Add syntax in try/catch to retrieve the stacktrace directly

This commit adds a new syntax for retrieving the stacktrace
without calling erlang:get_stacktrace/0. That allow us to
deprecate erlang:get_stacktrace/0 and ultimately remove it.

The problem with erlang:get_stacktrace/0 is that it can keep huge
terms in a process for an indefinite time after an exception. The
stacktrace can be huge after a 'function_clause' exception or a failed
call to a BIF or operator, because the arguments for the call will be
included in the stacktrace. For example:

1> catch abs(lists:seq(1, 1000)).
{'EXIT',{badarg,[{erlang,abs,
                         [[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20|...]],
                         []},
                 {erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,674}]},
                 {erl_eval,expr,5,[{file,"erl_eval.erl"},{line,431}]},
                 {shell,exprs,7,[{file,"shell.erl"},{line,687}]},
                 {shell,eval_exprs,7,[{file,"shell.erl"},{line,642}]},
                 {shell,eval_loop,3,[{file,"shell.erl"},{line,627}]}]}}
2> erlang:get_stacktrace().
[{erlang,abs,
         [[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,
           23,24|...]],
         []},
 {erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,674}]},
 {erl_eval,expr,5,[{file,"erl_eval.erl"},{line,431}]},
 {shell,exprs,7,[{file,"shell.erl"},{line,687}]},
 {shell,eval_exprs,7,[{file,"shell.erl"},{line,642}]},
 {shell,eval_loop,3,[{file,"shell.erl"},{line,627}]}]
3>

We can extend the syntax for clauses in try/catch to optionally bind
the stacktrace to a variable.

Here is an example using the current syntax:

    try
      Expr
    catch C:E ->
      Stk = erlang:get_stacktrace(),
      .
      .
      .

In the new syntax, it would look like:

    try
      Expr
    catch
      C:E:Stk ->
       .
       .
       .

Only a variable (not a pattern) is allowed in the stacktrace position,
to discourage matching of the stacktrace. (Matching would also be
expensive, because the raw format of the stacktrace would have to be
converted to the cooked form before matching.)

Note that:

    try
      Expr
    catch E ->
      .
      .
      .

is a shorthand for:

    try
      Expr
    catch throw:E ->
      .
      .
      .

If the stacktrace is to be retrieved for a throw, the 'throw:'
prefix must be explicitly included:

    try
      Expr
    catch throw:E:Stk ->
      .
      .
      .

1 files changed, 36 insertions, 8 deletions

diff --git a/lib/stdlib/src/erl_lint.erl b/lib/stdlib/src/erl_lint.erl
index f58cb35cea..1930c462e8 100644
--- a/lib/stdlib/src/erl_lint.erl
+++ b/lib/stdlib/src/erl_lint.erl
@@ -144,6 +144,7 @@ value_option(Flag, Default, On, OnVal, Off, OffVal, Opts) ->
                    :: dict:dict(ta(), #typeinfo{}),
                exp_types=gb_sets:empty()        %Exported types
                    :: gb_sets:set(ta()),
+               in_try_head=false :: boolean(),  %In a try head.
                catch_scope = none               %Inside/outside try or catch
                    :: catch_scope()
               }).
@@ -312,6 +313,10 @@ format_error({unused_var, V}) ->
     io_lib:format("variable ~w is unused", [V]);
 format_error({variable_in_record_def,V}) ->
     io_lib:format("variable ~w in record definition", [V]);
+format_error({stacktrace_guard,V}) ->
+    io_lib:format("stacktrace variable ~w must not be used in a guard", [V]);
+format_error({stacktrace_bound,V}) ->
+    io_lib:format("stacktrace variable ~w must not be previously bound", [V]);
 %% --- binaries ---
 format_error({undefined_bittype,Type}) ->
     io_lib:format("bit type ~tw undefined", [Type]);
@@ -3218,11 +3223,11 @@ is_module_dialyzer_option(Option) ->
 
 try_clauses(Scs, Ccs, In, Vt, St0) ->
     {Csvt0,St1} = icrt_clauses(Scs, Vt, St0),
-    St2 = St1#lint{catch_scope=try_catch},
+    St2 = St1#lint{catch_scope=try_catch,in_try_head=true},
     {Csvt1,St3} = icrt_clauses(Ccs, Vt, St2),
     Csvt = Csvt0 ++ Csvt1,
     UpdVt = icrt_export(Csvt, Vt, In, St3),
-    {UpdVt,St3}.
+    {UpdVt,St3#lint{in_try_head=false}}.
 
 %% icrt_clauses(Clauses, In, ImportVarTable, State) ->
 %%      {UpdVt,State}.
@@ -3239,12 +3244,29 @@ icrt_clauses(Cs, Vt, St) ->
     mapfoldl(fun (C, St0) -> icrt_clause(C, Vt, St0) end, St, Cs).
 
 icrt_clause({clause,_Line,H,G,B}, Vt0, #lint{catch_scope=Scope}=St0) ->
-    {Hvt,Binvt,St1} = head(H, Vt0, St0),
-    Vt1 = vtupdate(Hvt, Binvt),
-    {Gvt,St2} = guard(G, vtupdate(Vt1, Vt0), St1),
-    Vt2 = vtupdate(Gvt, Vt1),
-    {Bvt,St3} = exprs(B, vtupdate(Vt2, Vt0), St2),
-    {vtupdate(Bvt, Vt2),St3#lint{catch_scope=Scope}}.
+    Vt1 = taint_stack_var(Vt0, H, St0),
+    {Hvt,Binvt,St1} = head(H, Vt1, St0),
+    Vt2 = vtupdate(Hvt, Binvt),
+    Vt3 = taint_stack_var(Vt2, H, St0),
+    {Gvt,St2} = guard(G, vtupdate(Vt3, Vt0), St1#lint{in_try_head=false}),
+    Vt4 = vtupdate(Gvt, Vt2),
+    {Bvt,St3} = exprs(B, vtupdate(Vt4, Vt0), St2),
+    {vtupdate(Bvt, Vt4),St3#lint{catch_scope=Scope}}.
+
+taint_stack_var(Vt, Pat, #lint{in_try_head=true}) ->
+    [{tuple,_,[_,_,{var,_,Stk}]}] = Pat,
+    case Stk of
+        '_' ->
+            Vt;
+        _ ->
+            lists:map(fun({V,{bound,Used,Lines}}) when V =:= Stk ->
+                              {V,{stacktrace,Used,Lines}};
+                         (B) ->
+                              B
+                      end, Vt)
+    end;
+taint_stack_var(Vt, _Pat, #lint{in_try_head=false}) ->
+    Vt.
 
 icrt_export(Vts, Vt, {Tag,Attrs}, St) ->
     {_File,Loc} = loc(Attrs, St),
@@ -3484,6 +3506,9 @@ pat_var(V, Line, Vt, Bvt, St) ->
                     {[{V,{bound,used,Ls}}],[],
                      %% As this is matching, exported vars are risky.
                      add_warning(Line, {exported_var,V,From}, St)};
+                {ok,{stacktrace,_Usage,Ls}} ->
+                    {[{V,{bound,used,Ls}}],[],
+                     add_error(Line, {stacktrace_bound,V}, St)};
                 error when St#lint.recdef_top ->
                     {[],[{V,{bound,unused,[Line]}}],
                      add_error(Line, {variable_in_record_def,V}, St)};
@@ -3541,6 +3566,9 @@ expr_var(V, Line, Vt, St) ->
                 false ->
                     {[{V,{{export,From},used,Ls}}],St}
             end;
+        {ok,{stacktrace,_Usage,Ls}} ->
+            {[{V,{bound,used,Ls}}],
+             add_error(Line, {stacktrace_guard,V}, St)};
         error ->
             {[{V,{bound,used,[Line]}}],
              add_error(Line, {unbound_var,V}, St)}