Re: What's the correct new API to request pcap_linux to not open an eventfd

[Bill Fenner via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: Bill Fenner <fenner () gmail com>
>
> Date: Fri, 1 Jul 2022 13:55:30 -0400
>
> On Fri, May 20, 2022 at 6:10 PM Bill Fenner <fenner () gmail com> wrote:
>
> > On Fri, May 20, 2022 at 12:36 PM Guy Harris <gharris () sonic net> wrote:
> >
> > > If it's putting them in non-blocking mode, and using some
> > > select/poll/epoll/etc. mechanism in a single event loop, then the right
> > > name for the API is pcap_setnonblock().  There's no need for an eventfd to
> > > wake up the blocking poll() if there *is* no blocking poll(), so:
> > >
> > >         if non-blocking mode is on before pcap_activate() is called, no
> > > eventfd should be opened, and poll_breakloop_fd should be set to -1;
> > >
> > >         if non-blocking mode is turned on after pcap_activate() is
> > > called, the eventfd should be closed, and poll_breakloop_fd should be set
> > > to -1;
> > >
> > >         if non-blocking mode is turned *off* afterwards, an eventfd
> > > should be opened, and poll_breakloop_fd should be set to it;
> > >
> > >         if poll_breakloop_fd is -1, the poll() should only wait on the
> > > socket FD;
> > >
> > > so this can be handled without API changes.
> >
> > Thank you for the excellent observation, Guy.  It is indeed setting
> > non-block before pcap_activate().  I'll work on this plan.
>
> A slight variation of this plan is at
> https://github.com/the-tcpdump-group/libpcap/pull/1113
>
> I wrote a test program that doesn't do much, but does demonstrate that the
> blocking-ness API on Linux is at least a little weird.  If we set
> pcap_nonblock after pcap_create and before pcap_activate, we get -3 - which
> I don't get at all, unless, -3 means "you didn't activate the pcap yet".
> My naive reading of the Linux pcap_getnonblock code says it'll return the
> integer value of a bool, and I don't know how that can be -3.
>
> The sequence ends up being:
> pcap_create() -> open eventfd
> pcap_setnonblock() -> close eventfd
> pcap_activate()
>
> I didn't want to move the eventfd creation out of pcap_create without
> having a deeper understanding of the strangeness around the nonblock API.
>
> I ran 15,645 internal Arista tests using these changes, in an
> infrastructure that relies on nonblocking pcap for packet exchange, and
> they all passed.  Obviously this doesn't really say much about its general
> applicability, and kind of is a no-brainer "when you don't need the eventfd
> you don't notice if it's not there", but at least it says that things
> aren't drastically broken.
>
>   Bill
>
> --- End Message ---
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