Issue #10871 has been updated by Griffin Smith. I also would very much like a backport of this, as it effectively makes certain bits of "idiomatic" ruby inherently (and very unexpectedly) completely thread-unsafe - see https://github.com/puma/puma/issues/647 ---------------------------------------- Bug #10871: Sclass thread unsafe due to CREF sharing https://bugs.ruby-lang.org/issues/10871#change-52408 * Author: Evan Phoenix * Status: Open * Priority: High * Assignee: * ruby -v: 2.2.0p0, trunk * Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN ---------------------------------------- When entering an sclass, the context is tracked via the same cref mechanism used for class and module, specifically on the iseq->cref_stack. The bug is that the cref_stack is the wrong place to put the new cref because the scope is specific only to that sclass body. Mutating and using the iseq->cref_stack causes any code that reads the cref via this cref_stack to incorrectly pick up the sclass instance instead of the proper scope! This is major thread safety bug because it means that all uses of `class << obj` are thread-unsafe and can cause random code to fail. Here is a simple reproduction of the bug: https://gist.github.com/evanphx/6eef92f2c40662a4171b I attempted to fix the bug by treating an sclass body the same as an eval, which already has special handling for cref's but I don't understand the code enough to make that change quickly. I believe this is a major bug and hope that ruby-core can address it soon. Thank you! -- https://bugs.ruby-lang.org/