Challenge: The code review bug that gives me nightmares–The fix November 3, 2021 Author: Oren Eini, CEO RavenDB
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Challenge: The code review bug that gives me nightmares–The fix

by  Oren Eini
published: November 3, 2021 | updated: November 22, 2021
RavenDB News

After presenting the issue of how to return items to the array pool without creating a use after free bug, I asked you how you would fix that. There are several ways to try to do that, you can use reference counting scheme, or try to use locking, etc. All of those options are expensive, what is worse, they are expensive on a routine basis, not just for the free the buffer code path.

Instead, I changed the way we are handling returning the buffer. Take a look at the following code:

This may require some explanation. I’m using a ConditionaWeakTable here, that was added to the runtime to enable dynamic properties on objects. Basically, it creates a table that you can lookup by an object to get a key. The most important feature is that the runtime ensures that the associated reference lifetime match the key object lifetime. In other words, when we add the buffer in the eviction callback, we ensure that the ReturnBuffer we register will live at least as long as the buffer.

That means that we can let the GC do the verification job. We’ll now return the buffer back to the pool only after the GC has ensured that there are no outstanding references to it. Not a lot of code, and an elegant solution. This also ensures that we are only paying the code on eviction (likely rare), and not all the time.

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