When a page with a delta chain is re-read from the page service (e.g., after eviction or connection reopen), block_meta->cumulative_size is set to the raw size of the most recent block instead of the true cumulative total of base + all deltas.

The bug is in __block_disagg_read_multiple (block_disagg_read.c). The function receives the correct cumulative size via its size parameter (from cookie.size on the address cookie), but the loop overwrites size on each iteration with individual block sizes:

size = (uint32_t)current->size;

The block_meta->cumulative_size = size assignment was inside the result == last block, which executes on the first loop iteration – after size has already been overwritten. So cumulative_size ends up holding just the most recent block's raw size rather than the cumulative total.

After re-read, subsequent delta writes compute cookie.size incorrectly:

// In __wti_block_disagg_write (block_disagg_write.c)
cookie.size = block_meta->cumulative_size + size;
// Uses wrong (too small) cumulative_size from the re-read

When the delta chain eventually terminates, the discard path subtracts this too-small cookie.size from bytes_total, causing a permanent leak.

Concrete example:

1. Write base page (4KB): cumulative_size = 4KB, cookie.size = 4KB
2. Write delta1 (1KB): cumulative_size = 5KB, cookie.size = 5KB
3. Write delta2 (1KB): cumulative_size = 6KB, cookie.size = 6KB
4. Page evicted and re-read: cumulative_size = 1KB (BUG – should be 6KB)
5. Write delta3 (1KB): cookie.size = 1KB + 1KB = 2KB (should be 7KB)
6. Delta chain terminates, discard: bytes_total = 2KB instead of = 7KB – 5KB leaked

This leak compounds with every eviction/re-read cycle and is proportional to the size of the base page + early deltas in the chain.

Fix

Move the block_meta->cumulative_size assignment before the loop, while size still holds the function parameter (the correct cumulative from the cookie):

block_meta->cumulative_size = size;

for (result = last; result >= 0; result--) {
    ...
    size = (uint32_t)current->size;
    ...
}

Added a post-loop diagnostic assert that verifies the cookie's cumulative matches the sum of all individual block sizes returned by the page service:

WT_ASSERT(session, block_meta->cumulative_size == block_size_sum);