Warn for potentially unsafe use of get_stacktrace/0

erlang:get_stacktrace/0 returns the stacktrace for the latest
exception.  The problem is that the stacktrace is kept until the next
exception occurs. If the last exception was a 'function_clause' or a
'badarg', the arguments for the call are also kept forever. The
arguments can be terms of any size (potentially huge).

In a future release, we would like to only allow
erlang:get_stacktrace/0 from within a 'try' expression. That would
make it possible to clear the stacktrace when the 'try' expression is
exited.

The 'catch' expression has no natural end where the stacktrace could
be cleared. The stacktrace could be cleared at the end of the function
that the 'catch' occurs in, but that would cause problems in the
following scenario (from real life, but simplified):

try
  ...
catch _:_ ->
  io:format(...),
  io:format("~p\n", [erlang:get_stacktrace()])
end.

%% In io.erl.
format(Fmt, Args) ->
  Res = case ... of
     SomePattern ->
       catch...
       ...;
     SomeOtherPattern ->
       %% Output the formatted string here
       ...
  end,
  clear_stacktrace(),  %% Inserted by compiler.
  Res.

The call to io:format() would always clear the stacktrace before
it could be retrieved.

That problem could be solved by tightning the scope in which the
stacktrace is kept, but the rules for how long erlang:get_stacktrace/0
would work would become complicated.

Therefore, the solution we suggest for a future major release of
OTP is that erlang:get_stacktrace/0 will return [] if it is called
outside the 'catch' part of a 'try' expression.

To help users prepare, introduce a warning when it is likely that
erlang:get_stacktrace/0 will always return an empty list, for example
in this code:

  catch error(foo),
  Stk = erlang:get_stacktrace()

or in this code:

  try Expr
  catch _:_ -> ok end,
  Stk = erlang:get_stacktrace()

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/compiler/doc/src/compile.xml b/lib/compiler/doc/src/compile.xml
index fed68037c1..10164890f2 100644
--- a/lib/compiler/doc/src/compile.xml
+++ b/lib/compiler/doc/src/compile.xml
@@ -629,6 +629,14 @@ module.beam: module.erl \
             <p>Turns off warnings for unused record types. Default is to 
 	    emit warnings for unused locally defined record types.</p>
           </item>
+
+	  <tag><c>nowarn_get_stacktrace</c></tag>
+          <item>
+            <p>Turns off warnings for using <c>get_stacktrace/0</c> in a context
+	    where it will probably not work in a future release. For example,
+	    by default there will be a warning if <c>get_stacktrace/0</c> is
+	    used following a <c>catch</c> expression.</p>
+          </item>
         </taglist>
 
 	<p>Another class of warnings is generated by the compiler