Bug #4285

Updated by Koichi Sasada about 3 years ago

=begin

This issue was discovered during [Bug#4266] discussion.
Current timeout is racy.

Now, timeout module has following code.
-------------------------------------------------------------------------------
def timeout()
begin
x = Thread.current
y = Thread.start {
begin
sleep sec
rescue => e
x.raise e
else
x.raise exception, "execution expired" if x.alive?
end
}
return yield(sec)
rescue exception => e
raise Error, e.message, e.backtrace
ensure
if y and y.alive?
y.kill
y.join # make sure y is dead.
end
end
end
---------------------------------------------------------------

Unfortunatelly,

y = Thread.start {}

is not an atomic operation. Then, A following race can occur.


CPU0(thread x) CPU1(thread y) remark
---------------------------------------------------------------------------
enter begin block
[thread construct] but no assign y yet
sleep sec
wakeup from sleep
x.raise
if y return false. (see above)


Therefore, CPU0 don't call y.join and leak y's thread resource. C# have solved
this two-step-construction vs asynchrounous exception race by RAII.
(Plus, C#'s finally block is async exception safe automatically)

But unfortunately, Ruby don't have such language feature. So, We can't write
async-exception-safe code. One of solution is to move timeout module from ruby code
into c code as JRuby does. But I don't think timeout is only asynchrounos exception user.
we also have Interrupt class (for Ctrl-C) and I think we need to allow to write async
exception safe code by ruby.

So, My proposal is,
o for 1.9.x
=> Reimplement timeout.rb by C (as JRuby)

o for 2.0
=> Aim new feature for writing async exception safe code.


Or, Am I missing something?

=end

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