gistfile1.rb

# Some examples for why the
#
#    @obj ||= "some value"
#
# expression is threadsafe on MRI (should one be dumb enough to write
# such code)
#
# MRI will only preempt a Ruby thread when it is returning from a
# method call. If a code block contains no ruby method calls then
# then it will not be preempted. If it contains a single method
# call then the preemption point is predictable.
#
# This code:
#
#      @obj ||= "some value"
#
# expands to:
#
#     if @obj
#        @obj = "some value"
#     end
#
# and for this to be unsafe it should contain two method calls. There
# are none or one depending on how you count it.
#
# Now if I were to change the code to something like this
#
#     @obj ||= Whatever.new
#
# then now there is a function call between the CHECK and SET
# operations and it is no longer thread safe.
#
#
# Regardless of how safe the original code was, the best thing to do
# is avoiding shared state when working with threads:
#
#   Don't communicate by sharing memory; share memory by communicating
#   (Go Programming Language).


class DoesNotPreemptWithoutMethodCalls
  def run
    @whatever = nil
    @non_nil_count = 0
    @thread_count = 1_000
    @thread_count.times.map do
      Thread.new do
        if @whatever != nil
          @non_nil_count += 1
        end
        @whatever ||= 1         # ??? Thread safe ???
      end
    end.each(&:join)

    if @thread_count == @non_nil_count + 1
      puts "OK"
    else
      puts "BUM: Preempted without a method call!"
    end
  end
end
DoesNotPreemptWithoutMethodCalls.new.run


class HaveSomeMethodcallsForPreemtion
  def count_non_nils
    if @whatever != nil
      @non_nil_count += 1
    end
    # may preempt here
  end

  def run
    @whatever = nil
    @non_nil_count = 0
    @thread_count = 1_000
    @thread_count.times.map do
      Thread.new do
        count_non_nils          # may be preempted here
        @whatever ||= 1         # but not here
      end
    end.each(&:join)

    if @thread_count == @non_nil_count + 1
      puts "2 - OK"
    else
      puts "2 - BUM: Preempted without a method call!"
    end
  end
end
HaveSomeMethodcallsForPreemtion.new.run