kvdb-overlay/tests/database_overlay_state.rs

/* This file is part of DarkFi (https://dark.fi)
 *
 * Copyright (C) 2026-2026 Dyne.org foundation
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

//! Simulate the creation of a [`DatabaseOverlay`] on top of an entire
//! [`Database`] instance, and perform diffs and writes to verify
//! overlay's cache diff functionality.

use kvdb_overlay::{Database, DatabaseOverlay, Result};

const TREE_1: &str = "_tree1";
const TREE_2: &str = "_tree2";
const TREE_3: &str = "_tree3";
const TREE_4: &str = "_tree4";
const TREE_5: &str = "_tree5";
const TREE_6: &str = "_tree6";

#[test]
fn database_overlay_state() -> Result<()> {
    // Initialize database
    let (db, _folder) = Database::open_temp()?;

    // Initialize trees with some values
    let tree_1 = db.open_tree_default(TREE_1)?;
    tree_1.insert(b"key_a", b"val_a")?;
    let tree_4 = db.open_tree_default(TREE_4)?;
    tree_4.insert(b"key_g", b"val_g")?;
    tree_4.insert(b"key_j", b"val_j")?;

    // Initialize overlay
    let mut overlay = DatabaseOverlay::new(&db, vec![])?;

    // Open trees in the overlay
    overlay.open_tree_default(TREE_1, false)?;
    overlay.open_tree_default(TREE_3, false)?;
    overlay.open_tree_default(TREE_4, false)?;
    overlay.open_tree_default(TREE_6, false)?;

    // Make a vector to keep track of changes
    let mut sequence = vec![];

    // Perform some changes and grab their differences
    overlay.insert(TREE_1, b"key_b", b"val_b")?;
    overlay.insert(TREE_3, b"key_i", b"val_i")?;
    overlay.insert(TREE_4, b"key_k", b"val_k")?;
    overlay.remove(TREE_4, b"key_g")?;
    sequence.push(overlay.diff(&sequence)?);

    overlay.insert(TREE_1, b"key_b", b"val_bb")?;
    overlay.remove(TREE_1, b"key_a")?;
    overlay.open_tree_default(TREE_2, false)?;
    overlay.insert(TREE_2, b"key_d", b"val_d")?;
    overlay.insert(TREE_2, b"key_e", b"val_e")?;
    overlay.drop_tree(TREE_3)?;
    overlay.insert(TREE_4, b"key_k", b"val_kk")?;
    overlay.open_tree_default(TREE_5, false)?;
    overlay.insert(TREE_5, b"key_h", b"val_h")?;
    sequence.push(overlay.diff(&sequence)?);

    overlay.insert(TREE_1, b"key_a", b"val_a")?;
    overlay.remove(TREE_1, b"key_b")?;
    overlay.insert(TREE_1, b"key_c", b"val_c")?;
    overlay.remove(TREE_2, b"key_e")?;
    overlay.insert(TREE_2, b"key_f", b"val_f")?;
    overlay.insert(TREE_4, b"key_l", b"val_l")?;
    overlay.drop_tree(TREE_4)?;
    sequence.push(overlay.diff(&sequence)?);

    // Verify overlay has the correct state
    assert_eq!(overlay.state.initial_tree_names.len(), 2);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert_eq!(overlay.state.new_tree_names, [TREE_6, TREE_2, TREE_5]);
    assert_eq!(overlay.state.caches.len(), 4);
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 2);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_a".as_slice()),
        Some(&b"val_a".to_vec())
    );
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_c".as_slice()),
        Some(&b"val_c".to_vec())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_b".as_slice()),
        Some(&b"key_b".into())
    );
    let tree_6_cache = overlay.state.caches.get(TREE_6).unwrap();
    assert!(tree_6_cache.state.cache.is_empty());
    assert!(tree_6_cache.state.removed.is_empty());
    let tree_2_cache = overlay.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 2);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_d".as_slice()),
        Some(&b"val_d".to_vec())
    );
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".to_vec())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    let tree_5_cache = overlay.state.caches.get(TREE_5).unwrap();
    assert_eq!(tree_5_cache.state.cache.len(), 1);
    assert_eq!(
        tree_5_cache.state.cache.get(b"key_h".as_slice()),
        Some(&b"val_h".to_vec())
    );
    assert!(tree_5_cache.state.removed.is_empty());
    assert_eq!(overlay.state.dropped_trees.len(), 2);
    let dropped_tree_3_cache = overlay.state.dropped_trees.get(TREE_3).unwrap();
    assert!(dropped_tree_3_cache.cache.is_empty());
    assert!(dropped_tree_3_cache.removed.is_empty());
    let dropped_tree_4_cache = overlay.state.dropped_trees.get(TREE_4).unwrap();
    assert_eq!(dropped_tree_4_cache.cache.len(), 2);
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_g".as_slice()),
        Some(&(None, b"val_g".into()))
    );
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_j".as_slice()),
        Some(&(None, b"val_j".into()))
    );
    assert!(dropped_tree_4_cache.removed.is_empty());

    // Verify diffs sequence is correct
    assert_eq!(sequence.len(), 3);

    assert_eq!(sequence[0].initial_tree_names.len(), 2);
    assert!(sequence[0].initial_tree_names.contains(&TREE_1.into()));
    assert!(sequence[0].initial_tree_names.contains(&TREE_4.into()));
    assert_eq!(sequence[0].caches.len(), 4);
    let (tree_1_cache, drop) = sequence[0].caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.cache.len(), 1);
    assert_eq!(
        tree_1_cache.cache.get(b"key_b".as_slice()),
        Some(&(None, b"val_b".to_vec()))
    );
    assert!(tree_1_cache.removed.is_empty());
    assert!(!drop);
    let (tree_3_cache, drop) = sequence[0].caches.get(TREE_3).unwrap();
    assert_eq!(tree_3_cache.cache.len(), 1);
    assert_eq!(
        tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".to_vec()))
    );
    assert!(tree_3_cache.removed.is_empty());
    assert!(!drop);
    let (tree_4_cache, drop) = sequence[0].caches.get(TREE_4).unwrap();
    assert_eq!(tree_4_cache.cache.len(), 1);
    assert_eq!(
        tree_4_cache.cache.get(b"key_k".as_slice()),
        Some(&(None, b"val_k".to_vec()))
    );
    assert_eq!(tree_4_cache.removed.len(), 1);
    assert_eq!(
        tree_4_cache.removed.get(b"key_g".as_slice()),
        Some(&b"val_g".into())
    );
    assert!(!drop);
    let (tree_6_cache, drop) = sequence[0].caches.get(TREE_6).unwrap();
    assert!(tree_6_cache.cache.is_empty());
    assert!(tree_6_cache.removed.is_empty());
    assert!(!drop);
    assert!(sequence[0].dropped_trees.is_empty());
    assert_eq!(sequence[0], sequence[0].inverse().inverse());

    assert_eq!(sequence[1].initial_tree_names.len(), 4);
    assert!(sequence[1].initial_tree_names.contains(&TREE_1.into()));
    assert!(sequence[1].initial_tree_names.contains(&TREE_4.into()));
    assert!(sequence[1].initial_tree_names.contains(&TREE_6.into()));
    assert!(sequence[1].initial_tree_names.contains(&TREE_3.into()));
    assert_eq!(sequence[1].caches.len(), 4);
    let (tree_1_cache, drop) = sequence[1].caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.cache.len(), 1);
    assert_eq!(
        tree_1_cache.cache.get(b"key_b".as_slice()),
        Some(&(Some(b"val_b".into()), b"val_bb".into()))
    );
    assert_eq!(tree_1_cache.removed.len(), 1);
    assert_eq!(
        tree_1_cache.removed.get(b"key_a".as_slice()),
        Some(&b"val_a".into())
    );
    assert!(!drop);
    let (tree_2_cache, drop) = sequence[1].caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.cache.len(), 2);
    assert_eq!(
        tree_2_cache.cache.get(b"key_d".as_slice()),
        Some(&(None, b"val_d".into()))
    );
    assert_eq!(
        tree_2_cache.cache.get(b"key_e".as_slice()),
        Some(&(None, b"val_e".into()))
    );
    assert!(tree_2_cache.removed.is_empty());
    assert!(!drop);
    let (tree_4_cache, drop) = sequence[1].caches.get(TREE_4).unwrap();
    assert_eq!(tree_4_cache.cache.len(), 1);
    assert_eq!(
        tree_4_cache.cache.get(b"key_k".as_slice()),
        Some(&(Some(b"val_k".into()), b"val_kk".into()))
    );
    assert!(tree_4_cache.removed.is_empty());
    assert!(!drop);
    let (tree_5_cache, drop) = sequence[1].caches.get(TREE_5).unwrap();
    assert_eq!(tree_5_cache.cache.len(), 1);
    assert_eq!(
        tree_5_cache.cache.get(b"key_h".as_slice()),
        Some(&(None, b"val_h".into()))
    );
    assert!(tree_5_cache.removed.is_empty());
    assert!(!drop);
    assert_eq!(sequence[1].dropped_trees.len(), 1);
    let (dropped_tree_3_cache, restored) = sequence[1].dropped_trees.get(TREE_3).unwrap();
    assert_eq!(dropped_tree_3_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(dropped_tree_3_cache.removed.is_empty());
    assert!(!restored);
    assert_eq!(sequence[1], sequence[1].inverse().inverse());

    assert_eq!(sequence[2].initial_tree_names.len(), 5);
    assert!(sequence[2].initial_tree_names.contains(&TREE_1.into()));
    assert!(sequence[2].initial_tree_names.contains(&TREE_4.into()));
    assert!(sequence[2].initial_tree_names.contains(&TREE_6.into()));
    assert!(sequence[2].initial_tree_names.contains(&TREE_2.into()));
    assert!(sequence[2].initial_tree_names.contains(&TREE_5.into()));
    assert_eq!(sequence[2].caches.len(), 2);
    let (tree_1_cache, drop) = sequence[2].caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.cache.len(), 2);
    assert_eq!(
        tree_1_cache.cache.get(b"key_a".as_slice()),
        Some(&(None, b"val_a".into()))
    );
    assert_eq!(
        tree_1_cache.cache.get(b"key_c".as_slice()),
        Some(&(None, b"val_c".into()))
    );
    assert_eq!(tree_1_cache.removed.len(), 1);
    assert_eq!(
        tree_1_cache.removed.get(b"key_b".as_slice()),
        Some(&b"val_bb".into())
    );
    assert!(!drop);
    let (tree_2_cache, drop) = sequence[2].caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.cache.len(), 1);
    assert_eq!(
        tree_2_cache.cache.get(b"key_f".as_slice()),
        Some(&(None, b"val_f".into()))
    );
    assert_eq!(tree_2_cache.removed.len(), 1);
    assert_eq!(
        tree_2_cache.removed.get(b"key_e".as_slice()),
        Some(&b"val_e".into())
    );
    assert!(!drop);
    assert_eq!(sequence[2].dropped_trees.len(), 1);
    let (dropped_tree_4_cache, restored) = sequence[2].dropped_trees.get(TREE_4).unwrap();
    assert_eq!(dropped_tree_4_cache.cache.len(), 2);
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_j".as_slice()),
        Some(&(None, b"val_j".into()))
    );
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_k".as_slice()),
        Some(&(Some(b"val_k".into()), b"val_kk".into()))
    );
    assert!(dropped_tree_4_cache.removed.is_empty());
    assert!(!restored);
    assert_eq!(sequence[2], sequence[2].inverse().inverse());

    // Now we are going to apply each diff and check that the database
    // and the overlay have been mutated accordingly.
    // Don't forget to flush.
    overlay.apply_diff(&sequence[0])?;
    db.flush_default_mode()?;

    // All trees should be present in the database
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 6);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_3.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));
    assert!(db_tree_names.contains(&TREE_6.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_b".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert!(tree_2.is_empty()?);
    let tree_3 = db.open_tree_default(TREE_3)?;
    assert_eq!(tree_3.len()?, 1);
    assert_eq!(tree_3.get(b"key_i")?, Some(b"val_i".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 2);
    assert_eq!(tree_4.get(b"key_j")?, Some(b"val_j".into()));
    assert_eq!(tree_4.get(b"key_k")?, Some(b"val_k".into()));
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert!(tree_5.is_empty()?);
    let tree_6 = db.open_tree_default(TREE_6)?;
    assert!(tree_6.is_empty()?);

    assert_eq!(overlay.state.initial_tree_names.len(), 4);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_3.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_6.into()));
    assert_eq!(overlay.state.new_tree_names, [TREE_2, TREE_5]);
    // Tree 6 was stale so it should have been closed
    assert_eq!(overlay.state.caches.len(), 3);
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 2);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_a".as_slice()),
        Some(&b"val_a".into())
    );
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_c".as_slice()),
        Some(&b"val_c".into())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_b".as_slice()),
        Some(&b"key_b".into())
    );
    let tree_2_cache = overlay.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 2);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_d".as_slice()),
        Some(&b"val_d".into())
    );
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    let tree_5_cache = overlay.state.caches.get(TREE_5).unwrap();
    assert_eq!(tree_5_cache.state.cache.len(), 1);
    assert_eq!(
        tree_5_cache.state.cache.get(b"key_h".as_slice()),
        Some(&b"val_h".into())
    );
    assert!(tree_5_cache.state.removed.is_empty());
    assert_eq!(overlay.state.dropped_trees.len(), 2);
    let dropped_tree_3_cache = overlay.state.dropped_trees.get(TREE_3).unwrap();
    assert_eq!(dropped_tree_3_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(dropped_tree_3_cache.removed.is_empty());
    let dropped_tree_4_cache = overlay.state.dropped_trees.get(TREE_4).unwrap();
    assert_eq!(dropped_tree_4_cache.cache.len(), 2);
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_j".as_slice()),
        Some(&(None, b"val_j".into()))
    );
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_k".as_slice()),
        Some(&(None, b"val_k".into()))
    );
    assert!(dropped_tree_4_cache.removed.is_empty());

    overlay.apply_diff(&sequence[1])?;
    db.flush_default_mode()?;

    // All trees should be present in the database
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 5);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));
    assert!(db_tree_names.contains(&TREE_6.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 1);
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_bb".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 2);
    assert_eq!(tree_2.get(b"key_d")?, Some(b"val_d".into()));
    assert_eq!(tree_2.get(b"key_e")?, Some(b"val_e".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 2);
    assert_eq!(tree_4.get(b"key_j")?, Some(b"val_j".into()));
    assert_eq!(tree_4.get(b"key_k")?, Some(b"val_kk".into()));
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert_eq!(tree_5.len()?, 1);
    assert_eq!(tree_5.get(b"key_h")?, Some(b"val_h".into()));
    let tree_6 = db.open_tree_default(TREE_6)?;
    assert!(tree_6.is_empty()?);

    assert_eq!(overlay.state.initial_tree_names.len(), 5);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_5.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_6.into()));
    assert!(overlay.state.new_tree_names.is_empty());
    // Tree 5 was stale so it should have been closed
    assert_eq!(overlay.state.caches.len(), 2);
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 2);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_a".as_slice()),
        Some(&b"val_a".into())
    );
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_c".as_slice()),
        Some(&b"val_c".into())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_b".as_slice()),
        Some(&b"key_b".into())
    );
    let tree_2_cache = overlay.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 1);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    assert_eq!(overlay.state.dropped_trees.len(), 1);
    let dropped_tree_4_cache = overlay.state.dropped_trees.get(TREE_4).unwrap();
    assert_eq!(dropped_tree_4_cache.cache.len(), 2);
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_j".as_slice()),
        Some(&(None, b"val_j".into()))
    );
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_k".as_slice()),
        Some(&(Some(b"val_k".into()), b"val_kk".into()))
    );
    assert!(dropped_tree_4_cache.removed.is_empty());

    overlay.apply_diff(&sequence[2])?;
    db.flush_default_mode()?;

    // All trees should be present in the database
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 4);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));
    assert!(db_tree_names.contains(&TREE_6.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_c")?, Some(b"val_c".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 2);
    assert_eq!(tree_2.get(b"key_d")?, Some(b"val_d".into()));
    assert_eq!(tree_2.get(b"key_f")?, Some(b"val_f".into()));
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert_eq!(tree_5.len()?, 1);
    assert_eq!(tree_5.get(b"key_h")?, Some(b"val_h".into()));
    let tree_6 = db.open_tree_default(TREE_6)?;
    assert!(tree_6.is_empty()?);

    // Since we removed everything, current overlay must not have
    // diffs over the tree, therefore its safe to keep using it
    assert_eq!(overlay.state.initial_tree_names.len(), 4);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_5.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_6.into()));
    assert!(overlay.state.new_tree_names.is_empty());
    assert!(overlay.state.caches.is_empty());
    assert!(overlay.state.dropped_trees.is_empty());

    let diff = overlay.diff(&[])?;
    assert!(diff.caches.is_empty());
    assert!(diff.dropped_trees.is_empty());

    // We are going to make some changes that we want to revert
    // using the corresponding inverse diff
    overlay.open_tree_default(TREE_1, false)?;
    overlay.insert(TREE_1, b"key_a", b"val_aa")?;
    overlay.insert(TREE_1, b"key_b", b"val_b")?;
    overlay.remove(TREE_1, b"key_c")?;
    overlay.open_tree_default(TREE_2, false)?;
    overlay.remove(TREE_2, b"key_d")?;
    overlay.insert(TREE_2, b"key_f", b"val_ff")?;
    overlay.drop_tree(TREE_5)?;

    // Grab the diff, apply it and verify the database state
    let diff = overlay.diff(&[])?;
    overlay.apply_diff(&diff)?;
    db.flush_default_mode()?;
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 3);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_6.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_aa".into()));
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_b".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 1);
    assert_eq!(tree_2.get(b"key_f")?, Some(b"val_ff".into()));
    let tree_6 = db.open_tree_default(TREE_6)?;
    assert!(tree_6.is_empty()?);

    // Now we grab the diff inverse, apply it and verity the database
    // state.
    overlay.apply_diff(&diff.inverse())?;
    db.flush_default_mode()?;
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 4);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_6.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_c")?, Some(b"val_c".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 2);
    assert_eq!(tree_2.get(b"key_d")?, Some(b"val_d".into()));
    assert_eq!(tree_2.get(b"key_f")?, Some(b"val_f".into()));
    let tree_6 = db.open_tree_default(TREE_6)?;
    assert!(tree_6.is_empty()?);
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert_eq!(tree_5.len()?, 1);
    assert_eq!(tree_5.get(b"key_h")?, Some(b"val_h".into()));

    // Now we are going to revert the diffs sequence going backwards
    // and verify the database state mutates accordingly
    overlay.apply_diff(&sequence[2].inverse())?;
    db.flush_default_mode()?;

    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 5);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_6.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 1);
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_bb".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 2);
    assert_eq!(tree_2.get(b"key_d")?, Some(b"val_d".into()));
    assert_eq!(tree_2.get(b"key_e")?, Some(b"val_e".into()));
    let tree_6 = db.open_tree_default(TREE_6)?;
    assert!(tree_6.is_empty()?);
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 2);
    assert_eq!(tree_4.get(b"key_j")?, Some(b"val_j".into()));
    assert_eq!(tree_4.get(b"key_k")?, Some(b"val_kk".into()));
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert_eq!(tree_5.len()?, 1);
    assert_eq!(tree_5.get(b"key_h")?, Some(b"val_h".into()));

    overlay.apply_diff(&sequence[1].inverse())?;
    db.flush_default_mode()?;

    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 4);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_6.into()));
    assert!(db_tree_names.contains(&TREE_3.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_b".into()));
    let tree_6 = db.open_tree_default(TREE_6)?;
    assert!(tree_6.is_empty()?);
    let tree_3 = db.open_tree_default(TREE_3)?;
    assert_eq!(tree_3.len()?, 1);
    assert_eq!(tree_3.get(b"key_i")?, Some(b"val_i".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 2);
    assert_eq!(tree_4.get(b"key_j")?, Some(b"val_j".into()));
    assert_eq!(tree_4.get(b"key_k")?, Some(b"val_k".into()));

    overlay.apply_diff(&sequence[0].inverse())?;
    db.flush_default_mode()?;

    // Since we removed everything, current overlay must not have
    // diffs over the tree, therefore its safe to keep using it
    assert_eq!(overlay.state.initial_tree_names.len(), 2);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay.state.new_tree_names.is_empty());
    assert!(overlay.state.caches.is_empty());
    assert!(overlay.state.dropped_trees.is_empty());

    let diff = overlay.diff(&[])?;
    assert!(diff.caches.is_empty());
    assert!(diff.dropped_trees.is_empty());

    // The database has now reverted to its original state
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 2);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 1);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 2);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));
    assert_eq!(tree_4.get(b"key_j")?, Some(b"val_j".into()));

    Ok(())
}

#[test]
fn database_overlay_rebuild_state() -> Result<()> {
    // Initialize database
    let (db, _folder) = Database::open_temp()?;

    // Initialize trees with some values
    let tree_1 = db.open_tree_default(TREE_1)?;
    tree_1.insert(b"key_a", b"val_a")?;
    let tree_4 = db.open_tree_default(TREE_4)?;
    tree_4.insert(b"key_g", b"val_g")?;

    // Initialize overlay
    let mut overlay = DatabaseOverlay::new(&db, vec![])?;

    // Open trees in the overlay
    overlay.open_tree_default(TREE_1, false)?;
    overlay.open_tree_default(TREE_3, false)?;

    // Make a vector to keep track of changes
    let mut sequence = vec![];

    // Perform some changes and grab their differences
    overlay.insert(TREE_1, b"key_b", b"val_b")?;
    overlay.insert(TREE_3, b"key_i", b"val_i")?;
    sequence.push(overlay.diff(&sequence)?);

    overlay.insert(TREE_1, b"key_b", b"val_bb")?;
    overlay.remove(TREE_1, b"key_a")?;
    overlay.open_tree_default(TREE_2, false)?;
    overlay.insert(TREE_2, b"key_d", b"val_d")?;
    overlay.insert(TREE_2, b"key_e", b"val_e")?;
    overlay.drop_tree(TREE_3)?;
    sequence.push(overlay.diff(&sequence)?);

    overlay.insert(TREE_1, b"key_a", b"val_a")?;
    overlay.remove(TREE_1, b"key_b")?;
    overlay.insert(TREE_1, b"key_c", b"val_c")?;
    overlay.remove(TREE_2, b"key_e")?;
    overlay.insert(TREE_2, b"key_f", b"val_f")?;
    overlay.drop_tree(TREE_4)?;
    overlay.open_tree_default(TREE_5, false)?;
    overlay.insert(TREE_5, b"key_h", b"val_h")?;
    sequence.push(overlay.diff(&sequence)?);

    // Create a different overlay to rebuild
    // the previous one using the changes sequence
    let mut overlay2 = DatabaseOverlay::new(&db, vec![])?;
    // All trees should be present in the database
    assert_eq!(overlay2.state.initial_tree_names.len(), 5);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_3.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_5.into()));
    assert!(overlay2.state.new_tree_names.is_empty());
    assert!(overlay2.state.caches.is_empty());
    assert!(overlay2.state.dropped_trees.is_empty());

    // Add each diff from the sequence and verify
    // overlay has been mutated accordingly
    overlay2.add_diff(&sequence[0])?;
    assert_eq!(overlay2.state.initial_tree_names.len(), 2);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert_eq!(overlay2.state.new_tree_names, [TREE_3]);
    assert_eq!(overlay2.state.caches.len(), 2);
    let tree_1_cache = overlay2.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 1);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_b".as_slice()),
        Some(&b"val_b".into())
    );
    assert!(tree_1_cache.state.removed.is_empty());
    let tree_3_cache = overlay2.state.caches.get(TREE_3).unwrap();
    assert_eq!(tree_3_cache.state.cache.len(), 1);
    assert_eq!(
        tree_3_cache.state.cache.get(b"key_i".as_slice()),
        Some(&b"val_i".into())
    );
    assert!(tree_3_cache.state.removed.is_empty());
    assert!(overlay2.state.dropped_trees.is_empty());

    overlay2.add_diff(&sequence[1])?;
    assert_eq!(overlay2.state.initial_tree_names.len(), 2);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert_eq!(overlay2.state.new_tree_names, [TREE_2]);
    assert_eq!(overlay2.state.caches.len(), 2);
    let tree_1_cache = overlay2.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 1);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_b".as_slice()),
        Some(&b"val_bb".into())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_a".as_slice()),
        Some(&b"key_a".into())
    );
    let tree_2_cache = overlay2.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 2);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_d".as_slice()),
        Some(&b"val_d".into())
    );
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_e".as_slice()),
        Some(&b"val_e".into())
    );
    assert!(tree_2_cache.state.removed.is_empty());
    assert_eq!(overlay2.state.dropped_trees.len(), 1);
    let dropped_tree_3_cache = overlay2.state.dropped_trees.get(TREE_3).unwrap();
    assert_eq!(dropped_tree_3_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(dropped_tree_3_cache.removed.is_empty());

    // Deviate and create some new records in the overlay
    overlay2.insert(TREE_1, b"key_c", b"val_cc")?;
    overlay2.remove(TREE_2, b"key_e")?;
    overlay2.open_tree_default(TREE_4, false)?;
    overlay2.insert(TREE_4, b"key_f", b"val_f")?;
    overlay2.open_tree_default(TREE_6, false)?;
    overlay2.insert(TREE_6, b"key_h", b"val_h")?;
    overlay2.drop_tree(TREE_6)?;

    // Verify overlay2 has the correct state
    assert_eq!(overlay2.state.initial_tree_names.len(), 2);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert_eq!(overlay2.state.new_tree_names, [TREE_2]);
    assert_eq!(overlay2.state.caches.len(), 3);
    let tree_1_cache = overlay2.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 2);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_b".as_slice()),
        Some(&b"val_bb".into())
    );
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_c".as_slice()),
        Some(&b"val_cc".into())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_a".as_slice()),
        Some(&b"key_a".into())
    );
    let tree_2_cache = overlay2.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 1);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_d".as_slice()),
        Some(&b"val_d".into())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    let tree_4_cache = overlay2.state.caches.get(TREE_4).unwrap();
    assert_eq!(tree_4_cache.state.cache.len(), 1);
    assert_eq!(
        tree_4_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert!(tree_4_cache.state.removed.is_empty());
    assert_eq!(overlay2.state.dropped_trees.len(), 2);
    let dropped_tree_3_cache = overlay2.state.dropped_trees.get(TREE_3).unwrap();
    assert_eq!(dropped_tree_3_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(dropped_tree_3_cache.removed.is_empty());
    let dropped_tree_6_cache = overlay2.state.dropped_trees.get(TREE_6).unwrap();
    assert!(dropped_tree_6_cache.cache.is_empty());
    assert!(dropped_tree_6_cache.removed.is_empty());

    // Now we are going to apply each diff and check that the database
    // and the overlays have been mutated accordingly.
    // Don't forget to flush.
    overlay.apply_diff(&sequence[0])?;
    overlay2.remove_diff(&sequence[0]);
    db.flush_default_mode()?;

    // All trees should be present in the database
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 6);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_3.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));
    assert!(db_tree_names.contains(&TREE_6.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_b".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert!(tree_2.is_empty()?);
    let tree_3 = db.open_tree_default(TREE_3)?;
    assert_eq!(tree_3.len()?, 1);
    assert_eq!(tree_3.get(b"key_i")?, Some(b"val_i".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 1);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert!(tree_5.is_empty()?);
    let tree_6 = db.open_tree_default(TREE_6)?;
    assert!(tree_6.is_empty()?);

    assert_eq!(overlay.state.initial_tree_names.len(), 3);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_3.into()));
    assert_eq!(overlay.state.new_tree_names, [TREE_2, TREE_5]);
    assert_eq!(overlay.state.caches.len(), 3);
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 2);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_a".as_slice()),
        Some(&b"val_a".into())
    );
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_c".as_slice()),
        Some(&b"val_c".into())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_b".as_slice()),
        Some(&b"key_b".into())
    );
    let tree_2_cache = overlay.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 2);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_d".as_slice()),
        Some(&b"val_d".into())
    );
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    let tree_5_cache = overlay.state.caches.get(TREE_5).unwrap();
    assert_eq!(tree_5_cache.state.cache.len(), 1);
    assert_eq!(
        tree_5_cache.state.cache.get(b"key_h".as_slice()),
        Some(&b"val_h".into())
    );
    assert!(tree_5_cache.state.removed.is_empty());
    assert_eq!(overlay.state.dropped_trees.len(), 2);
    let dropped_tree_3_cache = overlay.state.dropped_trees.get(TREE_3).unwrap();
    assert_eq!(dropped_tree_3_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(dropped_tree_3_cache.removed.is_empty());
    let dropped_tree_4_cache = overlay.state.dropped_trees.get(TREE_4).unwrap();
    assert_eq!(dropped_tree_4_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_g".as_slice()),
        Some(&(None, b"val_g".into()))
    );
    assert!(dropped_tree_4_cache.removed.is_empty());

    assert_eq!(overlay2.state.initial_tree_names.len(), 3);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_3.into()));
    assert_eq!(overlay2.state.new_tree_names, [TREE_2]);
    assert_eq!(overlay2.state.caches.len(), 3);
    let tree_1_cache = overlay2.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 2);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_b".as_slice()),
        Some(&b"val_bb".into())
    );
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_c".as_slice()),
        Some(&b"val_cc".into())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_a".as_slice()),
        Some(&b"key_a".into())
    );
    let tree_2_cache = overlay2.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 1);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_d".as_slice()),
        Some(&b"val_d".into())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    let tree_4_cache = overlay2.state.caches.get(TREE_4).unwrap();
    assert_eq!(tree_4_cache.state.cache.len(), 1);
    assert_eq!(
        tree_4_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert!(tree_4_cache.state.removed.is_empty());
    assert_eq!(overlay2.state.dropped_trees.len(), 2);
    let dropped_tree_3_cache = overlay2.state.dropped_trees.get(TREE_3).unwrap();
    assert_eq!(dropped_tree_3_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(dropped_tree_3_cache.removed.is_empty());
    let dropped_tree_6_cache = overlay2.state.dropped_trees.get(TREE_6).unwrap();
    assert!(dropped_tree_6_cache.cache.is_empty());
    assert!(dropped_tree_6_cache.removed.is_empty());

    overlay.apply_diff(&sequence[1])?;
    overlay2.remove_diff(&sequence[1]);
    db.flush_default_mode()?;

    // All trees should be present in the database
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 5);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));
    assert!(db_tree_names.contains(&TREE_6.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 1);
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_bb".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 2);
    assert_eq!(tree_2.get(b"key_d")?, Some(b"val_d".into()));
    assert_eq!(tree_2.get(b"key_e")?, Some(b"val_e".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 1);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));

    assert_eq!(overlay.state.initial_tree_names.len(), 3);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert_eq!(overlay.state.new_tree_names, [TREE_5]);
    assert_eq!(overlay.state.caches.len(), 3);
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 2);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_a".as_slice()),
        Some(&b"val_a".into())
    );
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_c".as_slice()),
        Some(&b"val_c".into())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_b".as_slice()),
        Some(&b"key_b".into())
    );
    let tree_2_cache = overlay.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 1);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    let tree_5_cache = overlay.state.caches.get(TREE_5).unwrap();
    assert_eq!(tree_5_cache.state.cache.len(), 1);
    assert_eq!(
        tree_5_cache.state.cache.get(b"key_h".as_slice()),
        Some(&b"val_h".into())
    );
    assert!(tree_5_cache.state.removed.is_empty());
    assert_eq!(overlay.state.dropped_trees.len(), 1);
    let dropped_tree_4_cache = overlay.state.dropped_trees.get(TREE_4).unwrap();
    assert_eq!(dropped_tree_4_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_4_cache.cache.get(b"key_g".as_slice()),
        Some(&(None, b"val_g".into()))
    );
    assert!(dropped_tree_4_cache.removed.is_empty());

    assert_eq!(overlay2.state.initial_tree_names.len(), 3);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay2.state.new_tree_names.is_empty());
    assert_eq!(overlay2.state.caches.len(), 3);
    let tree_1_cache = overlay2.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 1);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_c".as_slice()),
        Some(&b"val_cc".into())
    );
    assert!(tree_1_cache.state.removed.is_empty());
    let tree_2_cache = overlay2.state.caches.get(TREE_2).unwrap();
    assert!(tree_2_cache.state.cache.is_empty());
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    let tree_4_cache = overlay2.state.caches.get(TREE_4).unwrap();
    assert_eq!(tree_4_cache.state.cache.len(), 1);
    assert_eq!(
        tree_4_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert!(tree_4_cache.state.removed.is_empty());
    assert_eq!(overlay2.state.dropped_trees.len(), 1);
    let dropped_tree_6_cache = overlay2.state.dropped_trees.get(TREE_6).unwrap();
    assert!(dropped_tree_6_cache.cache.is_empty());
    assert!(dropped_tree_6_cache.removed.is_empty());

    // We chose to follow the second overlay, so we apply its diff
    sequence[2] = overlay2.diff(&[])?;
    overlay2.apply_diff(&sequence[2])?;
    db.flush_default_mode()?;

    // Since we used overlay2, we must drop the new trees from original
    // overlay that are not present now, and stop using it.
    db.drop_tree(TREE_5)?;

    // All remaining trees should be present in the database
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 3);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_bb".into()));
    assert_eq!(tree_1.get(b"key_c")?, Some(b"val_cc".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 1);
    assert_eq!(tree_2.get(b"key_d")?, Some(b"val_d".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 2);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));
    assert_eq!(tree_4.get(b"key_f")?, Some(b"val_f".into()));

    // Since we removed everything, current overlay must not have
    // diffs over the tree, therefore its safe to keep using it
    assert_eq!(overlay2.state.initial_tree_names.len(), 3);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay2.state.new_tree_names.is_empty());
    assert!(overlay2.state.caches.is_empty());
    assert!(overlay2.state.dropped_trees.is_empty());

    let diff = overlay2.diff(&[])?;
    assert_eq!(diff.initial_tree_names.len(), 3);
    assert!(diff.initial_tree_names.contains(&TREE_1.into()));
    assert!(diff.initial_tree_names.contains(&TREE_2.into()));
    assert!(diff.initial_tree_names.contains(&TREE_4.into()));
    assert!(diff.caches.is_empty());
    assert!(diff.dropped_trees.is_empty());

    // Now we are going to revert the diffs sequence going backwards
    // and verify overlay state mutates accordingly
    overlay2.add_diff(&sequence[2].inverse())?;
    assert_eq!(overlay2.state.initial_tree_names.len(), 3);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay2.state.new_tree_names.is_empty());
    assert_eq!(overlay2.state.caches.len(), 3);
    let tree_1_cache = overlay2.state.caches.get(TREE_1).unwrap();
    assert!(tree_1_cache.state.cache.is_empty());
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_c".as_slice()),
        Some(&b"key_c".into())
    );
    let tree_2_cache = overlay2.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 1);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_e".as_slice()),
        Some(&b"val_e".into())
    );
    assert!(tree_2_cache.state.removed.is_empty());
    let tree_4_cache = overlay2.state.caches.get(TREE_4).unwrap();
    assert!(tree_4_cache.state.cache.is_empty());
    assert_eq!(tree_4_cache.state.removed.len(), 1);
    assert_eq!(
        tree_4_cache.state.removed.get(b"key_f".as_slice()),
        Some(&b"key_f".into())
    );
    assert!(overlay2.state.dropped_trees.is_empty());

    overlay2.add_diff(&sequence[1].inverse())?;
    assert_eq!(overlay2.state.initial_tree_names.len(), 2);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert_eq!(overlay2.state.new_tree_names, [TREE_3]);
    assert_eq!(overlay2.state.caches.len(), 3);
    let tree_1_cache = overlay2.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 2);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_a".as_slice()),
        Some(&b"val_a".into())
    );
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_b".as_slice()),
        Some(&b"val_b".into())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 1);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_c".as_slice()),
        Some(&b"key_c".into())
    );
    let tree_4_cache = overlay2.state.caches.get(TREE_4).unwrap();
    assert!(tree_4_cache.state.cache.is_empty());
    assert_eq!(tree_4_cache.state.removed.len(), 1);
    assert_eq!(
        tree_4_cache.state.removed.get(b"key_f".as_slice()),
        Some(&b"key_f".into())
    );
    let tree_3_cache = overlay2.state.caches.get(TREE_3).unwrap();
    assert_eq!(tree_3_cache.state.cache.len(), 1);
    assert_eq!(
        tree_3_cache.state.cache.get(b"key_i".as_slice()),
        Some(&b"val_i".into())
    );
    assert!(tree_3_cache.state.removed.is_empty());
    assert_eq!(overlay2.state.dropped_trees.len(), 1);
    let dropped_tree_2_cache = overlay2.state.dropped_trees.get(TREE_2).unwrap();
    assert!(dropped_tree_2_cache.cache.is_empty());
    assert_eq!(dropped_tree_2_cache.removed.len(), 2);
    assert_eq!(
        dropped_tree_2_cache.removed.get(b"key_d".as_slice()),
        Some(&b"val_d".into())
    );
    assert_eq!(
        dropped_tree_2_cache.removed.get(b"key_e".as_slice()),
        Some(&b"val_e".into())
    );

    overlay2.add_diff(&sequence[0].inverse())?;
    assert_eq!(overlay2.state.initial_tree_names.len(), 2);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay2.state.new_tree_names.is_empty());
    assert_eq!(overlay2.state.caches.len(), 2);
    let tree_1_cache = overlay2.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 1);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_a".as_slice()),
        Some(&b"val_a".into())
    );
    assert_eq!(tree_1_cache.state.removed.len(), 2);
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_b".as_slice()),
        Some(&b"key_b".into())
    );
    assert_eq!(
        tree_1_cache.state.removed.get(b"key_c".as_slice()),
        Some(&b"key_c".into())
    );
    let tree_4_cache = overlay2.state.caches.get(TREE_4).unwrap();
    assert!(tree_4_cache.state.cache.is_empty());
    assert_eq!(tree_4_cache.state.removed.len(), 1);
    assert_eq!(
        tree_4_cache.state.removed.get(b"key_f".as_slice()),
        Some(&b"key_f".into())
    );
    assert_eq!(overlay2.state.dropped_trees.len(), 2);
    let dropped_tree_2_cache = overlay2.state.dropped_trees.get(TREE_2).unwrap();
    assert!(dropped_tree_2_cache.cache.is_empty());
    assert_eq!(dropped_tree_2_cache.removed.len(), 2);
    assert_eq!(
        dropped_tree_2_cache.removed.get(b"key_d".as_slice()),
        Some(&b"val_d".into())
    );
    assert_eq!(
        dropped_tree_2_cache.removed.get(b"key_e".as_slice()),
        Some(&b"val_e".into())
    );
    let dropped_tree_3_cache = overlay2.state.dropped_trees.get(TREE_3).unwrap();
    assert!(dropped_tree_3_cache.cache.is_empty());
    assert_eq!(dropped_tree_3_cache.removed.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.removed.get(b"key_i".as_slice()),
        Some(&b"val_i".into())
    );

    // We are now going to apply the overlay and remove the complete diff
    let diff = overlay2.diff(&[])?;
    overlay2.apply()?;
    overlay2.remove_diff(&diff);
    db.flush_default_mode()?;

    // Since we removed everything, current overlay must not have
    // diffs over the tree, therefore its safe to keep using it
    assert_eq!(overlay2.state.initial_tree_names.len(), 2);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay2.state.new_tree_names.is_empty());
    assert!(overlay2.state.caches.is_empty());
    assert!(overlay2.state.dropped_trees.is_empty());

    let diff = overlay2.diff(&[])?;
    assert!(diff.caches.is_empty());
    assert!(diff.dropped_trees.is_empty());

    // Database has now reverted to its original state
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 2);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 1);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 1);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));

    Ok(())
}

#[test]
fn database_overlay_protected_trees() -> Result<()> {
    // Initialize database
    let (db, _folder) = Database::open_temp()?;

    // Initialize trees with some values
    let tree_1 = db.open_tree_default(TREE_1)?;
    tree_1.insert(b"key_a", b"val_a")?;
    let tree_4 = db.open_tree_default(TREE_4)?;
    tree_4.insert(b"key_g", b"val_g")?;

    // Initialize overlay
    let mut overlay = DatabaseOverlay::new(&db, vec![TREE_1.into(), TREE_4.into()])?;

    // Open trees in the overlay
    overlay.open_tree_default(TREE_1, false)?;
    overlay.open_tree_default(TREE_3, false)?;

    // Try to remove protected trees
    assert!(overlay.drop_tree(TREE_1).is_err());
    assert!(overlay.drop_tree(TREE_4).is_err());

    // Make a vector to keep track of changes
    let mut sequence = vec![];

    // Perform some changes and grab their differences
    overlay.insert(TREE_1, b"key_b", b"val_b")?;
    overlay.insert(TREE_3, b"key_i", b"val_i")?;
    sequence.push(overlay.diff(&sequence)?);

    overlay.open_tree_default(TREE_2, false)?;
    overlay.insert(TREE_2, b"key_d", b"val_d")?;
    overlay.insert(TREE_2, b"key_e", b"val_e")?;
    overlay.open_tree_default(TREE_5, false)?;
    overlay.insert(TREE_5, b"key_h", b"val_h")?;
    sequence.push(overlay.diff(&sequence)?);

    overlay.drop_tree(TREE_3)?;
    overlay.remove(TREE_2, b"key_e")?;
    overlay.insert(TREE_2, b"key_f", b"val_f")?;
    sequence.push(overlay.diff(&sequence)?);

    // Verify overlay has the correct state
    assert_eq!(overlay.state.initial_tree_names.len(), 2);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert_eq!(overlay.state.new_tree_names, [TREE_2, TREE_5]);
    assert_eq!(overlay.state.caches.len(), 3);
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.state.cache.len(), 1);
    assert_eq!(
        tree_1_cache.state.cache.get(b"key_b".as_slice()),
        Some(&b"val_b".into())
    );
    assert!(tree_1_cache.state.removed.is_empty());
    let tree_2_cache = overlay.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 2);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_d".as_slice()),
        Some(&b"val_d".into())
    );
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    let tree_5_cache = overlay.state.caches.get(TREE_5).unwrap();
    assert_eq!(tree_5_cache.state.cache.len(), 1);
    assert_eq!(
        tree_5_cache.state.cache.get(b"key_h".as_slice()),
        Some(&b"val_h".into())
    );
    assert!(tree_5_cache.state.removed.is_empty());
    assert_eq!(overlay.state.dropped_trees.len(), 1);
    let dropped_tree_3_cache = overlay.state.dropped_trees.get(TREE_3).unwrap();
    assert!(dropped_tree_3_cache.cache.is_empty());
    assert!(dropped_tree_3_cache.removed.is_empty());
    assert_eq!(overlay.state.protected_tree_names.len(), 2);
    assert!(overlay.state.protected_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.protected_tree_names.contains(&TREE_4.into()));

    // Verify diffs sequence is correct
    assert_eq!(sequence.len(), 3);

    assert_eq!(sequence[0].initial_tree_names.len(), 2);
    assert!(sequence[0].initial_tree_names.contains(&TREE_1.into()));
    assert!(sequence[0].initial_tree_names.contains(&TREE_4.into()));
    assert_eq!(sequence[0].caches.len(), 2);
    let (tree_1_cache, drop) = sequence[0].caches.get(TREE_1).unwrap();
    assert_eq!(tree_1_cache.cache.len(), 1);
    assert_eq!(
        tree_1_cache.cache.get(b"key_b".as_slice()),
        Some(&(None, b"val_b".into()))
    );
    assert!(tree_1_cache.removed.is_empty());
    assert!(!drop);
    let (tree_3_cache, drop) = sequence[0].caches.get(TREE_3).unwrap();
    assert_eq!(tree_3_cache.cache.len(), 1);
    assert_eq!(
        tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(tree_3_cache.removed.is_empty());
    assert!(!drop);
    assert!(sequence[0].dropped_trees.is_empty());
    assert_eq!(sequence[0], sequence[0].inverse().inverse());

    assert_eq!(sequence[1].initial_tree_names.len(), 3);
    assert!(sequence[1].initial_tree_names.contains(&TREE_1.into()));
    assert!(sequence[1].initial_tree_names.contains(&TREE_4.into()));
    assert!(sequence[1].initial_tree_names.contains(&TREE_3.into()));
    assert_eq!(sequence[1].caches.len(), 2);
    let (tree_2_cache, drop) = sequence[1].caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.cache.len(), 2);
    assert_eq!(
        tree_2_cache.cache.get(b"key_d".as_slice()),
        Some(&(None, b"val_d".into()))
    );
    assert_eq!(
        tree_2_cache.cache.get(b"key_e".as_slice()),
        Some(&(None, b"val_e".into()))
    );
    assert!(tree_2_cache.removed.is_empty());
    assert!(!drop);
    let (tree_5_cache, drop) = sequence[1].caches.get(TREE_5).unwrap();
    assert_eq!(tree_5_cache.cache.len(), 1);
    assert_eq!(
        tree_5_cache.cache.get(b"key_h".as_slice()),
        Some(&(None, b"val_h".into()))
    );
    assert!(tree_5_cache.removed.is_empty());
    assert!(!drop);
    assert!(sequence[1].dropped_trees.is_empty());
    assert_eq!(sequence[1], sequence[1].inverse().inverse());

    assert_eq!(sequence[2].initial_tree_names.len(), 5);
    assert!(sequence[2].initial_tree_names.contains(&TREE_1.into()));
    assert!(sequence[2].initial_tree_names.contains(&TREE_4.into()));
    assert!(sequence[2].initial_tree_names.contains(&TREE_3.into()));
    assert!(sequence[2].initial_tree_names.contains(&TREE_2.into()));
    assert!(sequence[2].initial_tree_names.contains(&TREE_5.into()));
    assert_eq!(sequence[2].caches.len(), 1);
    let (tree_2_cache, drop) = sequence[2].caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.cache.len(), 1);
    assert_eq!(
        tree_2_cache.cache.get(b"key_f".as_slice()),
        Some(&(None, b"val_f".into()))
    );
    assert_eq!(tree_2_cache.removed.len(), 1);
    assert_eq!(
        tree_2_cache.removed.get(b"key_e".as_slice()),
        Some(&b"val_e".into())
    );
    assert!(!drop);
    assert_eq!(sequence[2].dropped_trees.len(), 1);
    let (dropped_tree_3_cache, restored) = sequence[2].dropped_trees.get(TREE_3).unwrap();
    assert_eq!(dropped_tree_3_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(dropped_tree_3_cache.removed.is_empty());
    assert!(!restored);
    assert_eq!(sequence[2], sequence[2].inverse().inverse());

    // Now we are going to apply each diff and check that the database
    // and the overlay have been mutated accordingly.
    // Don't forget to flush.
    overlay.apply_diff(&sequence[0])?;
    db.flush_default_mode()?;

    // All trees should be present in the database
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 5);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_3.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_b".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert!(tree_2.is_empty()?);
    let tree_3 = db.open_tree_default(TREE_3)?;
    assert_eq!(tree_3.len()?, 1);
    assert_eq!(tree_3.get(b"key_i")?, Some(b"val_i".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 1);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert!(tree_5.is_empty()?);

    assert_eq!(overlay.state.initial_tree_names.len(), 3);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_3.into()));
    assert_eq!(overlay.state.new_tree_names, [TREE_2, TREE_5]);
    assert_eq!(overlay.state.caches.len(), 3);
    // Tree 1 here became stale, but since its protected it just got reset
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert!(tree_1_cache.state.cache.is_empty());
    assert!(tree_1_cache.state.removed.is_empty());
    let tree_2_cache = overlay.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 2);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_d".as_slice()),
        Some(&b"val_d".into())
    );
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    let tree_5_cache = overlay.state.caches.get(TREE_5).unwrap();
    assert_eq!(tree_5_cache.state.cache.len(), 1);
    assert_eq!(
        tree_5_cache.state.cache.get(b"key_h".as_slice()),
        Some(&b"val_h".into())
    );
    assert!(tree_5_cache.state.removed.is_empty());
    assert_eq!(overlay.state.dropped_trees.len(), 1);
    let dropped_tree_3_cache = overlay.state.dropped_trees.get(TREE_3).unwrap();
    assert_eq!(dropped_tree_3_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(dropped_tree_3_cache.removed.is_empty());
    assert_eq!(overlay.state.protected_tree_names.len(), 2);
    assert!(overlay.state.protected_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.protected_tree_names.contains(&TREE_4.into()));

    overlay.apply_diff(&sequence[1])?;
    db.flush_default_mode()?;

    // All trees should be present in the database
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 5);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_3.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_b".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 2);
    assert_eq!(tree_2.get(b"key_d")?, Some(b"val_d".into()));
    assert_eq!(tree_2.get(b"key_e")?, Some(b"val_e".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 1);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert_eq!(tree_5.len()?, 1);
    assert_eq!(tree_5.get(b"key_h")?, Some(b"val_h".into()));

    assert_eq!(overlay.state.initial_tree_names.len(), 5);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_3.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_5.into()));
    assert!(overlay.state.new_tree_names.is_empty());
    // Tree 5 was stale so it should have been closed
    assert_eq!(overlay.state.caches.len(), 2);
    // Tree 1 reference stays alive
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert!(tree_1_cache.state.cache.is_empty());
    assert!(tree_1_cache.state.removed.is_empty());
    let tree_2_cache = overlay.state.caches.get(TREE_2).unwrap();
    assert_eq!(tree_2_cache.state.cache.len(), 1);
    assert_eq!(
        tree_2_cache.state.cache.get(b"key_f".as_slice()),
        Some(&b"val_f".into())
    );
    assert_eq!(tree_2_cache.state.removed.len(), 1);
    assert_eq!(
        tree_2_cache.state.removed.get(b"key_e".as_slice()),
        Some(&b"key_e".into())
    );
    assert_eq!(overlay.state.dropped_trees.len(), 1);
    let dropped_tree_3_cache = overlay.state.dropped_trees.get(TREE_3).unwrap();
    assert_eq!(dropped_tree_3_cache.cache.len(), 1);
    assert_eq!(
        dropped_tree_3_cache.cache.get(b"key_i".as_slice()),
        Some(&(None, b"val_i".into()))
    );
    assert!(dropped_tree_3_cache.removed.is_empty());
    assert_eq!(overlay.state.protected_tree_names.len(), 2);
    assert!(overlay.state.protected_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.protected_tree_names.contains(&TREE_4.into()));

    overlay.apply_diff(&sequence[2])?;
    db.flush_default_mode()?;

    // All trees should be present in the database
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 4);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_b".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 2);
    assert_eq!(tree_2.get(b"key_d")?, Some(b"val_d".into()));
    assert_eq!(tree_2.get(b"key_f")?, Some(b"val_f".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 1);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert_eq!(tree_5.len()?, 1);
    assert_eq!(tree_5.get(b"key_h")?, Some(b"val_h".into()));

    // Since we removed everything, current overlay must not have
    // diffs over the tree, just the protected opened references,
    // therefore its safe to keep using it
    assert_eq!(overlay.state.initial_tree_names.len(), 4);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_5.into()));
    assert!(overlay.state.new_tree_names.is_empty());
    // Tree 1 reference stays alive
    assert_eq!(overlay.state.caches.len(), 1);
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert!(tree_1_cache.state.cache.is_empty());
    assert!(tree_1_cache.state.removed.is_empty());
    assert!(overlay.state.dropped_trees.is_empty());
    assert_eq!(overlay.state.protected_tree_names.len(), 2);
    assert!(overlay.state.protected_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.protected_tree_names.contains(&TREE_4.into()));

    let diff = overlay.diff(&[])?;
    // Even if Tree 1 reference is alive, it produces no diff over the
    // tree.
    assert!(diff.caches.is_empty());
    assert!(diff.dropped_trees.is_empty());

    // Now we are going to create an overlay where Tree 1 is not
    // protected, drop it, produce its diff and try to apply it to the
    // original overlay.
    let mut overlay2 = DatabaseOverlay::new(&db, vec![])?;
    overlay2.drop_tree(TREE_1)?;
    assert_eq!(overlay2.state.initial_tree_names.len(), 4);
    assert!(overlay2.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_2.into()));
    assert!(overlay2.state.initial_tree_names.contains(&TREE_5.into()));
    assert!(overlay2.state.new_tree_names.is_empty());
    assert!(overlay2.state.caches.is_empty());
    assert_eq!(overlay2.state.dropped_trees.len(), 1);
    let dropped_tree_1_cache = overlay2.state.dropped_trees.get(TREE_1).unwrap();
    assert_eq!(dropped_tree_1_cache.cache.len(), 2);
    assert_eq!(
        dropped_tree_1_cache.cache.get(b"key_a".as_slice()),
        Some(&(None, b"val_a".into()))
    );
    assert_eq!(
        dropped_tree_1_cache.cache.get(b"key_b".as_slice()),
        Some(&(None, b"val_b".into()))
    );
    assert!(dropped_tree_1_cache.removed.is_empty());
    assert!(overlay2.state.protected_tree_names.is_empty());
    let diff = overlay2.diff(&[])?;
    assert!(diff.caches.is_empty());
    assert_eq!(diff.dropped_trees.len(), 1);
    let (dropped_tree_1_cache, restored) = diff.dropped_trees.get(TREE_1).unwrap();
    assert_eq!(dropped_tree_1_cache.cache.len(), 2);
    assert_eq!(
        dropped_tree_1_cache.cache.get(b"key_a".as_slice()),
        Some(&(None, b"val_a".into()))
    );
    assert_eq!(
        dropped_tree_1_cache.cache.get(b"key_b".as_slice()),
        Some(&(None, b"val_b".into()))
    );
    assert!(dropped_tree_1_cache.removed.is_empty());
    assert!(!restored);
    assert_eq!(diff, diff.inverse().inverse());
    assert!(overlay.apply_diff(&diff).is_err());

    // Now we are going to revert the diffs sequence going backwards
    // and verify the database state mutates accordingly
    overlay.apply_diff(&sequence[2].inverse())?;
    db.flush_default_mode()?;

    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 5);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_2.into()));
    assert!(db_tree_names.contains(&TREE_3.into()));
    assert!(db_tree_names.contains(&TREE_5.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_b".into()));
    let tree_2 = db.open_tree_default(TREE_2)?;
    assert_eq!(tree_2.len()?, 2);
    assert_eq!(tree_2.get(b"key_d")?, Some(b"val_d".into()));
    assert_eq!(tree_2.get(b"key_e")?, Some(b"val_e".into()));
    let tree_3 = db.open_tree_default(TREE_3)?;
    assert_eq!(tree_3.len()?, 1);
    assert_eq!(tree_3.get(b"key_i")?, Some(b"val_i".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 1);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));
    let tree_5 = db.open_tree_default(TREE_5)?;
    assert_eq!(tree_5.len()?, 1);
    assert_eq!(tree_5.get(b"key_h")?, Some(b"val_h".into()));

    overlay.apply_diff(&sequence[1].inverse())?;
    db.flush_default_mode()?;

    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 3);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));
    assert!(db_tree_names.contains(&TREE_3.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 2);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    assert_eq!(tree_1.get(b"key_b")?, Some(b"val_b".into()));
    let tree_3 = db.open_tree_default(TREE_3)?;
    assert_eq!(tree_3.len()?, 1);
    assert_eq!(tree_3.get(b"key_i")?, Some(b"val_i".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 1);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));

    overlay.apply_diff(&sequence[0].inverse())?;
    db.flush_default_mode()?;

    // Since we removed everything, current overlay must not have
    // diffs over the tree, therefore its safe to keep using it
    assert_eq!(overlay.state.initial_tree_names.len(), 2);
    assert!(overlay.state.initial_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.initial_tree_names.contains(&TREE_4.into()));
    assert!(overlay.state.new_tree_names.is_empty());
    // Tree 1 reference stays alive
    assert_eq!(overlay.state.caches.len(), 1);
    let tree_1_cache = overlay.state.caches.get(TREE_1).unwrap();
    assert!(tree_1_cache.state.cache.is_empty());
    assert!(tree_1_cache.state.removed.is_empty());
    assert!(overlay.state.dropped_trees.is_empty());
    assert_eq!(overlay.state.protected_tree_names.len(), 2);
    assert!(overlay.state.protected_tree_names.contains(&TREE_1.into()));
    assert!(overlay.state.protected_tree_names.contains(&TREE_4.into()));

    let diff = overlay.diff(&[])?;
    assert!(diff.caches.is_empty());
    assert!(diff.dropped_trees.is_empty());

    // The database has now reverted to its original state
    let db_tree_names = db.tree_names()?;
    assert_eq!(db_tree_names.len(), 2);
    assert!(db_tree_names.contains(&TREE_1.into()));
    assert!(db_tree_names.contains(&TREE_4.into()));

    let tree_1 = db.open_tree_default(TREE_1)?;
    assert_eq!(tree_1.len()?, 1);
    assert_eq!(tree_1.get(b"key_a")?, Some(b"val_a".into()));
    let tree_4 = db.open_tree_default(TREE_4)?;
    assert_eq!(tree_4.len()?, 1);
    assert_eq!(tree_4.get(b"key_g")?, Some(b"val_g".into()));

    Ok(())
}