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## Problem Image layers may be nested inside in-memory layers as diagnosed [here](https://github.com/neondatabase/neon/issues/10720#issuecomment-2649419252). The read path doesn't support this and may skip over the image layer, resulting in a failure to reconstruct the page. ## Summary of changes We already support nesting of image layers inside delta layers. The logic lives in `LayerMap::select_layer`. The main goal of this PR is to propagate the candidate in-memory layer down to that point and update the selection logic. Important changes are: 1. Support partial reads for the in-memory layer. Previously, we could only specify the start LSN of the read. We need to control the end LSN too. 2. `LayerMap::ranged_search` considers in-memory layers too. Previously, the search for in-memory layers was done explicitly in `Timeline::get_reconstruct_data_timeline`. Note that `LayerMap::ranged_search` now returns a weak readable layer which the `LayerManager` can upgrade. This dance is such that we can unit test the layer selection logic. 3. Update `LayerMap::select_layer` to consider the candidate in-memory layer too Loosely related drive bys: 1. Remove the "keys not found" tracking in the ranged search. This wasn't used anywhere and it just complicates things. 2. Remove the difficulty map stuff from the layer map. Again, not used anywhere. Closes https://github.com/neondatabase/neon/issues/9185 Closes https://github.com/neondatabase/neon/issues/10720
Pageserver Benchmarks
How to run
To run all benchmarks:
cargo bench
To run a specific file:
cargo bench --bench bench_layer_map
To run a specific function:
cargo bench --bench bench_layer_map -- real_map_uniform_queries