Concurrent::Set is not thread-safe in CRuby #796
Description
Concurrent::Set returns Set in CRuby with this argument:
# Because MRI never runs code in parallel, the existing
# non-thread-safe structures should usually work fine.
::SetAs discussed in #793, that is not enough for thread-safety, you also need the interface to be atomic somehow. For Hash and Array this holds because they are written in C, but Set is partially written in Ruby.
To demonstrate Concurrent::Set is not thread-safe I have created this artificial example:
require "concurrent-ruby"
class MySet < Concurrent::Set
# Note include? is written in Ruby in Set, like delete?
def include?(o)
puts "include? call issued, 1 belongs to self, but there is a context switch"
sleep 2
super
end
end
set = MySet[1]
t = Thread.new do
sleep 1
puts "clearing the set in a different thread"
set.clear
end
p set.delete?(1) # should be self, but it is nil
t.joinBy the time delete? is invoked, the set has the element 1, therefore, that method should return self (the set). However, it returns nil because clear was called during a context switch in the middle of the method execution.