From: Brian Candler <b.candler@...>
Date: 2008-09-30T17:53:22+09:00
Subject: Re: exception in thread?

Sylvain Viart wrote:
> It seems that ruby have difficulties to catch exception in multi
> threaded mode, any hint?

For debugging purposes, maybe you want Thread.abort_on_exception = true 
(or just run ruby with -d flag)

Other than that I don't understand your problem. What behaviour do you 
see when you run your test program? What behaviour do you expect? Is no 
warning generated for the non-existent host?

>   # execute commands on all servers
>  begin
>     session.exec( "hostname" )
>  rescue Exception => e
>     p "main:#{e}"
>  end

That rescue won't catch exceptions in other threads. Each thread of 
execution is responsible for catching its own exceptions. If it doesn't, 
then the thread just terminates (unless Thread.abort_on_exception is 
set)

It *is* possible for one thread to raise an exception in another thread 
(Thread#raise), but this is extremely hairy asynchronous programming and 
I would strongly discourage it.

It would seem reasonable for session.exec to collect the status of each 
of the threads and return an array of them. I don't know if it does so. 
Perhaps you can use something like this:

    errs = []
    ...
      :on_error => lambda { |server| errs << server }
    ...
    session.exec "hostname"
    unless errs.empty?
      puts "The command failed on #{errs.size} hosts"
    end
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