Threads are hard

foo.rb

m = Mutex.new
t = []
1.upto(50) do |i|
  t << Thread.new(i) do |id|
    m.synchronize do
      puts i
      User.first
    end
  end
end
t.each { |th| th.join }

Result

$ rails/runner foo.rb

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
script/foo:11:in `join': deadlock detected (fatal)
	from script/foo:11:in `block in <top (required)>'
	from script/foo:11:in `each'
	from script/foo:11:in `<top (required)>'
	from /Users/sreeix/.rvm/gems/ruby-1.9.2-p180@spaghetti/gems/rails-2.3.11/lib/commands/runner.rb:46:in `eval'
	from /Users/sreeix/.rvm/gems/ruby-1.9.2-p180@spaghetti/gems/rails-2.3.11/lib/commands/runner.rb:46:in `<top (required)>'
	from script/runner:3:in `require'
	from script/runner:3:in `<main>'