Workaround for Ruby 2.1 not having a public thread_native.h header

Hopefully we can deprecate Ruby 2.1 soon.

.rubocop.yml

@@ -375,3 +375,6 @@ Style/TrailingCommaInHashLiteral:
 Style/GlobalVars:
   Exclude:
     - 'ext/ddtrace_profiling_native_extension/extconf.rb'
+
+Naming/VariableNumber:
+  Enabled: false

ext/ddtrace_profiling_native_extension/clock_id_from_pthread.c

@@ -9,7 +9,12 @@
 #include <errno.h>
 
 #include <ruby.h>
+
+#ifdef RUBY_2_1_WORKAROUND
+#include <thread_native.h>
+#else
 #include <ruby/thread_native.h>
+#endif
 
 #include "private_vm_api_access.h"
 

ext/ddtrace_profiling_native_extension/extconf.rb

@@ -116,10 +116,18 @@ def add_compiler_flag(flag)
   # This gem ships source code copies of these VM headers for the different Ruby VM versions;
   # see https://github.com/ruby-debug/debase-ruby_core_source for details
 
+  thread_native_for_ruby_2_1 = proc { true }
+  if RUBY_VERSION < '2.2'
+    # This header became public in Ruby 2.2, but we need to pull it from the private headers folder for 2.1
+    thread_native_for_ruby_2_1 = proc { have_header('thread_native.h') }
+    $defs << '-DRUBY_2_1_WORKAROUND'
+  end
+
   create_header
 
   require 'debase/ruby_core_source'
   dir_config('ruby') # allow user to pass in non-standard core include directory
 
-  Debase::RubyCoreSource.create_makefile_with_core(proc { have_header('vm_core.h') }, EXTENSION_NAME)
+  Debase::RubyCoreSource
+    .create_makefile_with_core(proc { have_header('vm_core.h') && thread_native_for_ruby_2_1.call }, EXTENSION_NAME)
 end

spec/ddtrace/contrib/rails/rack_spec.rb

@@ -24,7 +24,7 @@
     }
   end
 
-  let(:rails_older_than_3_2) do # rubocop:disable Naming/VariableNumber
+  let(:rails_older_than_3_2) do
     Gem::Version.new(Rails::VERSION::STRING) < Gem::Version.new('3.2')
   end