Make $$ writable, but still magical

This commit makes $$ writable again, as it was in 5.6, while preserv-
ing the magical pid-fetching added recently (post-5.14.0) by com-
mit 0e219455.

It does this by following Aristotle Pagaltzis’ brilliant suggestion in
<20110609145148.GD8471@klangraum.plasmasturm.org>; namely, to store
the PID in magic when $$ is written to, so that get-magic can detect
whether a fork() has occurred and reset $$ accordingly.  This makes it
seem as though the fork() code sets $$ itself (which it used to before
0e219455), while even working when C code outside of perl’s control
calls fork().

This restores compatibility with DBIx::Connector and PPerl.

1 files changed, 18 insertions, 1 deletions

diff --git a/mg.c b/mg.c
index 86f1eb6e4d..1bdf5c4cda 100644
--- a/mg.c
+++ b/mg.c
@@ -1080,7 +1080,13 @@ Perl_magic_get(pTHX_ SV *sv, MAGIC *mg)
 	    sv_copypv(sv, PL_ors_sv);
 	break;
     case '$': /* $$ */
-	sv_setiv(sv, (IV)PerlProc_getpid());
+	{
+	    IV const pid = (IV)PerlProc_getpid();
+	    if (isGV(mg->mg_obj) || SvIV(mg->mg_obj) != pid)
+		/* never set manually, or at least not since last fork */
+		sv_setiv(sv, pid);
+	    /* else a value has been assigned manually, so do nothing */
+	}
 	break;
 
     case '!':
@@ -2881,6 +2887,17 @@ Perl_magic_set(pTHX_ SV *sv, MAGIC *mg)
     case ':':
 	PL_chopset = SvPV_force(sv,len);
 	break;
+    case '$': /* $$ */
+	/* Store the pid in mg->mg_obj so we can tell when a fork has
+	   occurred.  mg->mg_obj points to *$ by default, so clear it. */
+	if (isGV(mg->mg_obj)) {
+	    if (mg->mg_flags & MGf_REFCOUNTED) /* probably never true */
+		SvREFCNT_dec(mg->mg_obj);
+	    mg->mg_flags |= MGf_REFCOUNTED;
+	    mg->mg_obj = newSViv((IV)PerlProc_getpid());
+	}
+	else sv_setiv(mg->mg_obj, (IV)PerlProc_getpid());
+	break;
     case '0':
 	LOCK_DOLLARZERO_MUTEX;
 #ifdef HAS_SETPROCTITLE