The doc_diff calculator is used on the pipeline update path for computing a diff between the update's pre-image and post-image. This code does roughly one allocation per diff-node, in order to build a diff tree.

This is rather inefficient and we should look into ways of avoiding the allocations. Namely,

• Restructure the code to avoid creating a tree at all. This may not be possible but is worth a look.
• Allocate space for the tree nodes all at once. Essentially a bump allocator. There may also be generic infrastructure for this type of pattern that already exists, and if so, we should try to use it.
  • This should be a good candidate for a bump allocator since we create the diff tree, generate a BSON from it, and then throw the diff tree away. The lifetime of the diff tree is relatively short and we dispose of the whole thing at once.