kern_fork.c: add kern.pdfork_implicit_nowaitpid sysctl

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D59349

sys/kern/kern_fork.c

@@ -91,6 +91,11 @@ dtrace_fork_func_t	dtrace_fasttrap_fork;
 SDT_PROVIDER_DECLARE(proc);
 SDT_PROBE_DEFINE3(proc, , , create, "struct proc *", "struct proc *", "int");
 
+static bool pdfork_implicit_nowaitpid;
+SYSCTL_BOOL(_kern, OID_AUTO, pdfork_implicit_nowaitpid, CTLFLAG_RWTUN,
+    &pdfork_implicit_nowaitpid, 0,
+    "PD_NOWAITPID is assumed to be always set");
+
 #ifndef _SYS_SYSPROTO_H_
 struct fork_args {
 	int     dummy;
@@ -560,7 +565,8 @@ do_fork(struct thread *td, struct fork_req *fr, struct proc *p2, struct thread *
 	    P2_LOGSIGEXIT_ENABLE);
 	if ((fr->fr_flags & RFPROCDESC) != 0) {
 		p2->p_zombieref = PZOMBIEREF_PROCDESC;
-		if ((fr->fr_pd_flags & PD_NOWAITPID) == 0 &&
+		if (((fr->fr_pd_flags & PD_NOWAITPID) == 0 &&
+		    !pdfork_implicit_nowaitpid) &&
 		    (fr->fr_flags & RFNOWAIT) == 0)
 			p2->p_zombieref |= (PZOMBIEREF_PARENT |
 			    PZOMBIEREF_NEEDPARENT);