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    // Copyright 2015-2017 Parity Technologies (UK) Ltd.
    // This file is part of Parity.

    // Parity is free software: you can redistribute it and/or modify
    // it under the terms of the GNU General Public License as published by
    // the Free Software Foundation, either version 3 of the License, or
    // (at your option) any later version.

    // Parity 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 General Public License for more details.

    // You should have received a copy of the GNU General Public License
    // along with Parity.  If not, see <http://www.gnu.org/licenses/>.

    //! Key-Value store abstraction with `RocksDB` backend.

    use std::io::ErrorKind;
    use common::*;
    use elastic_array::*;
    use std::default::Default;
    use std::path::PathBuf;
    use hashdb::DBValue;
    use rlp::{UntrustedRlp, RlpType, View, Compressible};
    use rocksdb::{DB, Writable, WriteBatch, WriteOptions, IteratorMode, DBIterator,
    	Options, DBCompactionStyle, BlockBasedOptions, Direction, Cache, Column, ReadOptions};
    #[cfg(target_os = "linux")]
    use regex::Regex;
    #[cfg(target_os = "linux")]
    use std::process::Command;
    #[cfg(target_os = "linux")]
    use std::fs::File;

    const DB_BACKGROUND_FLUSHES: i32 = 2;
    const DB_BACKGROUND_COMPACTIONS: i32 = 2;

    /// Write transaction. Batches a sequence of put/delete operations for efficiency.
    pub struct DBTransaction {
    	ops: Vec<DBOp>,
    }

    enum DBOp {
    	Insert {
    		col: Option<u32>,
    		key: ElasticArray32<u8>,
    		value: DBValue,
    	},
    	InsertCompressed {
    		col: Option<u32>,
    		key: ElasticArray32<u8>,
    		value: DBValue,
    	},
    	Delete {
    		col: Option<u32>,
    		key: ElasticArray32<u8>,
    	}
    }

    impl DBTransaction {
    	/// Create new transaction.
    	pub fn new(_db: &Database) -> DBTransaction {
    		DBTransaction {
    			ops: Vec::with_capacity(256),
    		}
    	}

    	/// Insert a key-value pair in the transaction. Any existing value value will be overwritten upon write.
    	pub fn put(&mut self, col: Option<u32>, key: &[u8], value: &[u8]) {
    		let mut ekey = ElasticArray32::new();
    		ekey.append_slice(key);
    		self.ops.push(DBOp::Insert {
    			col: col,
    			key: ekey,
    			value: DBValue::from_slice(value),
    		});
    	}

    	/// Insert a key-value pair in the transaction. Any existing value value will be overwritten upon write.
    	pub fn put_vec(&mut self, col: Option<u32>, key: &[u8], value: Bytes) {
    		let mut ekey = ElasticArray32::new();
    		ekey.append_slice(key);
    		self.ops.push(DBOp::Insert {
    			col: col,
    			key: ekey,
    			value: DBValue::from_vec(value),
    		});
    	}

    	/// Insert a key-value pair in the transaction. Any existing value value will be overwritten upon write.
    	/// Value will be RLP-compressed on flush
    	pub fn put_compressed(&mut self, col: Option<u32>, key: &[u8], value: Bytes) {
    		let mut ekey = ElasticArray32::new();
    		ekey.append_slice(key);
    		self.ops.push(DBOp::InsertCompressed {
    			col: col,
    			key: ekey,
    			value: DBValue::from_vec(value),
    		});
    	}

    	/// Delete value by key.
    	pub fn delete(&mut self, col: Option<u32>, key: &[u8]) {
    		let mut ekey = ElasticArray32::new();
    		ekey.append_slice(key);
    		self.ops.push(DBOp::Delete {
    			col: col,
    			key: ekey,
    		});
    	}
    }

    enum KeyState {
    	Insert(DBValue),
    	InsertCompressed(DBValue),
    	Delete,
    }

    /// Compaction profile for the database settings
    #[derive(Clone, Copy, PartialEq, Debug)]
    pub struct CompactionProfile {
    	/// L0-L1 target file size
    	pub initial_file_size: u64,
    	/// L2-LN target file size multiplier
    	pub file_size_multiplier: i32,
    	/// rate limiter for background flushes and compactions, bytes/sec, if any
    	pub write_rate_limit: Option<u64>,
    }

    impl Default for CompactionProfile {
    	/// Default profile suitable for most storage
    	fn default() -> CompactionProfile {
    		CompactionProfile::ssd()
    	}
    }

    /// Given output of df command return Linux rotational flag file path.
    #[cfg(target_os = "linux")]
    pub fn rotational_from_df_output(df_out: Vec<u8>) -> Option<PathBuf> {
    	str::from_utf8(df_out.as_slice())
    		.ok()
    		// Get the drive name.
    		.and_then(|df_str| Regex::new(r"/dev/(sd[:alpha:]{1,2})")
    			.ok()
    			.and_then(|re| re.captures(df_str))
    			.and_then(|captures| captures.at(1)))
    		// Generate path e.g. /sys/block/sda/queue/rotational
    		.map(|drive_path| {
    			let mut p = PathBuf::from("/sys/block");
    			p.push(drive_path);
    			p.push("queue/rotational");
    			p
    		})
    }

    impl CompactionProfile {
    	/// Attempt to determine the best profile automatically, only Linux for now.
    	#[cfg(target_os = "linux")]
    	pub fn auto(db_path: &Path) -> CompactionProfile {
    		let hdd_check_file = db_path
    			.to_str()
    			.and_then(|path_str| Command::new("df").arg(path_str).output().ok())
    			.and_then(|df_res| match df_res.status.success() {
    				true => Some(df_res.stdout),
    				false => None,
    			})
    			.and_then(rotational_from_df_output);
    		// Read out the file and match compaction profile.
    		if let Some(hdd_check) = hdd_check_file {
    			if let Ok(mut file) = File::open(hdd_check.as_path()) {
    				let mut buffer = [0; 1];
    				if file.read_exact(&mut buffer).is_ok() {
    					// 0 means not rotational.
    					if buffer == [48] { return Self::ssd(); }
    					// 1 means rotational.
    					if buffer == [49] { return Self::hdd(); }
    				}
    			}
    		}
    		// Fallback if drive type was not determined.
    		Self::default()
    	}

    	/// Just default for other platforms.
    	#[cfg(not(target_os = "linux"))]
    	pub fn auto(_db_path: &Path) -> CompactionProfile {
    		Self::default()
    	}

    	/// Default profile suitable for SSD storage
    	pub fn ssd() -> CompactionProfile {
    		CompactionProfile {
    			initial_file_size: 32 * 1024 * 1024,
    			file_size_multiplier: 2,
    			write_rate_limit: None,
    		}
    	}

    	/// Slow HDD compaction profile
    	pub fn hdd() -> CompactionProfile {
    		CompactionProfile {
    			initial_file_size: 192 * 1024 * 1024,
    			file_size_multiplier: 1,
    			write_rate_limit: Some(8 * 1024 * 1024),
    		}
    	}
    }

    /// Database configuration
    #[derive(Clone)]
    pub struct DatabaseConfig {
    	/// Max number of open files.
    	pub max_open_files: i32,
    	/// Cache sizes (in MiB) for specific columns.
    	pub cache_sizes: HashMap<Option<u32>, usize>,
    	/// Compaction profile
    	pub compaction: CompactionProfile,
    	/// Set number of columns
    	pub columns: Option<u32>,
    	/// Should we keep WAL enabled?
    	pub wal: bool,
    }

    impl DatabaseConfig {
    	/// Create new `DatabaseConfig` with default parameters and specified set of columns.
    	/// Note that cache sizes must be explicitly set.
    	pub fn with_columns(columns: Option<u32>) -> Self {
    		let mut config = Self::default();
    		config.columns = columns;
    		config
    	}

    	/// Set the column cache size in MiB.
    	pub fn set_cache(&mut self, col: Option<u32>, size: usize) {
    		self.cache_sizes.insert(col, size);
    	}
    }

    impl Default for DatabaseConfig {
    	fn default() -> DatabaseConfig {
    		DatabaseConfig {
    			cache_sizes: HashMap::new(),
    			max_open_files: 512,
    			compaction: CompactionProfile::default(),
    			columns: None,
    			wal: true,
    		}
    	}
    }

    /// Database iterator for flushed data only
    pub struct DatabaseIterator {
    	iter: DBIterator,
    }

    impl<'a> Iterator for DatabaseIterator {
    	type Item = (Box<[u8]>, Box<[u8]>);

        fn next(&mut self) -> Option<Self::Item> {
    		self.iter.next()
    	}
    }

    struct DBAndColumns {
    	db: DB,
    	cfs: Vec<Column>,
    }

    /// Key-Value database.
    pub struct Database {
    	db: RwLock<Option<DBAndColumns>>,
    	config: DatabaseConfig,
    	write_opts: WriteOptions,
    	read_opts: ReadOptions,
    	path: String,
    	// Dirty values added with `write_buffered`. Cleaned on `flush`.
    	overlay: RwLock<Vec<HashMap<ElasticArray32<u8>, KeyState>>>,
    	// Values currently being flushed. Cleared when `flush` completes.
    	flushing: RwLock<Vec<HashMap<ElasticArray32<u8>, KeyState>>>,
    	// Prevents concurrent flushes.
    	// Value indicates if a flush is in progress.
    	flushing_lock: Mutex<bool>,
    }

    impl Database {
    	/// Open database with default settings.
    	pub fn open_default(path: &str) -> Result<Database, String> {
    		Database::open(&DatabaseConfig::default(), path)
    	}

    	/// Open database file. Creates if it does not exist.
    	pub fn open(config: &DatabaseConfig, path: &str) -> Result<Database, String> {
    		// default cache size for columns not specified.
    		const DEFAULT_CACHE: usize = 2;

    		let mut opts = Options::new();
    		if let Some(rate_limit) = config.compaction.write_rate_limit {
    			opts.set_parsed_options(&format!("rate_limiter_bytes_per_sec={}", rate_limit))?;
    		}
    		opts.set_parsed_options(&format!("max_total_wal_size={}", 64 * 1024 * 1024))?;
    		opts.set_parsed_options("verify_checksums_in_compaction=0")?;
    		opts.set_max_open_files(config.max_open_files);
    		opts.create_if_missing(true);
    		opts.set_use_fsync(false);

    		opts.set_max_background_flushes(DB_BACKGROUND_FLUSHES);
    		opts.set_max_background_compactions(DB_BACKGROUND_COMPACTIONS);

    		// compaction settings
    		opts.set_compaction_style(DBCompactionStyle::DBUniversalCompaction);
    		opts.set_target_file_size_base(config.compaction.initial_file_size);
    		opts.set_target_file_size_multiplier(config.compaction.file_size_multiplier);

    		let mut cf_options = Vec::with_capacity(config.columns.unwrap_or(0) as usize);
    		let cfnames: Vec<_> = (0..config.columns.unwrap_or(0)).map(|c| format!("col{}", c)).collect();
    		let cfnames: Vec<&str> = cfnames.iter().map(|n| n as &str).collect();

    		for col in 0 .. config.columns.unwrap_or(0) {
    			let mut opts = Options::new();
    			opts.set_compaction_style(DBCompactionStyle::DBUniversalCompaction);
    			opts.set_target_file_size_base(config.compaction.initial_file_size);
    			opts.set_target_file_size_multiplier(config.compaction.file_size_multiplier);

    			let col_opt = config.columns.map(|_| col);

    			{
    				let cache_size = config.cache_sizes.get(&col_opt).cloned().unwrap_or(DEFAULT_CACHE);
    				let mut block_opts = BlockBasedOptions::new();
    				// all goes to read cache.
    				block_opts.set_cache(Cache::new(cache_size * 1024 * 1024));
    				opts.set_block_based_table_factory(&block_opts);
    			}

    			cf_options.push(opts);
    		}

    		let mut write_opts = WriteOptions::new();
    		if !config.wal {
    			write_opts.disable_wal(true);
    		}
    		let mut read_opts = ReadOptions::new();
    		read_opts.set_verify_checksums(false);

    		let mut cfs: Vec<Column> = Vec::new();
    		let db = match config.columns {
    			Some(columns) => {
    				match DB::open_cf(&opts, path, &cfnames, &cf_options) {
    					Ok(db) => {
    						cfs = cfnames.iter().map(|n| db.cf_handle(n)
    							.expect("rocksdb opens a cf_handle for each cfname; qed")).collect();
    						assert!(cfs.len() == columns as usize);
    						Ok(db)
    					}
    					Err(_) => {
    						// retry and create CFs
    						match DB::open_cf(&opts, path, &[], &[]) {
    							Ok(mut db) => {
    								cfs = cfnames.iter().enumerate().map(|(i, n)| db.create_cf(n, &cf_options[i])).collect::<Result<_, _>>()?;
    								Ok(db)
    							},
    							err @ Err(_) => err,
    						}
    					}
    				}
    			},
    			None => DB::open(&opts, path)
    		};

    		let db = match db {
    			Ok(db) => db,
    			Err(ref s) if s.starts_with("Corruption:") => {
    				info!("{}", s);
    				info!("Attempting DB repair for {}", path);
    				DB::repair(&opts, path)?;

    				match cfnames.is_empty() {
    					true => DB::open(&opts, path)?,
    					false => DB::open_cf(&opts, path, &cfnames, &cf_options)?
    				}
    			},
    			Err(s) => { return Err(s); }
    		};
    		let num_cols = cfs.len();
    		Ok(Database {
    			db: RwLock::new(Some(DBAndColumns{ db: db, cfs: cfs })),
    			config: config.clone(),
    			write_opts: write_opts,
    			overlay: RwLock::new((0..(num_cols + 1)).map(|_| HashMap::new()).collect()),
    			flushing: RwLock::new((0..(num_cols + 1)).map(|_| HashMap::new()).collect()),
    			flushing_lock: Mutex::new((false)),
    			path: path.to_owned(),
    			read_opts: read_opts,
    		})
    	}

    	/// Creates new transaction for this database.
    	pub fn transaction(&self) -> DBTransaction {
    		DBTransaction::new(self)
    	}

    	fn to_overlay_column(col: Option<u32>) -> usize {
    		col.map_or(0, |c| (c + 1) as usize)
    	}

    	/// Commit transaction to database.
    	pub fn write_buffered(&self, tr: DBTransaction) {
    		let mut overlay = self.overlay.write();
    		let ops = tr.ops;
    		for op in ops {
    			match op {
    				DBOp::Insert { col, key, value } => {
    					let c = Self::to_overlay_column(col);
    					overlay[c].insert(key, KeyState::Insert(value));
    				},
    				DBOp::InsertCompressed { col, key, value } => {
    					let c = Self::to_overlay_column(col);
    					overlay[c].insert(key, KeyState::InsertCompressed(value));
    				},
    				DBOp::Delete { col, key } => {
    					let c = Self::to_overlay_column(col);
    					overlay[c].insert(key, KeyState::Delete);
    				},
    			}
    		};
    	}

    	/// Commit buffered changes to database. Must be called under `flush_lock`
    	fn write_flushing_with_lock(&self, _lock: &mut MutexGuard<bool>) -> Result<(), String> {
    		match *self.db.read() {
    			Some(DBAndColumns { ref db, ref cfs }) => {
    				let batch = WriteBatch::new();
    				mem::swap(&mut *self.overlay.write(), &mut *self.flushing.write());
    				{
    					for (c, column) in self.flushing.read().iter().enumerate() {
    						for (ref key, ref state) in column.iter() {
    							match **state {
    								KeyState::Delete => {
    									if c > 0 {
    										batch.delete_cf(cfs[c - 1], &key)?;
    									} else {
    										batch.delete(&key)?;
    									}
    								},
    								KeyState::Insert(ref value) => {
    									if c > 0 {
    										batch.put_cf(cfs[c - 1], &key, value)?;
    									} else {
    										batch.put(&key, &value)?;
    									}
    								},
    								KeyState::InsertCompressed(ref value) => {
    									let compressed = UntrustedRlp::new(&value).compress(RlpType::Blocks);
    									if c > 0 {
    										batch.put_cf(cfs[c - 1], &key, &compressed)?;
    									} else {
    										batch.put(&key, &value)?;
    									}
    								}
    							}
    						}
    					}
    				}
    				db.write_opt(batch, &self.write_opts)?;
    				for column in self.flushing.write().iter_mut() {
    					column.clear();
    					column.shrink_to_fit();
    				}
    				Ok(())
    			},
    			None => Err("Database is closed".to_owned())
    		}
    	}

    	/// Commit buffered changes to database.
    	pub fn flush(&self) -> Result<(), String> {
    		let mut lock = self.flushing_lock.lock();
    		// If RocksDB batch allocation fails the thread gets terminated and the lock is released.
    		// The value inside the lock is used to detect that.
    		if *lock {
    			// This can only happen if another flushing thread is terminated unexpectedly.
    			return Err("Database write failure. Running low on memory perhaps?".to_owned());
    		}
    		*lock = true;
    		let result = self.write_flushing_with_lock(&mut lock);
    		*lock = false;
    		result
    	}

    	/// Commit transaction to database.
    	pub fn write(&self, tr: DBTransaction) -> Result<(), String> {
    		match *self.db.read() {
    			Some(DBAndColumns { ref db, ref cfs }) => {
    				let batch = WriteBatch::new();
    				let ops = tr.ops;
    				for op in ops {
    					match op {
    						DBOp::Insert { col, key, value } => {
    							col.map_or_else(|| batch.put(&key, &value), |c| batch.put_cf(cfs[c as usize], &key, &value))?
    						},
    						DBOp::InsertCompressed { col, key, value } => {
    							let compressed = UntrustedRlp::new(&value).compress(RlpType::Blocks);
    							col.map_or_else(|| batch.put(&key, &compressed), |c| batch.put_cf(cfs[c as usize], &key, &compressed))?
    						},
    						DBOp::Delete { col, key } => {
    							col.map_or_else(|| batch.delete(&key), |c| batch.delete_cf(cfs[c as usize], &key))?
    						},
    					}
    				}
    				db.write_opt(batch, &self.write_opts)
    			},
    			None => Err("Database is closed".to_owned())
    		}
    	}

    	/// Get value by key.
    	pub fn get(&self, col: Option<u32>, key: &[u8]) -> Result<Option<DBValue>, String> {
    		match *self.db.read() {
    			Some(DBAndColumns { ref db, ref cfs }) => {
    				let overlay = &self.overlay.read()[Self::to_overlay_column(col)];
    				match overlay.get(key) {
    					Some(&KeyState::Insert(ref value)) | Some(&KeyState::InsertCompressed(ref value)) => Ok(Some(value.clone())),
    					Some(&KeyState::Delete) => Ok(None),
    					None => {
    						let flushing = &self.flushing.read()[Self::to_overlay_column(col)];
    						match flushing.get(key) {
    							Some(&KeyState::Insert(ref value)) | Some(&KeyState::InsertCompressed(ref value)) => Ok(Some(value.clone())),
    							Some(&KeyState::Delete) => Ok(None),
    							None => {
    								col.map_or_else(
    									|| db.get_opt(key, &self.read_opts).map(|r| r.map(|v| DBValue::from_slice(&v))),
    									|c| db.get_cf_opt(cfs[c as usize], key, &self.read_opts).map(|r| r.map(|v| DBValue::from_slice(&v))))
    							},
    						}
    					},
    				}
    			},
    			None => Ok(None),
    		}
    	}

    	/// Get value by partial key. Prefix size should match configured prefix size. Only searches flushed values.
    	// TODO: support prefix seek for unflushed data
    	pub fn get_by_prefix(&self, col: Option<u32>, prefix: &[u8]) -> Option<Box<[u8]>> {
    		match *self.db.read() {
    			Some(DBAndColumns { ref db, ref cfs }) => {
    				let mut iter = col.map_or_else(|| db.iterator_opt(IteratorMode::From(prefix, Direction::Forward), &self.read_opts),
    					|c| db.iterator_cf_opt(cfs[c as usize], IteratorMode::From(prefix, Direction::Forward), &self.read_opts)
    						.expect("iterator params are valid; qed"));
    				match iter.next() {
    					// TODO: use prefix_same_as_start read option (not availabele in C API currently)
    					Some((k, v)) => if k[0 .. prefix.len()] == prefix[..] { Some(v) } else { None },
    					_ => None
    				}
    			},
    			None => None,
    		}
    	}

    	/// Get database iterator for flushed data.
    	pub fn iter(&self, col: Option<u32>) -> DatabaseIterator {
    		//TODO: iterate over overlay
    		match *self.db.read() {
    			Some(DBAndColumns { ref db, ref cfs }) => {
    				col.map_or_else(|| DatabaseIterator { iter: db.iterator_opt(IteratorMode::Start, &self.read_opts) },
    					|c| DatabaseIterator { iter: db.iterator_cf_opt(cfs[c as usize], IteratorMode::Start, &self.read_opts)
    						.expect("iterator params are valid; qed") })
    			},
    			None => panic!("Not supported yet") //TODO: return an empty iterator or change return type
    		}
    	}

    	/// Close the database
    	fn close(&self) {
    		*self.db.write() = None;
    		self.overlay.write().clear();
    		self.flushing.write().clear();
    	}

    	/// Restore the database from a copy at given path.
    	pub fn restore(&self, new_db: &str) -> Result<(), UtilError> {
    		self.close();

    		let mut backup_db = PathBuf::from(&self.path);
    		backup_db.pop();
    		backup_db.push("backup_db");

    		let existed = match fs::rename(&self.path, &backup_db) {
    			Ok(_) => true,
    			Err(e) => if let ErrorKind::NotFound = e.kind() {
    				false
    			} else {
    				return Err(e.into());
    			}
    		};

    		match fs::rename(&new_db, &self.path) {
    			Ok(_) => {
    				// clean up the backup.
    				if existed {
    					fs::remove_dir_all(&backup_db)?;
    				}
    			}
    			Err(e) => {
    				// restore the backup.
    				if existed {
    					fs::rename(&backup_db, &self.path)?;
    				}
    				return Err(e.into())
    			}
    		}

    		// reopen the database and steal handles into self
    		let db = Self::open(&self.config, &self.path)?;
    		*self.db.write() = mem::replace(&mut *db.db.write(), None);
    		*self.overlay.write() = mem::replace(&mut *db.overlay.write(), Vec::new());
    		*self.flushing.write() = mem::replace(&mut *db.flushing.write(), Vec::new());
    		Ok(())
    	}
    }

    impl Drop for Database {
    	fn drop(&mut self) {
    		// write all buffered changes if we can.
    		let _ = self.flush();
    	}
    }

    #[cfg(test)]
    mod tests {
    	use hash::H256;
    	use super::*;
    	use devtools::*;
    	use std::str::FromStr;

    	fn test_db(config: &DatabaseConfig) {
    		let path = RandomTempPath::create_dir();
    		let db = Database::open(config, path.as_path().to_str().unwrap()).unwrap();
    		let key1 = H256::from_str("02c69be41d0b7e40352fc85be1cd65eb03d40ef8427a0ca4596b1ead9a00e9fc").unwrap();
    		let key2 = H256::from_str("03c69be41d0b7e40352fc85be1cd65eb03d40ef8427a0ca4596b1ead9a00e9fc").unwrap();
    		let key3 = H256::from_str("01c69be41d0b7e40352fc85be1cd65eb03d40ef8427a0ca4596b1ead9a00e9fc").unwrap();

    		let mut batch = db.transaction();
    		batch.put(None, &key1, b"cat");
    		batch.put(None, &key2, b"dog");
    		db.write(batch).unwrap();

    		assert_eq!(&*db.get(None, &key1).unwrap().unwrap(), b"cat");

    		let contents: Vec<_> = db.iter(None).collect();
    		assert_eq!(contents.len(), 2);
    		assert_eq!(&*contents[0].0, &*key1);
    		assert_eq!(&*contents[0].1, b"cat");
    		assert_eq!(&*contents[1].0, &*key2);
    		assert_eq!(&*contents[1].1, b"dog");

    		let mut batch = db.transaction();
    		batch.delete(None, &key1);
    		db.write(batch).unwrap();

    		assert!(db.get(None, &key1).unwrap().is_none());

    		let mut batch = db.transaction();
    		batch.put(None, &key1, b"cat");
    		db.write(batch).unwrap();

    		let mut transaction = db.transaction();
    		transaction.put(None, &key3, b"elephant");
    		transaction.delete(None, &key1);
    		db.write(transaction).unwrap();
    		assert!(db.get(None, &key1).unwrap().is_none());
    		assert_eq!(&*db.get(None, &key3).unwrap().unwrap(), b"elephant");

    		assert_eq!(&*db.get_by_prefix(None, &key3).unwrap(), b"elephant");
    		assert_eq!(&*db.get_by_prefix(None, &key2).unwrap(), b"dog");

    		let mut transaction = db.transaction();
    		transaction.put(None, &key1, b"horse");
    		transaction.delete(None, &key3);
    		db.write_buffered(transaction);
    		assert!(db.get(None, &key3).unwrap().is_none());
    		assert_eq!(&*db.get(None, &key1).unwrap().unwrap(), b"horse");

    		db.flush().unwrap();
    		assert!(db.get(None, &key3).unwrap().is_none());
    		assert_eq!(&*db.get(None, &key1).unwrap().unwrap(), b"horse");
    	}

    	#[test]
    	fn kvdb() {
    		let path = RandomTempPath::create_dir();
    		let _ = Database::open_default(path.as_path().to_str().unwrap()).unwrap();
    		test_db(&DatabaseConfig::default());
    	}

    	#[test]
    	#[cfg(target_os = "linux")]
    	fn df_to_rotational() {
    		use std::path::PathBuf;
    		// Example df output.
    		let example_df = vec![70, 105, 108, 101, 115, 121, 115, 116, 101, 109, 32, 32, 32, 32, 32, 49, 75, 45, 98, 108, 111, 99, 107, 115, 32, 32, 32, 32, 32, 85, 115, 101, 100, 32, 65, 118, 97, 105, 108, 97, 98, 108, 101, 32, 85, 115, 101, 37, 32, 77, 111, 117, 110, 116, 101, 100, 32, 111, 110, 10, 47, 100, 101, 118, 47, 115, 100, 97, 49, 32, 32, 32, 32, 32, 32, 32, 54, 49, 52, 48, 57, 51, 48, 48, 32, 51, 56, 56, 50, 50, 50, 51, 54, 32, 32, 49, 57, 52, 52, 52, 54, 49, 54, 32, 32, 54, 55, 37, 32, 47, 10];
    		let expected_output = Some(PathBuf::from("/sys/block/sda/queue/rotational"));
    		assert_eq!(rotational_from_df_output(example_df), expected_output);
    	}
    }

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