pty tcdrain() bug in 2.6.27 to 2.6.30/current

["Luke-Jr" <luke@dashjr.org>]

tcdrain blocks on ptys since 2.6.27; at least 2.6.26 returns in a timely 
manner. The following test case reproduces the bug, and hangs only on affected 
kernels. Examination of 2.6.26 and 2.6.27 suggests the ioctl used by tcdrain 
underwent a rewrite for 2.6.27, and thus fixing this bug is beyond my 
capabilities at this time.

-------

#define _POSIX_C_SOURCE 200112L
#define _XOPEN_SOURCE

#include <assert.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

int
main() {
	int masterfd, slavefd;
	char test[] = "test";
	
	masterfd = posix_openpt(O_RDWR | O_NOCTTY);
	assert(masterfd != -1);
	grantpt(masterfd);
	unlockpt(masterfd);
	
	slavefd = open(ptsname(masterfd), O_NOCTTY | O_RDWR);
	assert(slavefd != -1);
	
	assert(sizeof(test) == write(slavefd, test, sizeof(test)));
	assert(!tcdrain(slavefd));
}