sh: Fix job pointer invalidation with trapsasync

Calling dotrap() can do almost anything, including reallocating the
jobtab array. Convert the job pointer to an index before calling
dotrap() and then restore a proper job pointer afterwards.

PR:		290330
Reported by:	bdrewery
Reviewed by:	bdrewery
Differential Revision:	https://reviews.freebsd.org/D53793

bin/sh/jobs.c

@@ -1078,6 +1078,7 @@ waitforjob(struct job *jp, int *signaled)
 #if JOBS
 	int propagate_int = jp->jobctl && jp->foreground;
 #endif
+	int jobindex;
 	int status;
 	int st;
 
@@ -1085,8 +1086,11 @@ waitforjob(struct job *jp, int *signaled)
 	TRACE(("waitforjob(%%%td) called\n", jp - jobtab + 1));
 	while (jp->state == 0)
 		if (dowait(DOWAIT_BLOCK | (Tflag ? DOWAIT_SIG |
-		    DOWAIT_SIG_TRAP : 0), jp) == -1)
+		    DOWAIT_SIG_TRAP : 0), jp) == -1) {
+			jobindex = jp - jobtab;
 			dotrap();
+			jp = jobtab + jobindex;
+		}
 #if JOBS
 	if (jp->jobctl) {
 		if (ttyfd >= 0 && tcsetpgrp(ttyfd, rootpid) < 0)

bin/sh/tests/execution/Makefile

@@ -18,6 +18,7 @@ ${PACKAGE}FILES+=		bg10.0 bg10.0.stdout
 ${PACKAGE}FILES+=		bg11.0
 ${PACKAGE}FILES+=		bg12.0
 ${PACKAGE}FILES+=		bg13.0
+${PACKAGE}FILES+=		bg14.0
 ${PACKAGE}FILES+=		env1.0
 ${PACKAGE}FILES+=		fork1.0
 ${PACKAGE}FILES+=		fork2.0

bin/sh/tests/execution/bg14.0

@@ -0,0 +1,9 @@
+T=`mktemp -d ${TMPDIR:-/tmp}/sh-test.XXXXXXXX`
+trap 'rm -rf "$T"' 0
+cd "$T" || exit 3
+mkfifo fifo1 || exit 3
+set -T
+trap "for i in 1 2 3 4; do sleep 1 & done" USR1
+sleep 1 &
+{ kill -USR1 "$$"; echo .; } >fifo1 &
+(read dummy <fifo1)