ChallengeFind the bug in the fix–answer

time to read 2 min | 347 words

I am writing this answer before people had a chance to answer the actual challenge, so I hope people caught it. This was neither easy nor obvious to catch, because it was hiding with a pile of other stuff and the bug is a monster to figure out.

In case you need a reminder, here is the before & after code:

Look at line 18 in the second part. If we tried to allocate native memory and failed, we would try again, this this with the requested amount.

The logic here is that we typically want to request memory in power of 2 increments. So if asked for 17MB, we’ll allocate 32MB. This code is actually part of our memory allocator, which request memory from the operating system, so it is fine if we allocate more, we’ll just use that in a bit. However, if we don’t have enough memory to allocate 32MB, maybe we do have enough to allocate 17MB. And in many cases, we do, which allow the system to carry on operating.

Everyone is happy, right? Look at line 21 in the second code snippet. We set the allocated size to the size we wanted to allocate, not the actual size we allocated.

We allocated 17MB, we think we allocated 32MB, and now everything can happen.

This is a nasty thing to figure out. If you are lucky, this will generate an access violation when trying to get to that memory you think you own. If you are not lucky, this memory was actually allocated to your process, which means that you are now corrupting some totally random part of memory in funny ways. And that means that in some other time you’ll be start seeing funny behaviors and impossible results and tear your hair out trying to figure it out.

To make things worse, this is something that only happens when you run out of memory, so you are already suspicious about pretty much everything that is going on there. Nasty, nasty, nasty.

I might need a new category of bugs: “Stuff that makes you want to go ARGH!”

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