Make the tracer thread-safe

[pocke]

Problem

This gem raises DiverDown::Trace::CallStack::StackEmptyError while investigating a Rails application.

I assumed the cause is thread-unsafe code, and I made a small code to reproduce it.
This code raises the StackEmptyError (it is not 100% occurred as with most thread-unsafe problems).

require 'diver_down-trace'

class M
  def f
    N.new.g
  end
end

class N
  def g
  end
end

defn = DiverDown::Trace::Tracer.new(module_set: { modules: [M] }).trace do
  10.times.map do
    Thread.new { 10_000.times { M.new.f } }
  end.each(&:join)
end

pp defn.to_h

$ ruby -Ilib/ tmp/test.rb
#<Thread:0x000000011ff92f48 tmp/test.rb:16 run> terminated with exception (report_on_exception is true):
/Users/kuwabara.masataka/ghq/github.com/alpaca-tc/diver_down/lib/diver_down/trace/call_stack.rb:52:in 'DiverDown::Trace::CallStack#pop': DiverDown::Trace::CallStack::StackEmptyError (DiverDown::Trace::CallStack::StackEmptyError)
        from /Users/kuwabara.masataka/ghq/github.com/alpaca-tc/diver_down/lib/diver_down/trace/session.rb:52:in 'block in DiverDown::Trace::Session#build_trace_point'
        from tmp/test.rb:16:in 'block (3 levels) in <main>'
/Users/kuwabara.masataka/ghq/github.com/alpaca-tc/diver_down/lib/diver_down/trace/call_stack.rb:52:in 'DiverDown::Trace::CallStack#pop': DiverDown::Trace::CallStack::StackEmptyError (DiverDown::Trace::CallStack::StackEmptyError)
        from /Users/kuwabara.masataka/ghq/github.com/alpaca-tc/diver_down/lib/diver_down/trace/session.rb:52:in 'block in DiverDown::Trace::Session#build_trace_point'
        from tmp/test.rb:16:in 'block (3 levels) in <main>'

Solution

This is because several threads update the same DiverDown::Trace::CallStack object.
So I separated the call stack object for each thread.

[alpaca-tc]

Thank you for the great PR!