[9fans] fun with rfork

[C H Forsyth <forsyth@vitanuova.com>]

here is part of an application of rfork(RFMEM|RFPROC ...).
most irrelevant code has been removed.
libinit was called early on before the process switched to
a stack on the malloc heap, and that's where it is running when it
calls oscmd.

what happens next?

	--------

void
libinit(char *imod)
{
	...
	int fd, n, pid;

	...
	/*
	 * guess at a safe stack for vstack
	 */
	ustack = (ulong)&fd;
	...
	/* switch to stack on the heap ... */
	tramp(sp+KSTACK, up->func, up->arg);
}

typedef struct Targ Targ;
struct Targ
{
	int	fd;
	int*	spin;
	char*	cmd;
};

void
exectramp(Targ *targ)
{
	*targ->spin = 0;
	...

	exec(argv[0], argv);
	exits("");
}

int
oscmd(char *cmd, int *rfd, int *sfd)
{
	Targ targ;
	int spin, *spinptr, fd[2];

	if(pipe(fd) < 0)
		return -1;

	spinptr = &spin;
	spin = 1;

	targ.fd = fd[0];
	targ.cmd = cmd;
	targ.spin = spinptr;

	switch(rfork(RFMEM|RFPROC|RFFDG|RFENVG|RFREND)) {
	case -1:
		return -1;
	case 0:
		vstack(&targ);			/* Never returns */
	default:
		while(*spinptr)
			;
		break;
	}
	close(fd[0]);

	*rfd = fd[1];
	*sfd = fd[1];
	return 0;
}

TEXT	vstack(SB),$0
	MOVL	arg+0(FP), AX
	MOVL	ustack(SB), SP
	PUSHL	AX
	CALL	exectramp(SB)
	POPL	AX			/* dammit ken! */
	RET