read builtin: Empty variables on timeout

This matches how a non-timeout error is handled.

Reviewed by:	jilles
MFC after:	2 weeks
Differential Revision: https://reviews.freebsd.org/D31876

bin/sh/miscbltin.c

@@ -231,6 +231,8 @@ readcmd(int argc __unused, char **argv __unused)
 		 * If there's nothing ready, return an error.
 		 */
 		if (status <= 0) {
+			while (*ap != NULL)
+				setvar(*ap++, "", 0);
 			sig = pendingsig;
 			return (128 + (sig != 0 ? sig : SIGALRM));
 		}

bin/sh/tests/builtins/Makefile

@@ -141,6 +141,8 @@ ${PACKAGE}FILES+=		read6.0
 ${PACKAGE}FILES+=		read7.0
 ${PACKAGE}FILES+=		read8.0
 ${PACKAGE}FILES+=		read9.0
+${PACKAGE}FILES+=		read10.0
+${PACKAGE}FILES+=		read11.0
 ${PACKAGE}FILES+=		return1.0
 ${PACKAGE}FILES+=		return2.1
 ${PACKAGE}FILES+=		return3.1

bin/sh/tests/builtins/read10.0

+
+set -e
+
+v=original_value
+r=0
+read v < /dev/null || r=$?
+[ "$r" -eq 1 ]
+[ -z "$v" ]

bin/sh/tests/builtins/read11.0

+
+set -e
+
+T=$(mktemp -d ${TMPDIR:-/tmp}/sh-test.XXXXXX)
+trap 'rm -rf "$T"' 0
+cd $T
+mkfifo fifo1
+# Open fifo1 for writing and then read block on a dummy fifo
+{ mkfifo fifo2; read dummy <fifo2; } >fifo1 &
+# Wait for the child to open fifo1 for writing
+exec 3<fifo1
+v=original_value
+r=0
+read -t 0 v <&3 || r=$?
+kill -TERM "$!" || :
+{ [ "$r" -gt 128 ] && [ "$(kill -l "$r")" = ALRM ]; } || exit
+[ -z "$v" ]