Make jobserver fds still open in subprocess

Intro.md

Steps to reproduce:

1. make x
2. make -j4 foo

Expected output:

./x
rjob is 3
wjob is 4
number of jobs is 1

Observed output:

./x
rjob is 3
wjob is 4
read make job token 43 +
read make job token 43 +
read make job token 43 +
number of jobs is 4

The description in http://mad-scientist.net/make/jobserver.html says that make will close the jobserver fds when it runs a job that's not another make, which it decides by looking for $(MAKE) in the command or + at the beginning of the command. My reading of the make source code agrees with this description, but I'm seeing different behavior. I'm confused.

Makefile

foo:
  ./x

x.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>

int
main()
{
  int rjob = 0, wjob = 0;
  int n = 1;
	char c, *s;
	s = strstr(getenv("MAKEFLAGS"), " --jobserver-fds=");
	if (!s) {
		puts("no jobserver flag");
	}

	s = s + 17; // length of " --jobserver-fds="
	rjob = (int)strtol(s, &s, 10);
	wjob = (int)strtol(s+1, NULL, 10); // skip comma
	printf("rjob is %d\n", rjob);
	printf("wjob is %d\n", wjob);

	if (rjob) {
		fcntl(rjob, F_SETFL, fcntl(3, F_GETFL)|O_NONBLOCK);
		while (read(rjob, &c, 1) > 0) {
			printf("read make job token %d %c\n", c, c);
			n++;
		}
	}
	printf("number of jobs is %d\n", n);

	c = '+';
	if (wjob) {
		fcntl(wjob, F_SETFL, fcntl(4, F_GETFL)|O_NONBLOCK);
		for (; n > 1; n--) {
			write(wjob, &c, 1); // ignore error; nothing we can do anyway
		}
	}

	return n;
}