From ef2e55c707a6c3d1f54f2acb9c8bfbe3fb7a8659 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 14 Nov 2017 09:05:31 +0100
Subject: 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 ->
      .
      .
      .
---
 lib/compiler/src/v3_core.erl | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

(limited to 'lib/compiler/src/v3_core.erl')

diff --git a/lib/compiler/src/v3_core.erl b/lib/compiler/src/v3_core.erl
index 20cb3343fb..cc8ea475d2 100644
--- a/lib/compiler/src/v3_core.erl
+++ b/lib/compiler/src/v3_core.erl
@@ -920,8 +920,9 @@ try_exception(Ecs0, St0) ->
     %% Note that Tag is not needed for rethrow - it is already in Info.
     {Evs,St1} = new_vars(3, St0), % Tag, Value, Info
     {Ecs1,Ceps,St2} = clauses(Ecs0, St1),
+    Ecs2 = try_build_stacktrace(Ecs1, hd(Evs)),
     [_,Value,Info] = Evs,
-    LA = case Ecs1 of
+    LA = case Ecs2 of
 	     [] -> [];
 	     [C|_] -> get_lineno_anno(C)
 	 end,
@@ -930,7 +931,7 @@ try_exception(Ecs0, St0) ->
 		  body=[#iprimop{anno=#a{},       %Must have an #a{}
 				 name=#c_literal{val=raise},
 				 args=[Info,Value]}]},
-    Hs = [#icase{anno=#a{anno=LA},args=[c_tuple(Evs)],clauses=Ecs1,fc=Ec}],
+    Hs = [#icase{anno=#a{anno=LA},args=[c_tuple(Evs)],clauses=Ecs2,fc=Ec}],
     {Evs,Ceps++Hs,St2}.
 
 try_after(As, St0) ->
@@ -946,6 +947,25 @@ try_after(As, St0) ->
     Hs = [#icase{anno=#a{},args=[c_tuple(Evs)],clauses=[],fc=Ec}],
     {Evs,Hs,St1}.
 
+try_build_stacktrace([#iclause{pats=Ps0,body=B0}=C0|Cs], RawStk) ->
+    [#c_tuple{es=[Class,Exc,Stk]}=Tup] = Ps0,
+    case Stk of
+        #c_var{name='_'} ->
+            %% Stacktrace variable is not used. Nothing to do.
+            [C0|try_build_stacktrace(Cs, RawStk)];
+        _ ->
+            %% Add code to build the stacktrace.
+            Ps = [Tup#c_tuple{es=[Class,Exc,RawStk]}],
+            Call = #iprimop{anno=#a{},
+                            name=#c_literal{val=build_stacktrace},
+                            args=[RawStk]},
+            Iset = #iset{var=Stk,arg=Call},
+            B = [Iset|B0],
+            C = C0#iclause{pats=Ps,body=B},
+            [C|try_build_stacktrace(Cs, RawStk)]
+    end;
+try_build_stacktrace([], _) -> [].
+
 %% expr_bin([ArgExpr], St) -> {[Arg],[PreExpr],St}.
 %%  Flatten the arguments of a bin. Do this straight left to right!
 %%  Note that ibinary needs to have its annotation wrapped in a #a{}
-- 
cgit v1.2.3