From: Benjamin Babut <benjamin@...>
Date: 2007-02-05T19:04:41+09:00
Subject: Exception never raised when threading an eval containing a malformed command [might be a bug ?]


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!

Ben