Fix crash when tying @DB::args

I was looking at diag.t to see what messages I could document.
‘av_reify called on tied array’ looked interesting, so I decided to
see whether I could trigger it.  I got something else:

./perl -Ilib -lwe '
  sub TIEARRAY{bless[]}
  sub CLEAR{}
  sub EXTEND{}
  tie @DB::args, "";
  package DB; sub {() = caller 0;}->(1,2,3);
'
Name "DB::args" used only once: possible typo at -e line 5.
Bus error

How exciting!

What’s happening is that Perl_init_dbargs turns off AvREAL after
clearing a real array.  Then pp_caller does av_extend and merrily
tries to copy into AvARRAY(PL_dbargs).  But AvARRAY has not been allo-
cated, because av_extend called EXTEND instead.

I fixed this by untying the array before turning off AvREAL.  I don’t
know whether that is the best fix.  Alternatives would be to croak
or to do the assignment in pp_caller differently for tied arrays (in
which case tying @DB::args would cause objects to leak unexpectedly,
until the next caller() call in the DB package).

1 files changed, 2 insertions, 0 deletions

diff --git a/perl.c b/perl.c
index 6c389cae60..b064da691b 100644
--- a/perl.c
+++ b/perl.c
@@ -3890,6 +3890,8 @@ Perl_init_dbargs(pTHX)
 	   "leak" until global destruction.  */
 	av_clear(args);
     }
+    if (SvTIED_mg((const SV *)args, PERL_MAGIC_tied))
+	sv_unmagic((const SV *)args, PERL_MAGIC_tied);
     AvREIFY_only(PL_dbargs);
 }