Don't build stacktrace until erlang:get_stacktrace() is called

Currently, dbg_istk:exception_stacktrace/2 does not do a very good job
imitating BEAM's stacktrace. The reason is that it need to be
relatively fast since a simulated stacktrace is constructed not only
when an exception oocurs, but also before every call to
non-interpreted code.

To prepare for a future more thorough (and slower) stacktrace
construction, refactor the building of the stacktrace so that
it only is done when erlang:get_stacktrace/0 is called.

3 files changed, 57 insertions, 28 deletions

diff --git a/lib/debugger/src/dbg_debugged.erl b/lib/debugger/src/dbg_debugged.erl
index 3732c40c73..76ed49fa93 100644
--- a/lib/debugger/src/dbg_debugged.erl
+++ b/lib/debugger/src/dbg_debugged.erl
@@ -76,8 +76,8 @@ msg_loop(Meta, Mref, SaveStacktrace) ->
 	    msg_loop(Meta, Mref, SaveStacktrace);
 
 	%% Meta needs something evaluated within context of real process
-	{sys, Meta, {command, Command, Stacktrace}} ->
-	    Reply = handle_command(Command, Stacktrace),
+	{sys, Meta, {command,Command}} ->
+	    Reply = handle_command(Command),
 	    Meta ! {sys, self(), Reply},
 	    msg_loop(Meta, Mref, SaveStacktrace);
 
@@ -93,11 +93,12 @@ msg_loop(Meta, Mref, SaveStacktrace) ->
 	    end
     end.
 
-handle_command(Command, Stacktrace) ->
-    try reply(Command)
+handle_command(Command) ->
+    try
+	reply(Command)
     catch Class:Reason ->
-	    Stacktrace2 = stacktrace_f(erlang:get_stacktrace()),
-	    {exception, {Class,Reason,Stacktrace2++Stacktrace}}
+	    Stacktrace = stacktrace_f(erlang:get_stacktrace()),
+	    {exception,{Class,Reason,Stacktrace}}
     end.
 
 reply({apply,M,F,As}) ->
diff --git a/lib/debugger/src/dbg_ieval.erl b/lib/debugger/src/dbg_ieval.erl
index 445eba3346..fe7f9af1a9 100644
--- a/lib/debugger/src/dbg_ieval.erl
+++ b/lib/debugger/src/dbg_ieval.erl
@@ -168,7 +168,7 @@ check_exit_msg(_Msg, _Bs, _Ieval) ->
 %% and then raise the exception.
 %%--------------------------------------------------------------------
 exception(Class, Reason, Bs, Ieval) ->
-    exception(Class, Reason, dbg_istk:exception_stacktrace(complete, Ieval),
+    exception(Class, Reason, dbg_istk:delayed_stacktrace(Ieval),
 	      Bs, Ieval).
 
 exception(Class, Reason, Stacktrace, Bs, #ieval{module=M, line=Line}) ->
@@ -226,8 +226,7 @@ meta(Int, Debugged, M, F, As) ->
 
 debugged_cmd(Cmd, Bs, Ieval) ->
     Debugged = get(self),
-    Stacktrace = dbg_istk:exception_stacktrace(no_current, Ieval),
-    Debugged ! {sys, self(), {command,Cmd,Stacktrace}},
+    Debugged ! {sys, self(), {command,Cmd}},
     meta_loop(Debugged, Bs, Ieval).
 
 meta_loop(Debugged, Bs, #ieval{level=Le} = Ieval) ->
@@ -240,12 +239,17 @@ meta_loop(Debugged, Bs, #ieval{level=Le} = Ieval) ->
 	    {value, Val, Bs};
 	{sys, Debugged, {value,Val,Bs2}} ->
 	    {value, Val, Bs2};
-	{sys, Debugged, {exception,{Class,Reason,Stacktrace}}} ->
+	{sys, Debugged, {exception,{Class,Reason,Stk}}} ->
 	    case get(exit_info) of
 
-		%% Error occured outside interpreted code
+		%% Error occurred outside of interpreted code.
 		undefined ->
-		    exception(Class,Reason,Stacktrace,Bs,Ieval);
+		    MakeStk0 = dbg_istk:delayed_stacktrace(),
+		    MakeStk = fun(Depth0) ->
+				      Depth = max(0, Depth0 - length(Stk)),
+				      Stk ++ MakeStk0(Depth)
+			      end,
+		    exception(Class, Reason, MakeStk, Bs, Ieval);
 
 		%% Error must have occured within a re-entry to
 		%% interpreted code, simply raise the exception
@@ -370,7 +374,7 @@ format_args1([]) ->
 
 %% Mimic catch behaviour
 catch_value(error, Reason) ->
-    {'EXIT',{Reason,get(stacktrace)}};
+    {'EXIT',{Reason,get_stacktrace()}};
 catch_value(exit, Reason) ->
     {'EXIT',Reason};
 catch_value(throw, Reason) ->
@@ -395,7 +399,7 @@ eval_mfa(Debugged, M, F, As, #ieval{level=Le}=Ieval0) ->
 	exit:{Int, Reason} ->
 	    exit(Reason);
 	Class:Reason ->
-	    {exception, {Class, Reason, get(stacktrace)}}
+	    {exception, {Class, Reason, get_stacktrace()}}
     end.
 
 eval_function(Mod, Name, As, Bs, Called, Ieval0) ->
@@ -782,7 +786,7 @@ expr({dbg,Line,self,[]}, Bs, #ieval{level=Le}) ->
     {value,Self,Bs};
 expr({dbg,Line,get_stacktrace,[]}, Bs, #ieval{level=Le}) ->
     trace(bif, {Le,Line,erlang,get_stacktrace,[]}),
-    Stacktrace = get(stacktrace),
+    Stacktrace = get_stacktrace(),
     trace(return, {Le,Stacktrace}),
     {value,Stacktrace,Bs};
 expr({dbg,Line,throw,As0}, Bs0, #ieval{level=Le}=Ieval0) ->
@@ -1501,3 +1505,19 @@ add_binding(N,Val,[B1|Bs]) ->
     [B1|add_binding(N,Val,Bs)];
 add_binding(N,Val,[]) ->
     [{N,Val}].
+
+%% get_stacktrace() -> Stacktrace
+%%  Return the latest stacktrace for the process.
+get_stacktrace() ->
+    case get(stacktrace) of
+	MakeStk when is_function(MakeStk, 1) ->
+	    %% The stacktrace has not been constructed before.
+	    %% Construct it and remember the result.
+	    Depth = erlang:system_flag(backtrace_depth, 8),
+	    erlang:system_flag(backtrace_depth, Depth),
+	    Stk = MakeStk(Depth),
+	    put(stacktrace, Stk),
+	    Stk;
+	Stk when is_list(Stk) ->
+	    Stk
+    end.
diff --git a/lib/debugger/src/dbg_istk.erl b/lib/debugger/src/dbg_istk.erl
index 2e7b114fab..2c4c2a3518 100644
--- a/lib/debugger/src/dbg_istk.erl
+++ b/lib/debugger/src/dbg_istk.erl
@@ -19,7 +19,7 @@
 -module(dbg_istk).
 -export([init/0,to_external/0,from_external/1,
 	 push/2,pop/0,pop/1,stack_level/0,
-	 exception_stacktrace/2,
+	 delayed_stacktrace/0,delayed_stacktrace/1,
 	 bindings/1,stack_frame/2,backtrace/2,
 	 in_use_p/2]).
 
@@ -106,25 +106,33 @@ stack_level() ->
 stack_level([]) -> 1;
 stack_level([#e{level=Le}|_]) -> Le.
 
-%% exception_stacktrace(HowMuch, #ieval{}) -> Stacktrace
-%%   HowMuch = complete | no_current
-%%   Stacktrace = [{M,F,Args|Arity} | {Fun,Args}]
-%% Convert internal stack format to an imitation of the
-%% regular stacktrace.
+%% delayed_stacktrace() -> CreateStacktraceFun
+%% delayed_stacktrace(#ieval{}) -> CreateStacktraceFun
+%%   CreateStacktraceFun = fun(NumberOfEntries)
+%%
+%% Return a fun that can convert the internal stack format to
+%% an imitation of the regular stacktrace.
 %%
 %% Max three elements, no repeated (recursive) calls to the same
 %% function and convert argument lists to arity for all but the topmost
 %% entry (and funs).
 
-exception_stacktrace(complete, #ieval{}=Ieval) ->
+delayed_stacktrace() ->
+    Stack = get(?STACK),
+    do_delayed_stacktrace(Stack).
+
+delayed_stacktrace(Ieval) ->
     #ieval{module=Mod,function=Name,arguments=As} = Ieval,
-    Stk = [#e{mfa={Mod,Name,As}}|get(?STACK)],
-    fix_stacktrace(Stk);
-exception_stacktrace(no_current, #ieval{}) ->
-    fix_stacktrace(get(?STACK)).
+    Stack = [#e{mfa={Mod,Name,As}}|get(?STACK)],
+    do_delayed_stacktrace(Stack).
+
+do_delayed_stacktrace(Stack) ->
+    fun(_NumEntries) ->
+	    fix_stacktrace(Stack)
+    end.
 
-fix_stacktrace(Stk) ->
-    case fix_stacktrace2(sublist(Stk, 1, 3)) of
+fix_stacktrace(Stack) ->
+    case fix_stacktrace2(sublist(Stack, 1, 3)) of
 	[] ->
 	    [];
 	[H|T] ->