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, 3 insertions, 0 deletions

diff --git a/gv.c b/gv.c
index 72cc9f6c45..a003adb67e 100644
--- a/gv.c
+++ b/gv.c
@@ -1469,6 +1469,9 @@ Perl_gv_fetchpvn_flags(pTHX_ const char *nambeg, STRLEN full_len, I32 flags,
 #endif
 	    goto magicalize;
 
+	case '$':		/* $$ */
+	    SvREADONLY_on(GvSVn(gv));
+	    goto magicalize;
 	case '!':		/* $! */
 	    GvMULTI_on(gv);
 	    /* If %! has been used, automatically load Errno.pm. */