Issue #6762 has been updated by ko1 (Koichi Sasada).


brent (Brent Roman) wrote:
> OK.  I see the logic in using the term "interrupt" if you are actually
>  trying to unify exceptions from other threads with handling of OS
>  signals.  However,  both of these are generally thought of as being
>  asynchronous events.

I agree.  They are "asynchronous events".

>  Try googling (with the quotes):
>  
>  "asynchronous interrupt*"  =>  1,130,000 results
>  "synchronous interrupt*" =>  180,000 results
>  
>  If you insist on the async_* prefix, you should apply it consistently.
>  But, Thread.control_async_interrupt(  is getting quite cumbersome, no?

You are right.

>  As someone who was writing ISRs for Intel 8080's and Zilog Z-80's in the
>  late 1970's, here are my suggestions for more conventional vocabulary:
>  
>  Thread.control_interrupt  becomes   Thread.interruptible
>    alternatives would be:
>       Thread.allow_interrupt or Thread.enable_interrupt
>  Any of these read better (to a native English speaker).
>  I like interruptible because it is a *property* of the thread being
>  assigned by the construct.  After all, nothing actually happens when
>  this construct is executed.  It affects what (might) happen later:
>  
>     th = Thread.new{
>        Thread.interruptible(RuntimeError => :on_blocking){
>            ...
>        }

"interruptible" makes sense for me.

But I feel it is ambiguous that this method returns only current interruptible flags.

How about to use `async_event' ?


>  In General:
>     Code within the block passed to the Thread.interruptible method may
>  or may not be interrupted according to the specification passed as its
>  Hash argument.
>  
>  In the example above, within the block passed to Thread.interruptible,
>  the thread becomes interruptible by any RuntimeError when/if
>  it waits for I/O or stops.
>  
>  =====
>  
>  The method :async_interrupted? would be better named:
>    :interrupts_pending?
>  A thread is not interrupted if it has interrupts being deferred.
>  The accepted idiom for this is is to say the thread has interrupts
>  pending for it.

As non-native English speaker, I'm not sure the difference with "pending_interrupt?". Yours is good?

>  The use case for defining interrupts_pending? method as Thread instance
>  method is summarized on one word:  debugging!

It makes sense.

>  If you have a complex application that has threads which seem to be
>  unresponsive, you'll want some way to tell whether those threads are
>  ignoring pending interrupts, or whether they are not even getting
>  interrupts delivered to them.
>  
>  I'd also suggest adding another method:
>  
>     Thread#interrupts_pending   #without the question mark
>  
>  This would return the number of pending interrupts for the thread.
>  A thread might normally have 0, 1 or 2 pending interrupts.  Seeing
>  dozens  pending would indicate a performance problem.  This would be
>  very useful information for debugging and optimization. A thread might
>  even decide to take some drastic action to if it discovers that it has
>  too many interrupts pending for itself.

I don't like this method.

I like Thread#interrupts_pending?(err_class) what current Thread#async_interrupt? do.

Number is important? I don't think so.


>  Making Thread.current.raise act like sending exceptions to any other
>  thread seemed more consistent to me because the method's behavior then
>  has no special case for Thread.current.  I have written low level code
>  what processed a hardware interrupt, but then decided it must defer it
>  for later and accomplished this by making the interrupt pending again,
>  in the controller chip, but masked the interrupt in the CPU.  However, I
>  can see where this might break existing code that currently relies on
>  Thread.current#raise being exactly synonymous with Kernel#raise
>  Either behavior is workable.

I agree with it. Non *exception* is easy to understand.

Guys: any problem on it?


----------------------------------------
Feature #6762: Control interrupt timing
https://bugs.ruby-lang.org/issues/6762#change-34348

Author: ko1 (Koichi Sasada)
Status: Feedback
Priority: High
Assignee: ko1 (Koichi Sasada)
Category: core
Target version: 2.0.0


=begin
= Abstract

Add asynchronous interrupt timing control feature.  Control with the following three modes:

* immediate: process interrupt immediately
* never: never process interrupt
* on_blocking: delay interrupt until blocking operation

  # example
  th = Thread.new do
    Thread.control_interrupt(RuntimeError => :never) do
      # in this block, thrown RuntimeError doesn't occur 
    end
    ... # raise thrown RuntimeError
  end
  ...
  th.raise "foo" 

= Background

== Terminology

* Interrupt: asynchronous interrupt and corresponding procedures
  * Thread#raise and occurring exception
  * signal and corresponding trap 
  * Thread#kill and thread termination
  * Main thread termination and thread termination
    (after main thread termination, all threads exit themselves)
* Interrupt checking: check interrupt
* Blocking operation: Possible to block the current thread such as IO read/write.  In CRuby implementation, it is nearly equals to tasks without GVL

== Current use-cases of Interrupt

There are several `Interrupt' in Ruby.

  # Example 1
  th = Thread.new{
    begin
      ...
    rescue FooError
      ...
    end
  }
  th.raise(FooError) #=> Raise FooError on thread `th'

  # Example 2
  q = Queue.new
  th1 = Thread.new{
    q << calc_in_algorithm1
  }
  th2 = Thread.new{
    q << calc_in_algorithm2
  }
  result = q.pop
  th1.raise(TerminateCalcError)
  th2.raise(TerminateCalcError)
  # Run two algorithms simultaneously.
  # If we get an answer from one algorithm,
  # kill them with TerminateCalcError
  # In this case, it is also okay with Thread#kill

  # Example 3
  trap(SIGINT){
    # do something
    # maybe termination process
  }
  trap(SIGHUP){
    # do something
    # maybe reloading configuration process
  }
  server_exec # server main process

In such interrupts are checked at several points such as:

* method invocation timing
* method returning timing
* move program counter
* before and after block operation

== Problem

Interrupt causes the following problems because we can't control occurring timing. 

* Un-safe ensure clause: Generally, ensure clause should not interrupt because it contains important tasks such as freeing resources.
* Un-safe resource allocation: If interrupt occurs between resource allocation and assign it to the variable, we can't free this object (however, this problem not too big because we have a gc and appropriate finalizer can free it).
* (other problems? please complement me)

I show an example below.

  # Example 4
  # this method is similar implementation of timeout()
  def timeout(sec)
    timer_thread = Thread.new(Thread.current){|parent|
      sleep(sec)
      parent.raise(TimeoutError)
    }
    begin
      yield
    ensure
      timer_thread.stop # close thread
    end
  end
  timeout(3){
    begin
      f = # point (a)
      open(...)  # of course, there are no problem with open(...){|f| ...} 
                 # but it is an example to show the problem
      ...
    ensure
      ... # point (b)
      f.close
    end
  }


On example 4, there are two problems.

Point (b) is easy to understand.  If interrupt was thrown at point (b), then `f.close()' isn't called.  It is problem.

On the point (a), it is a position between resource allocation (open()) and assignment `f = '.  It is very rare, but it is possible.  If we get interrupt before assignment, then we can't free resources (can't call f.close()) in ensure clause.  It is also problem.

The problem is we can't control interrupt timing.

= Proposal

Adding interrupt timing control feature to Thread.  Introduce two methods to Thread class.

* Thread.control_interrupt
* Thread.check_interrupt

Rdoc documents are:

Thread.control_interrupt():

  call-seq:
    Thread.control_interrupt(hash) { ... } -> result of the block
 
  Thread.control_interrupt controls interrupt timing.
 
  _interrupt_ means asynchronous event and corresponding procedure
  by Thread#raise, Thread#kill, signal trap (not supported yet)
  and main thread termination (if main thread terminates, then all
  other thread will be killed).
 
  _hash_ has pairs of ExceptionClass and TimingSymbol.  TimingSymbol
  is one of them:
  - :immediate   Invoke interrupt immediately.
  - :on_blocking Invoke interrupt while _BlockingOperation_.
  - :never       Never invoke interrupt.
 
  _BlockingOperation_ means that the operation will block the calling thread,
  such as read and write.  On CRuby implementation, _BlockingOperation_ is
  operation executed without GVL.
 
  Masked interrupts are delayed until they are enabled.
  This method is similar to sigprocmask(3).
 
  TODO (DOC): control_interrupt is stacked.
  TODO (DOC): check ancestors.
  TODO (DOC): to prevent all interrupt, {Object => :never} works.
 
  NOTE: Asynchronous interrupts are difficult to use.
        If you need to communicate between threads,
        please consider to use another way such as Queue.
        Or use them with deep understanding about this method.
 
 
    # example: Guard from Thread#raise
    th = Thread.new do
      Thead.control_interrupt(RuntimeError => :never) {
        begin
          # Thread#raise doesn't interrupt here.
          # You can write resource allocation code safely.
          Thread.control_interrupt(RuntimeError => :immediate) {
            # ...
            # It is possible to be interrupted by Thread#raise.
          }
        ensure
          # Thread#raise doesn't interrupt here.
          # You can write resource dealocation code safely.
        end
      }
    end
    Thread.pass
    # ...
    th.raise "stop"
 
    # example: Guard from TimeoutError
    require 'timeout'
    Thread.control_interrupt(TimeoutError => :never) {
      timeout(10){
        # TimeoutError doesn't occur here
        Thread.control_interrupt(TimeoutError => :on_blocking) {
          # possible to be killed by TimeoutError
          # while blocking operation
        }
        # TimeoutError doesn't occur here
      }
    }
 
    # example: Stack control settings
    Thread.control_interrupt(FooError => :never) {
      Thread.control_interrupt(BarError => :never) {
         # FooError and BarError are prohibited.
      }
    }
 
    # example: check ancestors
    Thread.control_interrupt(Exception => :never) {
      # all exceptions inherited from Exception are prohibited.
    }

Thread.check_interrupt():

  call-seq:
    Thread.check_interrupt() -> nil
 
  Check queued interrupts.
 
  If there are queued interrupts, process respective procedures.
 
  This method can be defined as the following Ruby code:
 
    def Thread.check_interrupt
      Thread.control_interrupt(Object => :immediate) {
        Thread.pass
      }
    end
 
  Examples:
 
    th = Thread.new{
      Thread.control_interrupt(RuntimeError => :on_blocking){
        while true
          ...
          # reach safe point to invoke interrupt
          Thread.check_interrupt
          ...
        end
      }
    }
    ...
    th.raise # stop thread
 
  NOTE: This example can be described by the another code.
        You need to keep to avoid asynchronous interrupts.
 
    flag = true
    th = Thread.new{
      Thread.control_interrupt(RuntimeError => :on_blocking){
        while true
          ...
          # reach safe point to invoke interrupt
          break if flag == false
          ...
        end
      }
    }
    ...
    flag = false # stop thread

I have already commit-ed these methods into trunk.
Please try it and discuss.

This commit is easy to revert :)

Naming is also problem as usual.  Good naming is also welcome.


= Acknowledgment

The base of this proposal is a discussion[1].

[1] Akira Tanaka "Re: Thread#raise, Thread#kill, and timeout.rb are
unsafe" ruty-talk (2008.3) <http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/294917>

Many dev-people help me to make up this proposal.

=end



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