[ART-380] Fix ArgumentError due to concurrent require and class use

[ivoanjo]

If for some reason multiple tracers are initializing concurrently (see customer issue in #1306 or the flaky test in tracer_integration_spec.rb also fixed by this PR), multiple threads can try to run default_statsd_client concurrently.

Because Ruby requires are not atomic (e.g. you can observe classes that are half-loaded), our conditional

require 'datadog/statsd' unless defined?(::Datadog::Statsd)
Datadog::Statsd.new(default_hostname, default_port)

could try to create a Statsd instance BEFORE the class was fully defined, leading to ArgumentError: wrong number of arguments (given 2, expected 0) when calling initialize. This error stems from the custom #initialize method not being available yet, so Ruby falls back to Object#initialize, which does not take any argument.

See also:

• https://slides.com/ianjo/spotting-unsafe-concurrent-ruby-patterns-fullstack-lx#/0/42/0
• https://gitlab.com/ivoanjo/unsafe-concurrent-ruby-patterns/-/blob/master/background_require.rb

The fix in this case is to always call require. Calling require repeatedly has little impact -- a given file is only loaded by require the first time AND concurrent requires are protected -- only one thread will get to execute the require, will all others wait until it finishes.

The follow example demonstrates this:

• lazy-require.rb:

def try_require
  puts "#{Thread.current} Going to require"
  require_relative './lazy-require-sleep-1'
  puts "#{Thread.current} After require"
end

2.times.map { Thread.new { try_require } }.map(&:join)

try_require

• lazy-require-sleep-1.rb:

puts "#{Thread.current} Running require"
sleep 1
puts "#{Thread.current} Require finished"

• Output:

$ ruby lazy-require.rb
#<Thread:0x00007fc08b9081e0 lazy-require.rb:7 run> Going to require
#<Thread:0x00007fc08b90be80 lazy-require.rb:7 run> Going to require
#<Thread:0x00007fc08b9081e0 lazy-require.rb:7 run> Running require
#<Thread:0x00007fc08b9081e0 lazy-require.rb:7 run> Require finished
#<Thread:0x00007fc08b9081e0 lazy-require.rb:7 run> After require
#<Thread:0x00007fc08b90be80 lazy-require.rb:7 run> After require
#<Thread:0x00007fc08b85fa90 run> Going to require
#<Thread:0x00007fc08b85fa90 run> After require

Notice how two threads tried to require, but only one actually ran the code on the other file, while the other was blocked and only woke up after the require finished.

Fixes #1306

[marcotc]

🧹