Re: EINTR in ftruncate()

Hi,

On 2022-07-06 21:29:41 +0200, Alvaro Herrera wrote:
> On 2022-Jul-05, Andres Freund wrote:
> 
> > I think we'd be better off disabling at least some signals during
> > dsm_impl_posix_resize(). I'm afraid we'll otherwise just find another
> > variation of these problems. I haven't checked the source of ftruncate, but
> > what Thomas dug up for fallocate makes it pretty clear that our current
> > approach of just retrying again and again isn't good enough. It's a bit more
> > obvious that it's a problem for fallocate, but I don't think it's worth having
> > different solutions for the two.
> 
> So what if we move the retry loop one level up?  As in the attached.
> Here, if we get EINTR then we retry both syscalls.

Doesn't really seem to address the problem to me. posix_fallocate()
takes some time (~1s for 3GB roughly), so if we signal at a higher rate,
we'll just get stuck.

I hacked a bit on a test program from Thomas, and it's pretty clearly
that with a 5ms timer interval you'll pretty much not make
progress. It's much easier to get fallocate() to get interrupted than
ftruncate(), but the latter gets interrupted e.g. when you do a strace
in the "wrong" moment (afaics SIGSTOP/SIGCONT trigger EINTR in
situations that are retried otherwise).

So I think we need: 1) block most signals, 2) a retry loop *without*
interrupt checks.

Greetings,

Andres Freund