POC: use shared ArrayBuffer for nextv()
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This makes `db.iterator()` with buffer encoding as fast as an iterator with utf8 encoding. The approach is: 1. In each `nextv()` call, create a `std::vector<char>` to hold the raw data of multiple entries 2. Copy LevelDB slices directly into that vector with `memcpy()` 3. Create an ArrayBuffer backed by the vector 4. In JS, split it into Buffers, each backed by the same ArrayBuffer but using a different offset. Apart from this being an incomplete implementation (it makes utf8 slower because the C++ side is buffer-only meaning JS has to transcode buffer to utf8), the approach has a downside: if userland code keeps a reference to just one of the Buffers, the entire ArrayBuffer is kept alive too. I.e. it costs memory. For now this PR is just a reference. The ideal solution (for this particular bottleneck) sits somewhere in between. For example, I might take just the ArrayBuffer concept, to replace use of `napi_create_buffer_copy()`.
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This makes
db.iterator()with buffer encoding as fast as an iterator with utf8 encoding. The approach is:nextv()call, create astd::vector<char>to hold the raw data of multiple entriesmemcpy()Apart from this being an incomplete implementation (it makes utf8 slower because the C++ side is buffer-only meaning JS has to transcode buffer to utf8), the approach has a downside: if userland code keeps a reference to just one of the Buffers, the entire ArrayBuffer is kept alive too. I.e. it costs memory.
For now this PR is just a reference. The ideal solution (for this particular bottleneck) sits somewhere in between. For example, I might take just the ArrayBuffer concept, to replace use of
napi_create_buffer_copy().