Fix race condition

Author: k-tsj

lib/power_assert.rb

@@ -65,13 +65,14 @@ def initialize(assertion_proc, assertion_method)
         @message ||= build_assertion_message(@line || '', methods || [], return_values, refs || [], assertion_proc.binding).freeze
       }
       @proc_local_variables = assertion_proc.binding.eval('local_variables').map(&:to_s)
+      target_thread = Thread.current
       @trace = TracePoint.new(:return, :c_return) do |tp|
         next if method_ids and ! method_ids.include?(tp.method_id)
         locs = tp.binding.eval('caller_locations')
         current_diff = locs.length - @base_caller_length
         target_diff = TARGET_CALLER_DIFF[tp.event]
         is_target_bmethod = current_diff < target_diff
-        if is_target_bmethod or current_diff == target_diff
+        if (is_target_bmethod or current_diff == target_diff) and Thread.current == target_thread
           idx = target_diff - TARGET_INDEX_OFFSET[is_target_bmethod ? :bmethod : :method]
           unless path
             path = locs[idx].path