DRb.thread.join in a Process.fork block?

Hi list,

I am trying to make my DRb server fork into a daemon, yet the DRbServer
thread joins immediately when called inside a fork block. I’m not
entirely
sure why this is happening but I’m coming to the conclusion that I’m
going
to have to resort to a sleeping while loop. I also tried blocking on a
Mutex
to keep the process running, but that resulted in an error about
blocking
the only thread (which isn’t true because I have a pool of them, plus
the
DRbServer thread which should still be running).
I did find a commit by matz some time ago about removing a “fork kills
threads” warning or some such, is this related?

Cheers,
Ian.

Ian L. wrote:

I did find a commit by matz some time ago about removing a “fork kills
threads” warning or some such, is this related?

Forking with threads is a little tricky. It causes all threads except
the thread calling #fork to be killed.

t1 = Thread.new {sleep}

Thread.new do
fork do
p t1
end
end

Process.wait

Output:

#<Thread:0xb7d986b8 dead>

This is probably sensible behavior, but it causes problems with mutexes.

If thread A does a Process.fork while thread B is holding a Mutex, then
in the child, all other threads, including thread B, will be dead. But
thread B is still holding the mutex. So if thread A tries to get the
mutex, the child process will deadlock.

There’s a safer implementation of Mutex and fork in the fsdb lib, with a
mini-test at the end (see
http://wiki.rubygarden.org/Ruby/page/show/ForkableMutex). It cleans up
dead threads that are holding mutexes, and it also fixes a race
condition in the waiters queue.

But it won’t help you keep the DRb threads alive after a fork (in the
child). Probably you need to start the service separately in the fork.

On Sat, 14 Oct 2006, Ian L. wrote:

threads" warning or some such, is this related?

Cheers,
Ian.

you have to fork first.

fork{
fork{
start_drb
}
exit!
}

fyi.

check out the code for slave - it does this and a bit more - though it’s
not
geared towards daemons.

-a

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