merge revision(s) 58696: [Backport #11384]

	autoload: always wait on loading thread

	We cannot assume autoload_provided/rb_feature_provided returning
	TRUE means it is safe to proceed without waiting.  Another
	thread may call rb_provide_feature before setting the constant
	(via autoload_const_set).  So we must wait until autoload is
	completed by another thread.

	Note: this patch was tested with an explicit rb_thread_schedule
	in rb_provide_feature to make the race condition more apparent
	as suggested by <s.wanabe@gmail.com>:
	> --- a/load.c
	> +++ b/load.c
	> @@ -563,6 +563,7 @@ rb_provide_feature(VALUE feature)
	>      rb_str_freeze(feature);
	>
	>      rb_ary_push(features, rb_fstring(feature));
	> +rb_thread_schedule();
	>      features_index_add(feature, INT2FIX(RARRAY_LEN(features)-1));
	>      reset_loaded_features_snapshot();
	>  }

	* variable.c (check_autoload_required): do not assume a provided
	  feature means autoload is complete, always wait if autoload is
	  being performed by another thread.
	  [ruby-core:81105] [Bug #11384] Thanks to <s.wanabe@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59303 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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