From: Tanaka Akira <akr@...>
Date: 2008-03-18T14:04:40+09:00
Subject: Re: Thread#raise, Thread#kill, and timeout.rb are unsafe

In article <E1JaM9H-0000jj-AB@x61.netlab.jp>,
  Yukihiro Matsumoto <matz@ruby-lang.org> writes:

> What should safe point declaration mechanism look like?

Basically, asynchronous events should be queued.

* make a queue for each thread.
* Thread#raise(exc) enqueue exc to the queue.
* Thread.check_interrupt(klass) checks an exception which is
  a kind of klass in the queue of the current thread.  If it
  exists, the exception is raised.
* other queue examining mechanism, such as sigpending, etc,
  may be useful.

Thread.check_interrupt is a safe point.

However safe points is not only Thread.check_interrupt.
Blocking operations, such as I/O, may or may not be safe
points.  It is because Thread.check_interrupt with blocking
operations causes race conditions.  So application must
choose that make a blocking operation interruption safe or
uninterruptible.

* Thread.blocking_interruptible(klass) { ... }
* Thread.blocking_uninterruptible(klass) { ... }

Blocking operations in Thread.blocking_interruptible are
safe points.

If a thread has no resources to release, it is safe to kill
asynchronously.  Other than that, safe points should be
declared explicitly.  When a resource is acquired,
application should declare it.

* Thread.delay_interrupt(klass) { ... }

It is safe points that out of outermost
Thread.delay_interrupt.

Another idea is about ensure clause.  Since an ensure
clause is used to release a resource, it may delay
asynchronous events as in Thread.delay_interrupt.
-- 
Tanaka Akira