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Merge 'Pager: clear overflow cells when freeing page' from Jussi Saurio

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## Background
The `balance_non_root` procedure can end up freeing a page if the pages
to be balanced can fit the required combined number of cells in less
pages, even if the page that triggered balancing is overfull. This can
then free the originally overfull pages, leaving a non-zero
`overflow_cells` on the in-mem representation of the page.
```rust
balance_non_root: page=305, overflow_cells=0
balance_non_root: page=304, overflow_cells=0
balance_non_root: page=302, overflow_cells=1
pre_edit_page(page=304, page_idx=0, new_cells=4, old_cells=1, cells_per_page_old=[1, 3, 9, 0, 0], cells_per_page_new=[4, 9, 9, 0, 0], cell_array_count=9)
edit_page start_old_cells=0 start_new_cells=0 number_new_cells=4 cell_array=9 end_old_cells=1 end_new_cells=4
pre_edit_page(page=305, page_idx=1, new_cells=4, old_cells=1, cells_per_page_old=[1, 3, 9, 0, 0], cells_per_page_new=[4, 9, 9, 0, 0], cell_array_count=9)
edit_page start_old_cells=2 start_new_cells=5 number_new_cells=4 cell_array=9 end_old_cells=3 end_new_cells=9
balance_non_root: sibling_count_new=2, sibling_count=3

// Custom assertion to demonstrate this:
thread 'main' panicked at core/storage/pager.rs:1127:29:
Pager::free_page: In memory page with id 302 has overflow cells
```
## Why is this a problem
Right now this is not an immediate problem, because we always allocate
brand new pages. However, in #2233 we begin to reuse pages from the
freelist for page allocation to improve performance and reduce database
size bloat. In that PR, the `balance_non_root` procedure will calculate
cell counts incorrectly in `edit_page()` and panic if: 1. a new
allocated page is taken from the freelist, 2. the page is still in
memory, and 3. and it still contains `overflow_cells`.
## Solution
Clear `page_contents.overflow_cells` when an in-memory page is freed.

Reviewed-by: Pere Diaz Bou <pere-altea@homail.com>

Closes #2238
This commit is contained in:
Jussi Saurio
2025-07-23 13:38:31 +03:00
parent ab92aabb70 f98a9e8939
commit 63f488a1cc
1 changed files with 11 additions and 1 deletions
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12
core/storage/pager.rs
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@@ -1140,7 +1140,17 @@ impl Pager {
let page = match page.clone() {
Some(page) => {
assert_eq!(page.get().id, page_id, "Page id mismatch");
assert_eq!(
page.get().id,
page_id,
"Pager::free_page: Page id mismatch: expected {} but got {}",
page_id,
page.get().id
);
if page.is_loaded() {
let page_contents = page.get_contents();
page_contents.overflow_cells.clear();
}
page
}
None => self.read_page(page_id)?,