Turn $$ into a magical readonly variable that always fetches getpid() instead of caching it

The intent is that by not caching $$, we eliminate one opportunity for bugs:
If one embeds Perl or uses XS and calls fork(3) from C, Perls notion of $$
may go out of sync with what getpid() returns. By always fetching the
value of $$ via getpid(), this bug opportunity is eliminated. The overhead
of always fetching $$ should be small and is likely only used for tempfile
creation, which should be dwarfed by file system accesses.

1 files changed, 4 insertions, 0 deletions

diff --git a/mg.c b/mg.c
index 85b546175b..af2c6474bc 100644
--- a/mg.c
+++ b/mg.c
@@ -1114,6 +1114,10 @@ Perl_magic_get(pTHX_ SV *sv, MAGIC *mg)
 	if (PL_ors_sv)
 	    sv_copypv(sv, PL_ors_sv);
 	break;
+    case '$': /* $$ */
+	sv_setiv(sv, (IV)PerlProc_getpid());
+	break;
+
     case '!':
 	{
 	dSAVE_ERRNO;