[GitHub] [accumulo] ctubbsii commented on issue #1064: Native maps crash (segfault) when calling LinkedBlockAllocator.deleteLast()

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ctubbsii commented on issue #1064: Native maps crash (segfault) when calling LinkedBlockAllocator.deleteLast()
URL: https://github.com/apache/accumulo/issues/1064#issuecomment-478173489
 
 
   So, it looks like the problem is that we do not guard our vector against calls to `back()` when it is empty. When empty, `vector.back()` is undefined. Apparently, this changed in behavior between libtstdc++ 8.2 and 8.3... but it shouldn't have mattered. We should not have been trying to call `back()` when the vector was empty.
   
   Something in our code assumes that `bigBlocks` will not be empty when `LinkedBlockAllocator::deleteLast` is called, but that assumption is wrong in some circumstances. I haven't yet figured out which circumstances, but the following guard does cause the test to work:
   
   ```patch
   diff --git a/server/native/src/main/c++/nativeMap/BlockAllocator.h b/server/native/src/main/c++/nativeMap/BlockAllocator.h
   index b7eb60e22d..da8e53fa0d 100644
   --- a/server/native/src/main/c++/nativeMap/BlockAllocator.h
   +++ b/server/native/src/main/c++/nativeMap/BlockAllocator.h
   @@ -124,6 +124,8 @@ struct LinkedBlockAllocator {
            blocks.back().rollback(p);
            lastAlloc = NULL;
            return;
   +      }else if(bigBlocks.empty()){
   +        return;
          }else if(bigBlocks.back().ptr == p){
            memused -= (sizeof(BigBlock) + bigBlocks.back().length);
            bigBlocks.pop_back();
   ```

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