Comment 0 for bug 1397976

From https://lkml.org/lkml/2014/10/10/345

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Commit f95499c3030f ("n_tty: Don't wait for buffer work in read() loop")
introduces a race window where a pty master can be signalled that the ptyslave was closed before all the data that the slave wrote is delivered.Commit f8747d4a466a ("tty: Fix pty master read() after slave closes") fixed theproblem in case of n_tty_read, but the problem still exists for n_tty_poll.This can be seen by running 'for ((i=0; i<100;i++));do ./test.py ;done'where test.py is:

import os, select, pty
(pid, pty_fd) = pty.fork()

if pid == 0:
   os.write(1, 'This string should be received by parent')
else:
   poller = select.epoll()
   poller.register( pty_fd, select.EPOLLIN )
   ready = poller.poll( 1 * 1000 )
   for fd, events in ready:
      if not events & select.EPOLLIN:
         print 'missed POLLIN event'
      else:
         print os.read(fd, 100)
   poller.close()

The string from the slave is missed several times.
This patch takes the same approach as the fix for read and special casesthis condition for poll.
Tested on 3.16.
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This is has been merged to Linus Torvalds branch: https://github.com/torvalds/linux/commit/c4dc304677e8d566572c4738d95c48be150c6606

This would be needed to be implemented in to 3.13 kernel too. As both 12.04 and 14.04 is currently running the Trusty LTS it affectes both.

br,

Björn