Potential Logic Issue in Prune Shard Handling for u64::MAX Shards

[AshinGau]

Problem Description

There appears to be a logical inconsistency in the prune shard handling code in crates/prune/prune/src/segments/user/history.rs, specifically in the prune_shard function around lines 114-143.

Current Logic

When a shard becomes empty after pruning (i.e., higher_blocks.is_empty()), the code distinguishes between two cases:

1. Regular shards (highest_block_number != u64::MAX): Simply delete the shard
2. Last shards (highest_block_number == u64::MAX): Check for previous shard and potentially merge

if higher_blocks.is_empty() {
    if key.as_ref().highest_block_number == u64::MAX {
        // Complex logic to check previous shard and potentially merge
        let prev_row = cursor.prev()?;
        match prev_row {
            Some((prev_key, prev_value)) if key_matches(&prev_key, &key) => {
                cursor.delete_current()?;
                cursor.upsert(RawKey::new(key), &prev_value)?;
                Ok(PruneShardOutcome::Updated)
            }
            _ => {
                cursor.delete_current()?;
                Ok(PruneShardOutcome::Deleted)
            }
        }
    } else {
        // Simple deletion for non-u64::MAX shards
        cursor.delete_current()?;
        Ok(PruneShardOutcome::Deleted)
    }
}

The Issue

The logic seems inconsistent because:

1. Pruning filter: blocks.iter().skip_while(|block| *block <= to_block)
2. Shard ordering: Shards are ordered by highest_block_number
3. Logical consequence: If the last shard (u64::MAX) is filtered empty, all previous shards should also be filtered empty

Why This Should Be True

• Shards are ordered by highest_block_number
• The pruning filter removes all blocks <= to_block
• If the last shard (with highest highest_block_number) is empty after filtering, it means all its blocks were <= to_block
• Since previous shards have lower highest_block_number, they should contain even more blocks that are <= to_block
• Therefore, previous shards should also be empty after filtering

Proposed Solution

Simplify the logic by removing the special case for u64::MAX shards:

if higher_blocks.is_empty() {
    // Simply delete the shard regardless of whether it's u64::MAX or not
    cursor.delete_current()?;
    Ok(PruneShardOutcome::Deleted)
}

Benefits of Simplification

1. Consistency: All empty shards are handled the same way
2. Correctness: If u64::MAX shard is empty, previous shards should also be empty
3. Simplicity: Removes complex logic that may be unnecessary
4. Performance: Eliminates unnecessary database operations (prev/next cursor movements)

Questions for Discussion

1. Are there any edge cases where the current logic is actually necessary?
2. Is there a specific reason why u64::MAX shards need special handling?
3. Could there be scenarios where previous shards still contain valid data even when the last shard is empty?

Files Affected

• crates/prune/prune/src/segments/user/history.rs (lines 114-143)
• Potentially affects account history and storage history pruning

Testing Considerations

The proposed change should be tested with:

1. Various pruning scenarios
2. Edge cases with shard boundaries
3. Performance comparison between current and simplified logic

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Note: This analysis is based on the assumption that shards are properly ordered by highest_block_number and that the pruning logic is consistent. If there are special cases or edge conditions that justify the current logic, they should be documented and explained.