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

ruby-talk
1

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.

2

Ian Leitch wrote:

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?

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.

···

--
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

3

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

···

On Sat, 14 Oct 2006, Ian Leitch wrote:

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.

--
my religion is very simple. my religion is kindness. -- the dalai lama