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ChallengeFind the bug in the fix
time to read 1 min | 72 words
We are working on improving the reliability of RavenDB under a host of scenarios This week it is low memory conditions. We made some fixes, and introduced a horrible bug.
Here is the code, can you see what the error is? Here are the first and second versions of the code. The second version is meant to be more robust to running under low memory conditions, but it is actually much worse.
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Comments
_allocated = newSize; In case of exception must be _allocated = requestedSize;
I can guess, that Bits.PowerOfTwo(requestedSize) returns nearest size as power of two, to avoid having many different size allocated buffers, as they are probably reused. So in case of exception you would create buffer of size, that is not power of two and probably some logic won't be able to find it for reuse, so under memory pressure you will waste more of it.
a nitpicker:
could be changed to
It's still allocating in the catch block, and not on a power of two.
Tom obviously gave the correct answer, but note that both of your snippets leak the previously allocated buffer.
(the real code doesn't leak, it keeps the pointer around to free it at a later time)
Bug is you cannot handle an out of memory exception.
Lucas, You are correct. I missed that when I stripped the code to the bare essentials
Dan, why on earth not? That is pretty common and in some case, explicitly something that you want to do.
Sorry, should have clarified better. If the framework is throwing the exception, they recommend to fail immediately.
https://msdn.microsoft.com/en-us/library/system.outofmemoryexception(v=vs.110).aspx
OutOfMemoryException is pre-allocated by the runtime to make sure instance exists even if no memory is left to create it - and then this creates some mess when there is another allocation inside catch block?
And also, looks like your code doesn't care about the previous buffer (both version #1 and #2) referenced by _ptrStart and _ptrCurrent - you allocate a new buffer and discard the old one. But inside GrowArena, if your mem allocation fails, you still hold on to the old buffer and dont allow the GC to collect it. If your intent is to throw it away it would be nice to let GC collect the old buffer before allocating a bigger one, increasing a chance of successful allocation
Dan, In this case, this depend on what exactly is throwing it. But even in the CLR, if the pre-allocated exception is thrown, there are stuff you can do. The recommendation there is mostly from the time we build primarily business applications with .NET Now that we are building servers, handling low resource utilization scenarios became much more important.
Rafal, This is actually called from my code that tries to "effectively" do
malloc, so nothing to do with the runtime.Rafal, Yes, I simplified the code a bit too much. And note that this is allocating native memory
If the allocation is done in the catch block, it should be
Possible effects ranging from 'simple' access-violations and server crashes (paradoxically best-case) to buffer-overrun vulnerabilities, or worst of all, data corruption in case another buffer sits next to it in memory.
Kurbien, Yes, and +10 on the effects
Understood. Thanks for sharing!
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