Re: CommitFest 2009-07: Yay, Kevin! Thanks, reviewers!

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> On 19/08/10 19:57, Tom Lane wrote:
>> Hmm, but couldn't you still do that inside pg_usleep?  Signal handlers
>> that do that couldn't know if they were interrupting a sleep per se,
>> so this would have to be a backend-wide convention.

> I don't understand, do what inside pg_usleep()?

I was envisioning still using pg_usleep, and having this interaction
between signal handlers and the delay be private to pg_usleep, rather
than putting such ugly code into forty-nine different places.  There
are a *lot* of places where we have loops that break down delays
into at-most-one-second pg_usleep calls, and if we're going to have a
hack like this we should fix them all, not just two or three that SR
cares about.

But I'm still not seeing how this self-pipe hack avoids a race
condition.  If the signal handler is sending a byte whenever it
executes, then you could have bytes already stacked up in the pipe
from previous interrupts that didn't happen to come while inside
pg_usleep.  If you clear those before sleeping, you have a race
condition, and if you don't, then you fail to sleep the intended
amount of time even though there was no interrupt this time.
        regards, tom lane