normal	2017-05-13 06:52:04 +0900 (Sat, 13 May 2017)

  New Revision: 58696

  https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=58696

  Log:
    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@g...>:
    > --- 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@g...>

  Modified files:
    trunk/variable.c
Index: variable.c
===================================================================
--- variable.c	(revision 58695)
+++ variable.c	(revision 58696)
@@ -1987,6 +1987,17 @@ check_autoload_required(VALUE mod, ID id https://github.com/ruby/ruby/blob/trunk/variable.c#L1987
     if (!RSTRING_LEN(file) || !*RSTRING_PTR(file)) {
 	rb_raise(rb_eArgError, "empty file name");
     }
+
+    /*
+     * if somebody else is autoloading, we MUST wait for them, since
+     * rb_provide_feature can provide a feature before autoload_const_set
+     * completes.  We must wait until autoload_const_set finishes in
+     * the other thread.
+     */
+    if (ele->state && ele->state->thread != rb_thread_current()) {
+	return load;
+    }
+
     loading = RSTRING_PTR(file);
     safe = rb_safe_level();
     rb_set_safe_level_force(0);

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