[ruby-core:68191] [Ruby trunk - Bug #10871] [Open] Sclass thread unsafe due to CREF sharing

Issue #10871 has been reported by Evan Phoenix.

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Bug #10871: Sclass thread unsafe due to CREF sharing
https://bugs.ruby-lang.org/issues/10871

* 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
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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!



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