truss(1) locked in kernel with 8.0-BETA2
Nathan Lay
nslay at comcast.net
Sun Aug 23 04:22:29 UTC 2009
Ed Schouten wrote:
> Hi Jeremie,
>
> * Jeremie Le Hen <jeremie at le-hen.org> wrote:
>
>> I've upgraded my laptop to 8.0-BETA2 and ran portupgrade in script(1).
>> But according to top, it seems script(1) is going crazy, even after I've
>> hit ^C:
>>
>
> The fact that script(1) is going crazy, is a known issue. I have been
> pointed to this issue earlier, but unfortunately I don't know what to
> do. A certain Colin introduced this bug about 6 years ago. ;-)
>
> It's basically a shortcoming of pseudo-terminals in general. script(1)
> wants to behave in a way which cannot be implemented using
> pseudo-terminals; when it receives a hangup on its standard input (on
> the outside), it wants to propagate the end-of-file condition and wants
> to continue until the child processes are finished, instead of shutting
> down immediately. So a couple of milliseconds later on, it calls
> select(2) again, but because the TTY it uses on the outside is still in
> a hangup condition, select(2) returns immediately.
>
> This can easily be reproduced as follows:
>
> script < /dev/null
>
> I think the only way we can sanely fix this, is by adding a special flag
> to instruct script(1) to keep going on, even if stdin disappears. I
> wrote a patch for this back in May:
>
> http://80386.nl/pub/script.diff
>
> Thanks for reminding me. I should contact re@ about this.
>
>
Instead of zeroing and setting master and STDIN_FILENO on the fd_set
every iteration, why not have two fd_sets: the original and temporary.
At every iteration the original is copied into the temporary for
select() to destroy. Once the hangup condition is detected, the
STDIN_FILENO is cleared from the original fd_set so it will not be
examined by select() later. For example:
fd_set rfds, tmpfds;
FD_ZERO(&rfds);
FD_SET(master,&rfds);
FD_SET(STDIN_FILENO,&rfds);
for (;;) {
tmpfds = rfds; /* Not sure if this assignment is portable */
n = select(master+1, &tmpfds, NULL, NULL, tvp);
if (hangup condition) {
FD_CLR(STDIN_FILENO,&rfds);
}
/* Do other stuff here */
}
Best Regards,
Nathan Lay
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