fix(pruner): history indices cursor.prev() emptiness check

[shekhirin]

Problem

First row in AccountHistory and StorageHistory tables wasn't pruned, but instead the row after it was. The issue is in this snippet:

reth/crates/prune/src/pruner.rs

Lines 690 to 709 in 34b68de

	if let Some(prev_value) = cursor
	.prev()?
	.filter(|(prev_key, _)| key_matches(prev_key, &key))
	.map(|(_, prev_value)| prev_value)
	{
	// If current shard is the last shard for the sharded key that has
	// previous shards, replace it with the previous shard.
	cursor.delete_current()?;
	// Upsert will replace the last shard for this sharded key with the
	// previous value.
	cursor.upsert(key.clone(), prev_value)?;
	} else {
	// If there's no previous shard for this sharded key,
	// just delete last shard completely.
	// Jump back to the original last shard.
	cursor.next()?;
	// Delete shard.
	cursor.delete_current()?;
	}

When we move a cursor to the previous row, it can be the first row of the table, so the cursor will not be actually moved and the cursor.prev() will return None. We assumed that we always move it, so the next movement forward via cursor.next() was actually going forward too much.

Solution

Check that previous row wasn't empty and we actually moved the cursor:

reth/crates/prune/src/pruner.rs

Lines 706 to 708 in 434d0a0

	if prev_row.is_some() {
	cursor.next()?;
	}

Also, save on a no-op seek_exact if we were already at the last shard:

reth/crates/prune/src/pruner.rs

Lines 724 to 727 in 4a5d8d2

	// Jump to the last shard for this key, if current key isn't already the last shard.
	if key.as_ref().highest_block_number != u64::MAX {
	cursor.seek_exact(last_key(&key))?;
	}

The rest is just reformatting of the comments.

[codecov]

Codecov Report

Merging #4301 (4a5d8d2) into main (34b68de) will increase coverage by 0.03%.
The diff coverage is 37.50%.

Files Changed	Coverage Δ
crates/prune/src/pruner.rs	78.63% <37.50%> (-0.08%)	⬇️

... and 9 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	16.84% <0.00%> (+<0.01%)	⬆️
unit-tests	63.80% <37.50%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	26.12% <ø> (ø)
blockchain tree	82.56% <ø> (ø)
pipeline	90.07% <ø> (ø)
storage (db)	74.77% <ø> (ø)
trie	94.81% <ø> (ø)
txpool	48.22% <ø> (ø)
networking	77.71% <ø> (+0.16%)	⬆️
rpc	58.81% <ø> (-0.01%)	⬇️
consensus	63.53% <ø> (ø)
revm	32.01% <ø> (ø)
payload builder	6.82% <ø> (ø)
primitives	86.17% <ø> (+0.02%)	⬆️

[joshieDo]

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