[perl #71806] perldb does not setup %dbline with the shebang option -d

The first time gv_fetchfile is called for a particular file, it
creates the glob and, if debugging is on, creates an AV. If the glob
already exists (i.e., in subsequent calls), the AV is not created. The
attached patch moves the check for debugging mode and the creation of
the AV outside the if-block that checks whether the glob exists.

This bug seems to have existed for a very long time and has been
intermittent. It seems that many different things can change the order
in which #!perl -d and gv_fetchfile occur. Whether compilation options
affect it I do not know. I can reproduce it in 5.6.2, 5.8.[123456]
(non-threaded) and 5.11.3 (both threaded and non-threaded), but not
5.8.[789] or 5.10.[01] (threaded).

gv.c

@@ -121,9 +121,9 @@ Perl_gv_fetchfile_flags(pTHX_ const char *const name, const STRLEN namelen,
 #else
 	sv_setpvn(GvSV(gv), name, namelen);
 #endif
-	if (PERLDB_LINE || PERLDB_SAVESRC)
-	    hv_magic(GvHVn(gv_AVadd(gv)), NULL, PERL_MAGIC_dbfile);
     }
+    if ((PERLDB_LINE || PERLDB_SAVESRC) && !GvAV(gv))
+	    hv_magic(GvHVn(gv_AVadd(gv)), NULL, PERL_MAGIC_dbfile);
     if (tmpbuf != smallbuf)
 	Safefree(tmpbuf);
     return gv;

pod/perldelta.pod

@@ -387,6 +387,13 @@ Parsing Perl code (either with string C<eval> or by loading modules) from
 within a C<UNITCHECK> block no longer causes the interpreter to crash
 L<[perl #70614]|http://rt.perl.org/rt3//Public/Bug/Display.html?id=70614>.
 
+=item *
+
+When C<-d> is used on the shebang (C<#!>) line, the debugger now has access
+to the lines of the main program. In the past, this sometimes worked and
+sometimes did not, depending on what order things happened to be arranged
+in memory.
+
 =back
 
 =head1 Known Problems

t/run/switchd.t

@@ -9,7 +9,7 @@ BEGIN { require "./test.pl"; }
 
 # This test depends on t/lib/Devel/switchd.pm.
 
-plan(tests => 2);
+plan(tests => 3);
 
 my $r;
 
@@ -44,3 +44,16 @@ __SWDTEST__
     like($r, qr/^sub<Devel::switchd::import>;import<Devel::switchd a 42>;DB<main,$::tempfile_regexp,9>;sub<Foo::foo>;DB<Foo,$::tempfile_regexp,5>;DB<Foo,$::tempfile_regexp,6>;DB<Foo,$::tempfile_regexp,6>;sub<Bar::bar>;DB<Bar,$::tempfile_regexp,2>;sub<Bar::bar>;DB<Bar,$::tempfile_regexp,2>;sub<Bar::bar>;DB<Bar,$::tempfile_regexp,2>;$/);
 }
 
+# [perl #71806]
+cmp_ok(
+  runperl(       # less is useful for something :-)
+   switches => [ '"-Mless ++INC->{q-Devel/_.pm-}"' ],
+   progs    => [
+    '#!perl -d:_',
+    'sub DB::DB{} print scalar @{q/_</.__FILE__}',
+   ],
+  ),
+ '>',
+  0,
+ 'The debugger can see the lines of the main program under #!perl -d',
+);