Add compat support for closefrom cribbed from libbsd

We had compat support in older dhcpcd but we want it faster
when linux supports faster, because, you know, fast.

compat/closefrom.c

@@ -0,0 +1,77 @@
+/*
+ * SPDX-License-Identifier: ISC
+ *
+ * Copyright (c) 2004-2005, 2007, 2010, 2012-2015, 2017-2018
+ *	Todd C. Miller <Todd.Miller@sudo.ws>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include "config.h"
+
+#ifdef __linux__
+# include <sys/syscall.h>
+# if defined(__NR_close_range) && !defined(SYS_close_range)
+#  define SYS_close_range __NR_close_range
+# endif
+#endif
+
+#include <fcntl.h>
+#include <unistd.h>
+
+#if defined(__linux__) && defined(SYS_close_range)
+static inline int
+sys_close_range(unsigned int fd, unsigned int max_fd, unsigned int flags)
+{
+
+	return (int)syscall(SYS_close_range, fd, max_fd, flags);
+}
+#endif
+
+/*
+ * Close all file descriptors greater than or equal to lowfd.
+ * This is the expensive (fallback) method.
+ */
+static int
+closefrom_fallback(int lowfd)
+{
+	int fd, maxfd;
+
+#ifdef _SC_OPEN_MAX
+	maxfd = (int)sysconf(_SC_OPEN_MAX);
+#else
+	maxfd = getdtablesize();
+#endif
+	if (maxfd == -1)
+		return -1;
+
+	for (fd = lowfd; fd < maxfd; fd++)
+	       close(fd);
+	return 0;
+}
+
+/*
+ *  * Close all file descriptors greater than or equal to lowfd.
+ *   * We try the fast way first, falling back on the slow method.
+ *    */
+void
+closefrom(int lowfd)
+{
+
+#if defined(__linux__) && defined(SYS_close_range)
+	if (sys_close_range((unsigned int)lowfd, UINT_MAX, 0) == 0)
+		return;
+#endif
+
+	closefrom_fallback(lowfd);
+}

compat/closefrom.h

@@ -0,0 +1,24 @@
+/*
+ * SPDX-License-Identifier: ISC
+ *
+ * Copyright (c) 2004-2005, 2007, 2010, 2012-2015, 2017-2018
+ *	Todd C. Miller <Todd.Miller@sudo.ws>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+#ifndef CLOSEFROM_H
+#define CLOSEFROM_H
+
+void closefrom(int);
+
+#endif

configure

@@ -811,6 +811,27 @@ fi
 rm -f _clock_gettime.c _clock_gettime
 $abort && exit 1
 
+if [ -z "$CLOSEFROM" ]; then
+	printf "Testing for closefrom ... "
+	cat <<EOF >_closefrom.c
+#include <unistd.h>
+int main(void) {
+	return closefrom(3);
+}
+EOF
+	if $XCC _closefrom.c -o _closefrom 2>&3; then
+		CLOSEFROM=yes
+	else
+		CLOSEFROM=no
+	fi
+	echo "$CLOSEFROM"
+fi
+rm -f _closefrom.c _closefrom
+if [ "$CLOSEFROM" = no ]; then
+	echo "COMPAT_SRCS+=	compat/closefrom.c" >>$CONFIG_MK
+	echo "#include			\"compat/closefrom.h\"" >>$CONFIG_H
+fi
+
 printf "Testing ioctl request type ... "
 cat <<EOF >_ioctl.c
 #include <sys/ioctl.h>