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Zookeeper >> mail # user >> How to deal with fork() properly when using the zkc mt lib


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Jonathan Simms 2012-05-10, 19:45
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Michi Mutsuzaki 2012-05-10, 22:41
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Jonathan Simms 2012-05-10, 23:11
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Re: How to deal with fork() properly when using the zkc mt lib
I've had a similar problem as well, but I've been using the single threaded
async library - I actually find it simpler to use than the mt library.

The way I do it is this:

During session connect -
 1. Grab the file descriptor from the C library via zookeeper_interest()
 2. If this is the first time I saw this file descriptor, and it's valid,
do a setsockopt() on it to set SO_NOSIGPIPE to 1.

When I need to "suspend" the session
 1. close() the file descriptor
 2. call zookeeper_close() on the handle

zookeeper_close() will try to send the close session message at step 2
here. Normally, that would cause a SIGPIPE and your app would crash - but
this time it won't because you've set SO_NOSIGPIPE on the socket. Instead,
the Zookeeper library will see a regular error from its send operation and
it'll free up the handle peacefully without closing the session.

Best Regards,
Martin Kou

On Thu, May 10, 2012 at 4:11 PM, Jonathan Simms <[EMAIL PROTECTED]> wrote:

> Michi, fair point, I actually just looked into it, there doesn't seem
> to be a way through the api to re-establish the session. If you call
> zookeeper_close on the handle:
>
>  "After this call, the client session will no longer be valid. The
> function will flush any outstanding send requests before return. As a
> result it may block."
>
> I tried:
>
> * establish session with handle A
> * copy clientid_t from handle A
> * zookeeper_close handle A
> * construct handle B using clientid_t values from handle A
>
> I get back a SESSION_EXPIRED from the server. (debug from mt lib here:
> https://gist.github.com/3b7e4060746d03cef287)
>
> It would be *really* useful if i could basically "suspend" a session
> while i forked, then reconnect and pick up where i left off. Is this
> not possible?
>
> On Thu, May 10, 2012 at 6:41 PM, Michi Mutsuzaki <[EMAIL PROTECTED]>
> wrote:
> > Hi Jonathan,
> >
> > It would be very difficult to share multi-threaded zk handle with
> > child process. I'm surprised it actually works on mac. I think saving
> > session id/password and re-establishing the session in the child
> > process is more robust and platform independent.
> >
> > Thanks!
> > --Michi
>
>
> >
> > On Thu, May 10, 2012 at 12:45 PM, Jonathan Simms <[EMAIL PROTECTED]>
> wrote:
> >> Hi all,
> >>
> >> I'm the maintainer of the ruby zookeeper library, and I'm having
> >> trouble getting consistent behavior when a user calls fork(). When
> >> developing it on MacOS (using 3.3.5), I was able to fork, then
> >> immediately call zookeeper_close() in the child, and then create a new
> >> handle. Testing on Linux, the behavior is much more unpredictable.
> >> Regularly, it seems there are segfaults when calling zookeeper_close.
> >> https://gist.github.com/22338464cd47e0e50970
> >>
> >>
> >> So I guess my question is, is there any safe way to fork() while the
> >> client is running?
> >>
> >> Another possibility i thought of is to note the session id/passwd,
> >> close the client, fork, then re-open with the same id/passwd to
> >> re-establish the session in the parent.
> >>
> >> Any recommendations?
>
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Martin Kou 2012-05-10, 23:53
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Jonathan Simms 2012-05-13, 03:06
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Jonathan Simms 2012-05-11, 01:46
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Jonathan Simms 2012-05-11, 04:19
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Martin Kou 2012-05-11, 06:41
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Jonathan Simms 2012-05-13, 02:42
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Michi Mutsuzaki 2012-05-10, 23:49