Re: [HACKERS] Weaker shmem interlock w/o postmaster.pid

On Thu, Apr 11, 2019 at 12:48:35PM +1200, Thomas Munro wrote:
> On Mon, Apr 8, 2019 at 6:42 PM Noah Misch <noah(at)leadboat(dot)com> wrote:
> > - AIX animals failed two ways. First, I missed a "use" statement such that
> > poll_start() would fail if it needed more than one attempt. Second, I
> > assumed $pid would be gone as soon as kill(9, $pid) returned[1].
>
> > [1] POSIX says "sig or at least one pending unblocked signal shall be
> > delivered to the sending thread before kill() returns." I doubt the
> > postmaster had another signal pending often enough to explain the failures, so
> > AIX probably doesn't follow POSIX in this respect.
>
> It looks like you fixed this, but I was curious about this obversation
> as someone interested in learning more about kernel stuff and
> portability... Maybe I misunderstood, but isn't POSIX referring to
> kill(sig, $YOUR_OWN_PID) there? That is, if you signal *yourself*,
> and no other thread exists that could handle the signal, it will be
> handled by the sending thread, and in the case of SIGKILL it will
> therefore never return. But here, you were talking about a perl
> script that kills the postmaster, no? If so, that passage doesn't
> seem to apply.

You're right. I revoke the footnote.

> In any case, regardless of whether the signal handler
> has run to completion when kill() returns, doesn't the pid have to
> continue to exist in the process table until it is reaped by its
> parent (possibly in response to SIGCHLD), with one of the wait*()
> family of system calls?

True. I'll add that to the code comment.