Re: Fatal: Ipc::Mem::Segment::create failed to shm_open.

On Fri, 21 Oct 2011 17:02:22 -0600, Alex Rousskov <rousskov_at_measurement-factory.com> wrote:
> On 10/21/2011 02:29 PM, Alex Rousskov wrote:
> > On 10/21/2011 01:54 PM, Alex Rousskov wrote:
> >> On 10/20/2011 07:53 PM, Amos Jeffries wrote:
> >>> Anyone got a fix for this?
> >>> It is currently blocking trunk snapshots.
> >>
> >> I am sure it is something I or Dmitry broke, but I do not get this error
> >> in my environment. Any specifics? I know bug #3361 mentioned this error,
> >> but I think it moved on to different problems after recent changes.
> >
> > I now see that error in fresh "Build failed" reports kicked by Christos'
> > commit. I assume that is what you referred to. Will try to fix.
>
>
> I suspect that shm_open(path) on FreeBSD fails with a "permission
> denied" error because FreeBSD wants an absolute path to a real file
> while Linux wants a name starting with a slash (and containing no other
> slashes). Linux converts its argument into an absolute path internally.
> FreeBSD is not that "smart".
>

You are right.

> We will need to add a wrapper for FreeBSD and possibly other OSes to
> prefix the shared memory segment path with some directory where Squid
> can write. Should we use DEFAULT_STATEDIR (PREFIX/var/run/squid) for
> that prefix? That is where we store IPC sockets now.
>

That should be the right place for shm.

>
> Any good ideas on how to automatically detect whether the shm_open()
> path should be prefixed in a given build environment? Here are some
> not-so-good ones:
>
> 0) Use a list of OSes known [not] to require absolute paths.
>
> 1) Try to shm_open("/squid-unique") and hope that no FreeBSD-like system
> would let us write into "/", but I suspect some would allow that because
> folks may be building as "root".
>
> 2) Try to shm_open("/tmp/squid-unique") and hope that no Linux-like
> system would let us use a name with an embedded slash (because there is
> no tmp/ directory wherever Linux creates the actual handle). I tested
> this on one Linux box and it worked, but others may be more lenient.
>
> 3) Use (2) but if it does not fail, try to confirm with (1), and
> resorting to (0) if the combined results are inconclusive.
>

IMHO trying to open /PREFIX/var/run/squid/unique and falling back to
/squid-unique is the best option because it provides better portability.
Runtime penalty should be negligible (and segments are opened only at
startup).

Note that both FreeBSD and Linux require (well, at least work with)
absolute paths (though they have different semantics), so (0)
description is not accurate. The difference is whether to use real path
name or any ID beginning with a slash and not containing other slashes.

Regards,
  Dmitry

>
> Thank you,
>
> Alex.
Received on Fri Oct 21 2011 - 23:39:40 MDT