[ruby-core:60527] [ruby-trunk - Bug #9494] [Open] Race condition in autoload of Digest::SHA256, etc.

Issue #9494 has been reported by Conrad Irwin.

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Bug #9494: Race condition in autoload of Digest::SHA256, etc.
https://bugs.ruby-lang.org/issues/9494

* Author: Conrad Irwin
* Status: Open
* Priority: Normal
* Assignee: 
* Category: ext
* Target version: 
* ruby -v: 2.0.0p247
* Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN, 2.1: UNKNOWN
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At the moment the digest extension uses const_missing to implement autoload. Unfortunately there's a race condition: another thread can run after the constant is defined, but before it is initialized. (http://git.io/YW3bbA)

There's a minimal test case here: http://git.io/5umBqQ — this was happening to us regularly in production because each sidekiq worker would race to create a hash.

The work around we have deployed is to require digest/sha2.so pre-emptively. And the attached patch does that.

An alternative solution would be to use `autoload`, but that is deprecated; or to change the way meta-data is stored for Digest classes so that it can be accessed immediately. There's no way to use a mutex inside const_missing to fix this, because the bug is that the constant becomes not-missing before it is fully defined.

---Files--------------------------------
require-digest-upfront.patch (1006 Bytes)


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