From: Jan Svitok <jan.svitok@...>
Date: 2007-02-05T19:19:47+09:00
Subject: Re: Exception never raised when threading an eval containing a malformed command [might be a bug ?]

On 2/5/07, Benjamin Babut <benjamin@coolsand-tech.fr> wrote:
>
> Hello everyone!
>
> I've encountered a strange issue when coding a tool extending ruby, and
> was wondering if I was missing something... I'm working under windows with
> the last stable release of ruby (1.8.5-21) and did not find a way to debug
> ruby sources to observe the behaviour. This is reproductible with just the
> common ruby interpreter. Let me explain the problem :
>
> First let's consider this small script :
>
>         #!/usr/bin/env ruby
>         $FOO+2
>
> We get the evident result ($FOO is not defined, an exception's raised) :
>
>         test.rb:2: undefined method `+' for nil:NilClass (NoMethodError)
>
> Then let's consider this small script :
>
>         #!/usr/bin/env ruby
>         eval("puts \"foo")
>
> You can see that the string given to puts is malformed, so we get again an
> exception :
>
>         (eval):1: compile error (SyntaxError)
>         (eval):1: unterminated string meets end of file
>
> Ok. Now let's thread this. Let's consider the following script :
>
>         #!/usr/bin/env ruby
>         def newthread()
>                 toret = Thread.new      {
>                 puts "START"
>                 begin
>                         $FOO+2
>                 rescue
>                         puts("EXCEPTION !!!!!!!!")
>                 end
>                 puts "END"
>                 }
>                 return toret
>         end
>
>         $TESTTHREAD = newthread()
>         2.times {
>                 if(!$TESTTHREAD.alive?) then
>                         puts "DEAD, RESURRECTING"
>                         $TESTTHREAD = newthread()
>                         sleep(4)
>                 end
>         }
>
> We create a thread, provoke an exception, catch it, restart the thread,
> and so on twice again. We get the following expected behaviour :
>
>         START
>         EXCEPTION !!!!!!!!
>         END
>         DEAD, RESURRECTING
>         START
>         EXCEPTION !!!!!!!!
>         END
>         DEAD, RESURRECTING
>         START
>         EXCEPTION !!!!!!!!
>         END
>
> But now, let's consider this (here comes the issue) : we do exactly the
> same thing (we only replace the $FOO+2 line by our malformed eval-puts) :
>
>         #!/usr/bin/env ruby
>         def newthread()
>                 toret = Thread.new      {
>                 puts "START"
>                 begin
>                         eval("puts \"foo")
>                 rescue
>                         puts("EXCEPTION !!!!!!!!")
>                 end
>                 puts "END"
>                 }
>                 return toret
>         end
>
>         $TESTTHREAD = newthread()
>         2.times {
>                 if(!$TESTTHREAD.alive?) then
>                         puts "DEAD, RESURRECTING"
>                         $TESTTHREAD = newthread()
>                         sleep(4)
>                 end
>         }
>
> And then we get...
>
>         START
>         DEAD, RESURRECTING
>         START
>         DEAD, RESURRECTING
>         START
>
> (!!) It simply seems that the ruby thread dies inside the eval, and the
> exception is never raised (?). Has someone an explanation for this ?
> (found a bug somewhere ?) :)
>
> Many thanks by advance for those who'd work on that matter!

Hi,

it's easy:
$FOO+2 raises NoMethodError < StandardError
while eval(..) raises SyntaxError !< StandardError

empty rescue == rescue StandardError

If you want to catch ALL exceptions, use rescue Exception.

For exceptions hierarchy see
http://www.zenspider.com/Languages/Ruby/QuickRef.html#34