallbackMode</c> selects the
-	  <seealso marker="#type-callback_mode">callback_mode()</seealso>.
+	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>.
 	  of the <c>gen_statem</c>.
 	  <c>State</c> is the initial
 	  <seealso marker="#type-state">state()</seealso>
@@ -1344,8 +1341,8 @@ handle_event(_, _, State, Data) ->
 	  Whenever a <c>gen_statem</c> receives an event from
           <seealso marker="#call/2">call/2</seealso>,
           <seealso marker="#cast/2">cast/2</seealso> or
-	  as a normal process message one of these functions is called.
-	  If <seealso marker="#type-callback_mode">callback_mode</seealso>
+	  as a normal process message one of these functions is called.  If
+	  <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
 	  is <c>state_functions</c> then <c>Module:StateName/3</c> is called,
 	  and if it is <c>handle_event_function</c>
 	  then <c>Module:handle_event/4</c> is called.
@@ -1468,7 +1465,11 @@ handle_event(_, _, State, Data) ->
         <v>&nbsp;&nbsp;Vsn = term()</v>
         <v>OldState = NewState = term()</v>
         <v>Extra = term()</v>
-	<v>Result = {ok,NewState,NewData} | Reason</v>
+	<v>Result = {NewCallbackMode,NewState,NewData} | Reason</v>
+	<v>
+	  NewCallbackMode =
+	  <seealso marker="#type-callback_mode">callback_mode()</seealso>
+	</v>
 	<v>
 	  OldState = NewState =
 	  <seealso marker="#type-state">state()</seealso>
@@ -1484,8 +1485,9 @@ handle_event(_, _, State, Data) ->
 	  This function is called by a <c>gen_statem</c> when it should
           update its internal state during a release upgrade/downgrade,
           that is when the instruction <c>{update,Module,Change,...}</c>
-          where <c>Change={advanced,Extra}</c> is given in
-          the <c>appup</c> file. See
+          where <c>Change={advanced,Extra}</c> is given in the
+	  <seealso marker="sasl:appup"><c>appup</c></seealso>
+	  file. See
           <seealso marker="doc/design_principles:release_handling#instr">
 	    OTP Design Principles
 	  </seealso>
@@ -1499,6 +1501,21 @@ handle_event(_, _, State, Data) ->
           <c>Module</c>. If no such attribute is defined, the version
           is the checksum of the BEAM file.
 	</p>
+	<note>
+	  <p>
+	    If you would dare to change
+	    <seealso marker="#type-callback_mode"><em>callback mode</em></seealso>
+	    during release upgrade/downgrade, the upgrade is no problem
+	    since the new code surely knows what <em>callback mode</em>
+	    it needs, but for a downgrade this function will have to
+	    know from the <c>Extra</c> argument that comes from the
+	    <seealso marker="sasl:appup"><c>appup</c></seealso>
+	    file what <em>callback mode</em> the old code did use.
+	    It may also be possible to figure this out
+	    from the <c>{down,Vsn}</c> argument since <c>Vsn</c>
+	    in effect defines the old callback module version.
+	  </p>
+	</note>
         <p>
 	  <c>OldState</c> and <c>OldData</c> is the internal state
 	  of the <c>gen_statem</c>.
@@ -1510,7 +1527,7 @@ handle_event(_, _, State, Data) ->
         <p>
 	  If successful, the function shall return the updated
           internal state in an
-	  <c>{ok,NewState,NewData}</c> tuple.
+	  <c>{NewCallbackMode,NewState,NewData}</c> tuple.
 	</p>
 	<p>
 	  If the function returns <c>Reason</c>, the ongoing
diff --git a/lib/stdlib/src/gen_statem.erl b/lib/stdlib/src/gen_statem.erl
index b3a149a569..c85c521d8e 100644
--- a/lib/stdlib/src/gen_statem.erl
+++ b/lib/stdlib/src/gen_statem.erl
@@ -219,7 +219,9 @@
 	    OldState :: state(),
 	    OldData :: data(),
 	    Extra :: term()) ->
-    {ok, NewState :: state(), NewData :: data()}.
+    {NewCallbackMode :: callback_mode(),
+     NewState :: state(),
+     NewData :: data()}.
 
 %% Format the callback module state in some sensible that is
 %% often condensed way.  For StatusOption =:= 'normal' the perferred
@@ -491,17 +493,10 @@ enter_loop(Module, Opts, CallbackMode, State, Data, Server_or_Actions) ->
 	Actions :: [action()] | action()) ->
 			no_return().
 enter_loop(Module, Opts, CallbackMode, State, Data, Server, Actions) ->
-    case is_atom(Module) andalso callback_mode(CallbackMode) of
-	true ->
-	    Parent = gen:get_parent(),
-	    enter(
-	      Module, Opts, CallbackMode, State, Data, Server,
-	      Actions, Parent);
-	false ->
-	    error(
-	      badarg,
-	      [Module,Opts,CallbackMode,State,Data,Server,Actions])
-    end.
+    is_atom(Module) orelse error({atom,Module}),
+    callback_mode(CallbackMode) orelse error({callback_mode,CallbackMode}),
+    Parent = gen:get_parent(),
+    enter(Module, Opts, CallbackMode, State, Data, Server, Actions, Parent).
 
 %%---------------------------------------------------------------------------
 %% API helpers
@@ -580,15 +575,13 @@ init_result(Starter, Parent, ServerRef, Module, Result, Opts) ->
     case Result of
 	{CallbackMode,State,Data} ->
 	    case callback_mode(CallbackMode) of
-		true
-		  when CallbackMode =:= state_functions, is_atom(State);
-		       CallbackMode =/= state_functions ->
+		true ->
 		    proc_lib:init_ack(Starter, {ok,self()}),
 		    enter(
 		      Module, Opts, CallbackMode, State, Data,
 		      ServerRef, [], Parent);
 		false ->
-		    Error = {bad_return_value,Result},
+		    Error = {callback_mode,CallbackMode},
 		    proc_lib:init_ack(Starter, {error,Error}),
 		    exit(Error)
 	    end;
@@ -600,7 +593,7 @@ init_result(Starter, Parent, ServerRef, Module, Result, Opts) ->
 		      Module, Opts, CallbackMode, State, Data,
 		      ServerRef, Actions, Parent);
 		false ->
-		    Error = {bad_return_value,Result},
+		    Error = {callback_mode,CallbackMode},
 		    proc_lib:init_ack(Starter, {error,Error}),
 		    exit(Error)
 	    end;
@@ -638,11 +631,12 @@ system_code_change(
 	    Result -> Result
 	end
     of
-	{ok,NewState,NewData} ->
-	    {ok,
-	     S#{
-	       state := NewState,
-	       data := NewData}};
+	{NewCallbackMode,NewState,NewData} ->
+	    callback_mode(NewCallbackMode) orelse
+		error({callback_mode,NewCallbackMode}),
+	    {ok,S#{state := NewState, data := NewData}};
+	{ok,_} = Error ->
+	    error({case_clause,Error});
 	Error ->
 	    Error
     end.
@@ -825,6 +819,18 @@ loop_events(
 	Result ->
 	    loop_event_result(
 	      Parent, Debug, S, Events, Event, Result);
+	error:badarg when CallbackMode =:= state_functions ->
+	    case erlang:get_stacktrace() of
+		[{erlang,apply,[Module,State,_],_}|Stacktrace] ->
+		    Args = [Type,Content,Data],
+		    terminate(
+		      error,
+		      {undef_state_function,{Module,State,Args}},
+		      Stacktrace,
+		      Debug, S, Q);
+		Stacktrace ->
+		    terminate(error, badarg, Stacktrace, Debug, S, Q)
+	    end;
 	error:undef ->
 	    %% Process an undef to check for the simple mistake
 	    %% of calling a nonexistent state function
@@ -861,7 +867,7 @@ loop_events(
 %% Interpret all callback return variants
 loop_event_result(
   Parent, Debug,
-  #{callback_mode := CallbackMode, state := State, data := Data} = S,
+  #{state := State, data := Data} = S,
   Events, Event, Result) ->
     case Result of
 	stop ->
@@ -887,15 +893,11 @@ loop_event_result(
 		  exit, Reason, ?STACKTRACE(), Debug, NewS, Q, Replies),
 	    %% Since we got back here Replies was bad
 	    terminate(Class, NewReason, Stacktrace, NewDebug, NewS, Q);
-	{next_state,NextState,NewData}
-	  when CallbackMode =:= state_functions, is_atom(NextState);
-	       CallbackMode =/= state_functions ->
+	{next_state,NextState,NewData} ->
 	    loop_event_actions(
 	      Parent, Debug, S, Events, Event,
 	      State, NextState, NewData, []);
-	{next_state,NextState,NewData,Actions}
-	  when CallbackMode =:= state_functions, is_atom(NextState);
-	       CallbackMode =/= state_functions ->
+	{next_state,NextState,NewData,Actions} ->
 	    loop_event_actions(
 	      Parent, Debug, S, Events, Event,
 	      State, NextState, NewData, Actions);
diff --git a/lib/stdlib/test/gen_statem_SUITE.erl b/lib/stdlib/test/gen_statem_SUITE.erl
index 27fb7d9650..3deb5fd986 100644
--- a/lib/stdlib/test/gen_statem_SUITE.erl
+++ b/lib/stdlib/test/gen_statem_SUITE.erl
@@ -637,9 +637,9 @@ code_change(Config) ->
     {ok,Pid} = gen_statem:start(?MODULE, start_arg(Config, []), []),
     {idle,data} = sys:get_state(Pid),
     sys:suspend(Pid),
-    sys:change_code(Pid, ?MODULE, old_vsn, extra),
+    sys:change_code(Pid, ?MODULE, old_vsn, state_functions),
     sys:resume(Pid),
-    {idle,{old_vsn,data,extra}} = sys:get_state(Pid),
+    {idle,{old_vsn,data,state_functions}} = sys:get_state(Pid),
     stop_it(Pid).
 
 call_format_status(Config) ->
@@ -1561,8 +1561,8 @@ wrap_result(Result) ->
 
 
 
-code_change(OldVsn, State, Data, Extra) ->
-    {ok,State,{OldVsn,Data,Extra}}.
+code_change(OldVsn, State, Data, CallbackMode) ->
+    {CallbackMode,State,{OldVsn,Data,CallbackMode}}.
 
 format_status(terminate, [_Pdict,State,Data]) ->
     {formatted,State,Data};