Abstract database behind generic interface and implement alternative drivers (#322)

- [x] Design interface
- [x] Implement Sled backend
  - [x] Re-implement the SledCountedTree hack ~~on Sled backend~~ on all backends (i.e. over the abstraction)
- [x] Convert Garage code to use generic interface
- [x] Proof-read converted Garage code
- [ ] Test everything well
- [x] Implement sqlite backend
- [x] Implement LMDB backend
- [ ] (Implement Persy backend?)
- [ ] (Implement other backends? (like RocksDB, ...))
- [x] Implement backend choice in config file and garage server module
- [x] Add CLI for converting between DB formats
- Exploit the new interface to put more things in transactions
  - [x] `.updated()` trigger on Garage tables

Fix #284

**Bugs**

- [x] When exporting sqlite, trees iterate empty??
- [x] LMDB doesn't work

**Known issues for various back-ends**

- Sled:
  - Eats all my RAM and also all my disk space
  - `.len()` has to traverse the whole table
  - Is actually quite slow on some operations
  - And is actually pretty bad code...
- Sqlite:
  - Requires a lock to be taken on all operations. The lock is also taken when iterating on a table with `.iter()`, and the lock isn't released until the iterator is dropped. This means that we must be VERY carefull to not do anything else inside a `.iter()` loop or else we will have a deadlock! Most such cases have been eliminated from the Garage codebase, but there might still be some that remain. If your Garage-over-Sqlite seems to hang/freeze, this is the reason.
  - (adapter uses a bunch of unsafe code)
- Heed (LMDB):
  - Not suited for 32-bit machines as it has to map the whole DB in memory.
  - (adpater uses a tiny bit of unsafe code)

**My recommendation:** avoid 32-bit machines and use LMDB as much as possible.

**Converting databases** is actually quite easy. For example from Sled to LMDB:

```bash
cd src/db
cargo run --features cli --bin convert -- -i path/to/garage/meta/db -a sled -o path/to/garage/meta/db.lmdb -b lmdb
```

Then, just add this to your `config.toml`:

```toml
db_engine = "lmdb"
```

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/322
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

src/db/Cargo.toml

@@ -0,0 +1,36 @@
+[package]
+name = "garage_db"
+version = "0.8.0"
+authors = ["Alex Auvolat <alex@adnab.me>"]
+edition = "2018"
+license = "AGPL-3.0"
+description = "Abstraction over multiple key/value storage engines that supports transactions"
+repository = "https://git.deuxfleurs.fr/Deuxfleurs/garage"
+readme = "../../README.md"
+
+[lib]
+path = "lib.rs"
+
+[[bin]]
+name = "convert"
+path = "bin/convert.rs"
+required-features = ["cli"]
+
+[dependencies]
+err-derive = "0.3"
+hexdump = "0.1"
+log = "0.4"
+
+heed = "0.11"
+rusqlite = { version = "0.27", features = ["bundled"] }
+sled = "0.34"
+
+# cli deps
+clap = { version = "3.1.18", optional = true, features = ["derive", "env"] }
+pretty_env_logger = { version = "0.4", optional = true }
+
+[dev-dependencies]
+mktemp = "0.4"
+
+[features]
+cli = ["clap", "pretty_env_logger"]

src/db/bin/convert.rs

@@ -0,0 +1,76 @@
+use std::path::PathBuf;
+
+use garage_db::*;
+
+use clap::Parser;
+
+/// K2V command line interface
+#[derive(Parser, Debug)]
+#[clap(author, version, about, long_about = None)]
+struct Args {
+	/// Input DB path
+	#[clap(short = 'i')]
+	input_path: PathBuf,
+	/// Input DB engine
+	#[clap(short = 'a')]
+	input_engine: String,
+
+	/// Output DB path
+	#[clap(short = 'o')]
+	output_path: PathBuf,
+	/// Output DB engine
+	#[clap(short = 'b')]
+	output_engine: String,
+}
+
+fn main() {
+	let args = Args::parse();
+	pretty_env_logger::init();
+
+	match do_conversion(args) {
+		Ok(()) => println!("Success!"),
+		Err(e) => eprintln!("Error: {}", e),
+	}
+}
+
+fn do_conversion(args: Args) -> Result<()> {
+	let input = open_db(args.input_path, args.input_engine)?;
+	let output = open_db(args.output_path, args.output_engine)?;
+	output.import(&input)?;
+	Ok(())
+}
+
+fn open_db(path: PathBuf, engine: String) -> Result<Db> {
+	match engine.as_str() {
+		"sled" => {
+			let db = sled_adapter::sled::Config::default().path(&path).open()?;
+			Ok(sled_adapter::SledDb::init(db))
+		}
+		"sqlite" | "sqlite3" | "rusqlite" => {
+			let db = sqlite_adapter::rusqlite::Connection::open(&path)?;
+			Ok(sqlite_adapter::SqliteDb::init(db))
+		}
+		"lmdb" | "heed" => {
+			std::fs::create_dir_all(&path).map_err(|e| {
+				Error(format!("Unable to create LMDB data directory: {}", e).into())
+			})?;
+
+			let map_size = if u32::MAX as usize == usize::MAX {
+				eprintln!(
+					"LMDB is not recommended on 32-bit systems, database size will be limited"
+				);
+				1usize << 30 // 1GB for 32-bit systems
+			} else {
+				1usize << 40 // 1TB for 64-bit systems
+			};
+
+			let db = lmdb_adapter::heed::EnvOpenOptions::new()
+				.max_dbs(100)
+				.map_size(map_size)
+				.open(&path)
+				.unwrap();
+			Ok(lmdb_adapter::LmdbDb::init(db))
+		}
+		e => Err(Error(format!("Invalid DB engine: {}", e).into())),
+	}
+}

src/db/counted_tree_hack.rs

@@ -0,0 +1,127 @@
+//! This hack allows a db tree to keep in RAM a counter of the number of entries
+//! it contains, which is used to call .len() on it.  This is usefull only for
+//! the sled backend where .len() otherwise would have to traverse the whole
+//! tree to count items.  For sqlite and lmdb, this is mostly useless (but
+//! hopefully not harmfull!). Note that a CountedTree cannot be part of a
+//! transaction.
+
+use std::sync::{
+	atomic::{AtomicUsize, Ordering},
+	Arc,
+};
+
+use crate::{Result, Tree, TxError, Value, ValueIter};
+
+#[derive(Clone)]
+pub struct CountedTree(Arc<CountedTreeInternal>);
+
+struct CountedTreeInternal {
+	tree: Tree,
+	len: AtomicUsize,
+}
+
+impl CountedTree {
+	pub fn new(tree: Tree) -> Result<Self> {
+		let len = tree.len()?;
+		Ok(Self(Arc::new(CountedTreeInternal {
+			tree,
+			len: AtomicUsize::new(len),
+		})))
+	}
+
+	pub fn len(&self) -> usize {
+		self.0.len.load(Ordering::SeqCst)
+	}
+
+	pub fn is_empty(&self) -> bool {
+		self.len() == 0
+	}
+
+	pub fn get<K: AsRef<[u8]>>(&self, key: K) -> Result<Option<Value>> {
+		self.0.tree.get(key)
+	}
+
+	pub fn first(&self) -> Result<Option<(Value, Value)>> {
+		self.0.tree.first()
+	}
+
+	pub fn iter(&self) -> Result<ValueIter<'_>> {
+		self.0.tree.iter()
+	}
+
+	// ---- writing functions ----
+
+	pub fn insert<K, V>(&self, key: K, value: V) -> Result<Option<Value>>
+	where
+		K: AsRef<[u8]>,
+		V: AsRef<[u8]>,
+	{
+		let old_val = self.0.tree.insert(key, value)?;
+		if old_val.is_none() {
+			self.0.len.fetch_add(1, Ordering::SeqCst);
+		}
+		Ok(old_val)
+	}
+
+	pub fn remove<K: AsRef<[u8]>>(&self, key: K) -> Result<Option<Value>> {
+		let old_val = self.0.tree.remove(key)?;
+		if old_val.is_some() {
+			self.0.len.fetch_sub(1, Ordering::SeqCst);
+		}
+		Ok(old_val)
+	}
+
+	pub fn compare_and_swap<K, OV, NV>(
+		&self,
+		key: K,
+		expected_old: Option<OV>,
+		new: Option<NV>,
+	) -> Result<bool>
+	where
+		K: AsRef<[u8]>,
+		OV: AsRef<[u8]>,
+		NV: AsRef<[u8]>,
+	{
+		let old_some = expected_old.is_some();
+		let new_some = new.is_some();
+
+		let tx_res = self.0.tree.db().transaction(|mut tx| {
+			let old_val = tx.get(&self.0.tree, &key)?;
+			let is_same = match (&old_val, &expected_old) {
+				(None, None) => true,
+				(Some(x), Some(y)) if x == y.as_ref() => true,
+				_ => false,
+			};
+			if is_same {
+				match &new {
+					Some(v) => {
+						tx.insert(&self.0.tree, &key, v)?;
+					}
+					None => {
+						tx.remove(&self.0.tree, &key)?;
+					}
+				}
+				tx.commit(())
+			} else {
+				tx.abort(())
+			}
+		});
+
+		match tx_res {
+			Ok(()) => {
+				match (old_some, new_some) {
+					(false, true) => {
+						self.0.len.fetch_add(1, Ordering::SeqCst);
+					}
+					(true, false) => {
+						self.0.len.fetch_sub(1, Ordering::SeqCst);
+					}
+					_ => (),
+				}
+				Ok(true)
+			}
+			Err(TxError::Abort(())) => Ok(false),
+			Err(TxError::Db(e)) => Err(e),
+		}
+	}
+}

src/db/lib.rs

@@ -0,0 +1,400 @@
+pub mod lmdb_adapter;
+pub mod sled_adapter;
+pub mod sqlite_adapter;
+
+pub mod counted_tree_hack;
+
+#[cfg(test)]
+pub mod test;
+
+use core::ops::{Bound, RangeBounds};
+
+use std::borrow::Cow;
+use std::cell::Cell;
+use std::sync::Arc;
+
+use err_derive::Error;
+
+#[derive(Clone)]
+pub struct Db(pub(crate) Arc<dyn IDb>);
+
+pub struct Transaction<'a>(&'a mut dyn ITx);
+
+#[derive(Clone)]
+pub struct Tree(Arc<dyn IDb>, usize);
+
+pub type Value = Vec<u8>;
+pub type ValueIter<'a> = Box<dyn std::iter::Iterator<Item = Result<(Value, Value)>> + 'a>;
+pub type TxValueIter<'a> = Box<dyn std::iter::Iterator<Item = TxOpResult<(Value, Value)>> + 'a>;
+
+// ----
+
+#[derive(Debug, Error)]
+#[error(display = "{}", _0)]
+pub struct Error(pub Cow<'static, str>);
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+#[derive(Debug, Error)]
+#[error(display = "{}", _0)]
+pub struct TxOpError(pub(crate) Error);
+pub type TxOpResult<T> = std::result::Result<T, TxOpError>;
+
+pub enum TxError<E> {
+	Abort(E),
+	Db(Error),
+}
+pub type TxResult<R, E> = std::result::Result<R, TxError<E>>;
+
+impl<E> From<TxOpError> for TxError<E> {
+	fn from(e: TxOpError) -> TxError<E> {
+		TxError::Db(e.0)
+	}
+}
+
+pub fn unabort<R, E>(res: TxResult<R, E>) -> TxOpResult<std::result::Result<R, E>> {
+	match res {
+		Ok(v) => Ok(Ok(v)),
+		Err(TxError::Abort(e)) => Ok(Err(e)),
+		Err(TxError::Db(e)) => Err(TxOpError(e)),
+	}
+}
+
+// ----
+
+impl Db {
+	pub fn engine(&self) -> String {
+		self.0.engine()
+	}
+
+	pub fn open_tree<S: AsRef<str>>(&self, name: S) -> Result<Tree> {
+		let tree_id = self.0.open_tree(name.as_ref())?;
+		Ok(Tree(self.0.clone(), tree_id))
+	}
+
+	pub fn list_trees(&self) -> Result<Vec<String>> {
+		self.0.list_trees()
+	}
+
+	pub fn transaction<R, E, F>(&self, fun: F) -> TxResult<R, E>
+	where
+		F: Fn(Transaction<'_>) -> TxResult<R, E>,
+	{
+		let f = TxFn {
+			function: fun,
+			result: Cell::new(None),
+		};
+		let tx_res = self.0.transaction(&f);
+		let ret = f
+			.result
+			.into_inner()
+			.expect("Transaction did not store result");
+
+		match tx_res {
+			Ok(()) => {
+				assert!(matches!(ret, Ok(_)));
+				ret
+			}
+			Err(TxError::Abort(())) => {
+				assert!(matches!(ret, Err(TxError::Abort(_))));
+				ret
+			}
+			Err(TxError::Db(e2)) => match ret {
+				// Ok was stored -> the error occured when finalizing
+				// transaction
+				Ok(_) => Err(TxError::Db(e2)),
+				// An error was already stored: that's the one we want to
+				// return
+				Err(TxError::Db(e)) => Err(TxError::Db(e)),
+				_ => unreachable!(),
+			},
+		}
+	}
+
+	pub fn import(&self, other: &Db) -> Result<()> {
+		let existing_trees = self.list_trees()?;
+		if !existing_trees.is_empty() {
+			return Err(Error(
+				format!(
+					"destination database already contains data: {:?}",
+					existing_trees
+				)
+				.into(),
+			));
+		}
+
+		let tree_names = other.list_trees()?;
+		for name in tree_names {
+			let tree = self.open_tree(&name)?;
+			if tree.len()? > 0 {
+				return Err(Error(format!("tree {} already contains data", name).into()));
+			}
+
+			let ex_tree = other.open_tree(&name)?;
+
+			let tx_res = self.transaction(|mut tx| {
+				let mut i = 0;
+				for item in ex_tree.iter().map_err(TxError::Abort)? {
+					let (k, v) = item.map_err(TxError::Abort)?;
+					tx.insert(&tree, k, v)?;
+					i += 1;
+					if i % 1000 == 0 {
+						println!("{}: imported {}", name, i);
+					}
+				}
+				tx.commit(i)
+			});
+			let total = match tx_res {
+				Err(TxError::Db(e)) => return Err(e),
+				Err(TxError::Abort(e)) => return Err(e),
+				Ok(x) => x,
+			};
+
+			println!("{}: finished importing, {} items", name, total);
+		}
+		Ok(())
+	}
+}
+
+#[allow(clippy::len_without_is_empty)]
+impl Tree {
+	#[inline]
+	pub fn db(&self) -> Db {
+		Db(self.0.clone())
+	}
+
+	#[inline]
+	pub fn get<T: AsRef<[u8]>>(&self, key: T) -> Result<Option<Value>> {
+		self.0.get(self.1, key.as_ref())
+	}
+	#[inline]
+	pub fn len(&self) -> Result<usize> {
+		self.0.len(self.1)
+	}
+
+	#[inline]
+	pub fn first(&self) -> Result<Option<(Value, Value)>> {
+		self.iter()?.next().transpose()
+	}
+	#[inline]
+	pub fn get_gt<T: AsRef<[u8]>>(&self, from: T) -> Result<Option<(Value, Value)>> {
+		self.range((Bound::Excluded(from), Bound::Unbounded))?
+			.next()
+			.transpose()
+	}
+
+	/// Returns the old value if there was one
+	#[inline]
+	pub fn insert<T: AsRef<[u8]>, U: AsRef<[u8]>>(
+		&self,
+		key: T,
+		value: U,
+	) -> Result<Option<Value>> {
+		self.0.insert(self.1, key.as_ref(), value.as_ref())
+	}
+	/// Returns the old value if there was one
+	#[inline]
+	pub fn remove<T: AsRef<[u8]>>(&self, key: T) -> Result<Option<Value>> {
+		self.0.remove(self.1, key.as_ref())
+	}
+
+	#[inline]
+	pub fn iter(&self) -> Result<ValueIter<'_>> {
+		self.0.iter(self.1)
+	}
+	#[inline]
+	pub fn iter_rev(&self) -> Result<ValueIter<'_>> {
+		self.0.iter_rev(self.1)
+	}
+
+	#[inline]
+	pub fn range<K, R>(&self, range: R) -> Result<ValueIter<'_>>
+	where
+		K: AsRef<[u8]>,
+		R: RangeBounds<K>,
+	{
+		let sb = range.start_bound();
+		let eb = range.end_bound();
+		self.0.range(self.1, get_bound(sb), get_bound(eb))
+	}
+	#[inline]
+	pub fn range_rev<K, R>(&self, range: R) -> Result<ValueIter<'_>>
+	where
+		K: AsRef<[u8]>,
+		R: RangeBounds<K>,
+	{
+		let sb = range.start_bound();
+		let eb = range.end_bound();
+		self.0.range_rev(self.1, get_bound(sb), get_bound(eb))
+	}
+}
+
+#[allow(clippy::len_without_is_empty)]
+impl<'a> Transaction<'a> {
+	#[inline]
+	pub fn get<T: AsRef<[u8]>>(&self, tree: &Tree, key: T) -> TxOpResult<Option<Value>> {
+		self.0.get(tree.1, key.as_ref())
+	}
+	#[inline]
+	pub fn len(&self, tree: &Tree) -> TxOpResult<usize> {
+		self.0.len(tree.1)
+	}
+
+	/// Returns the old value if there was one
+	#[inline]
+	pub fn insert<T: AsRef<[u8]>, U: AsRef<[u8]>>(
+		&mut self,
+		tree: &Tree,
+		key: T,
+		value: U,
+	) -> TxOpResult<Option<Value>> {
+		self.0.insert(tree.1, key.as_ref(), value.as_ref())
+	}
+	/// Returns the old value if there was one
+	#[inline]
+	pub fn remove<T: AsRef<[u8]>>(&mut self, tree: &Tree, key: T) -> TxOpResult<Option<Value>> {
+		self.0.remove(tree.1, key.as_ref())
+	}
+
+	#[inline]
+	pub fn iter(&self, tree: &Tree) -> TxOpResult<TxValueIter<'_>> {
+		self.0.iter(tree.1)
+	}
+	#[inline]
+	pub fn iter_rev(&self, tree: &Tree) -> TxOpResult<TxValueIter<'_>> {
+		self.0.iter_rev(tree.1)
+	}
+
+	#[inline]
+	pub fn range<K, R>(&self, tree: &Tree, range: R) -> TxOpResult<TxValueIter<'_>>
+	where
+		K: AsRef<[u8]>,
+		R: RangeBounds<K>,
+	{
+		let sb = range.start_bound();
+		let eb = range.end_bound();
+		self.0.range(tree.1, get_bound(sb), get_bound(eb))
+	}
+	#[inline]
+	pub fn range_rev<K, R>(&self, tree: &Tree, range: R) -> TxOpResult<TxValueIter<'_>>
+	where
+		K: AsRef<[u8]>,
+		R: RangeBounds<K>,
+	{
+		let sb = range.start_bound();
+		let eb = range.end_bound();
+		self.0.range_rev(tree.1, get_bound(sb), get_bound(eb))
+	}
+
+	// ----
+
+	#[inline]
+	pub fn abort<R, E>(self, e: E) -> TxResult<R, E> {
+		Err(TxError::Abort(e))
+	}
+
+	#[inline]
+	pub fn commit<R, E>(self, r: R) -> TxResult<R, E> {
+		Ok(r)
+	}
+}
+
+// ---- Internal interfaces
+
+pub(crate) trait IDb: Send + Sync {
+	fn engine(&self) -> String;
+	fn open_tree(&self, name: &str) -> Result<usize>;
+	fn list_trees(&self) -> Result<Vec<String>>;
+
+	fn get(&self, tree: usize, key: &[u8]) -> Result<Option<Value>>;
+	fn len(&self, tree: usize) -> Result<usize>;
+
+	fn insert(&self, tree: usize, key: &[u8], value: &[u8]) -> Result<Option<Value>>;
+	fn remove(&self, tree: usize, key: &[u8]) -> Result<Option<Value>>;
+
+	fn iter(&self, tree: usize) -> Result<ValueIter<'_>>;
+	fn iter_rev(&self, tree: usize) -> Result<ValueIter<'_>>;
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>>;
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>>;
+
+	fn transaction(&self, f: &dyn ITxFn) -> TxResult<(), ()>;
+}
+
+pub(crate) trait ITx {
+	fn get(&self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>>;
+	fn len(&self, tree: usize) -> TxOpResult<usize>;
+
+	fn insert(&mut self, tree: usize, key: &[u8], value: &[u8]) -> TxOpResult<Option<Value>>;
+	fn remove(&mut self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>>;
+
+	fn iter(&self, tree: usize) -> TxOpResult<TxValueIter<'_>>;
+	fn iter_rev(&self, tree: usize) -> TxOpResult<TxValueIter<'_>>;
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>>;
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>>;
+}
+
+pub(crate) trait ITxFn {
+	fn try_on(&self, tx: &mut dyn ITx) -> TxFnResult;
+}
+
+pub(crate) enum TxFnResult {
+	Ok,
+	Abort,
+	DbErr,
+}
+
+struct TxFn<F, R, E>
+where
+	F: Fn(Transaction<'_>) -> TxResult<R, E>,
+{
+	function: F,
+	result: Cell<Option<TxResult<R, E>>>,
+}
+
+impl<F, R, E> ITxFn for TxFn<F, R, E>
+where
+	F: Fn(Transaction<'_>) -> TxResult<R, E>,
+{
+	fn try_on(&self, tx: &mut dyn ITx) -> TxFnResult {
+		let res = (self.function)(Transaction(tx));
+		let res2 = match &res {
+			Ok(_) => TxFnResult::Ok,
+			Err(TxError::Abort(_)) => TxFnResult::Abort,
+			Err(TxError::Db(_)) => TxFnResult::DbErr,
+		};
+		self.result.set(Some(res));
+		res2
+	}
+}
+
+// ----
+
+fn get_bound<K: AsRef<[u8]>>(b: Bound<&K>) -> Bound<&[u8]> {
+	match b {
+		Bound::Included(v) => Bound::Included(v.as_ref()),
+		Bound::Excluded(v) => Bound::Excluded(v.as_ref()),
+		Bound::Unbounded => Bound::Unbounded,
+	}
+}

src/db/lmdb_adapter.rs

@@ -0,0 +1,329 @@
+use core::ops::Bound;
+use core::ptr::NonNull;
+
+use std::collections::HashMap;
+use std::convert::TryInto;
+use std::sync::{Arc, RwLock};
+
+use heed::types::ByteSlice;
+use heed::{BytesDecode, Env, RoTxn, RwTxn, UntypedDatabase as Database};
+
+use crate::{
+	Db, Error, IDb, ITx, ITxFn, Result, TxError, TxFnResult, TxOpError, TxOpResult, TxResult,
+	TxValueIter, Value, ValueIter,
+};
+
+pub use heed;
+
+// -- err
+
+impl From<heed::Error> for Error {
+	fn from(e: heed::Error) -> Error {
+		Error(format!("LMDB: {}", e).into())
+	}
+}
+
+impl From<heed::Error> for TxOpError {
+	fn from(e: heed::Error) -> TxOpError {
+		TxOpError(e.into())
+	}
+}
+
+// -- db
+
+pub struct LmdbDb {
+	db: heed::Env,
+	trees: RwLock<(Vec<Database>, HashMap<String, usize>)>,
+}
+
+impl LmdbDb {
+	pub fn init(db: Env) -> Db {
+		let s = Self {
+			db,
+			trees: RwLock::new((Vec::new(), HashMap::new())),
+		};
+		Db(Arc::new(s))
+	}
+
+	fn get_tree(&self, i: usize) -> Result<Database> {
+		self.trees
+			.read()
+			.unwrap()
+			.0
+			.get(i)
+			.cloned()
+			.ok_or_else(|| Error("invalid tree id".into()))
+	}
+}
+
+impl IDb for LmdbDb {
+	fn engine(&self) -> String {
+		"LMDB (using Heed crate)".into()
+	}
+
+	fn open_tree(&self, name: &str) -> Result<usize> {
+		let mut trees = self.trees.write().unwrap();
+		if let Some(i) = trees.1.get(name) {
+			Ok(*i)
+		} else {
+			let tree = self.db.create_database(Some(name))?;
+			let i = trees.0.len();
+			trees.0.push(tree);
+			trees.1.insert(name.to_string(), i);
+			Ok(i)
+		}
+	}
+
+	fn list_trees(&self) -> Result<Vec<String>> {
+		let tree0 = match self.db.open_database::<heed::types::Str, ByteSlice>(None)? {
+			Some(x) => x,
+			None => return Ok(vec![]),
+		};
+
+		let mut ret = vec![];
+		let tx = self.db.read_txn()?;
+		for item in tree0.iter(&tx)? {
+			let (tree_name, _) = item?;
+			ret.push(tree_name.to_string());
+		}
+		drop(tx);
+
+		let mut ret2 = vec![];
+		for tree_name in ret {
+			if self
+				.db
+				.open_database::<ByteSlice, ByteSlice>(Some(&tree_name))?
+				.is_some()
+			{
+				ret2.push(tree_name);
+			}
+		}
+
+		Ok(ret2)
+	}
+
+	// ----
+
+	fn get(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+
+		let tx = self.db.read_txn()?;
+		let val = tree.get(&tx, key)?;
+		match val {
+			None => Ok(None),
+			Some(v) => Ok(Some(v.to_vec())),
+		}
+	}
+
+	fn len(&self, tree: usize) -> Result<usize> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		Ok(tree.len(&tx)?.try_into().unwrap())
+	}
+
+	fn insert(&self, tree: usize, key: &[u8], value: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let mut tx = self.db.write_txn()?;
+		let old_val = tree.get(&tx, key)?.map(Vec::from);
+		tree.put(&mut tx, key, value)?;
+		tx.commit()?;
+		Ok(old_val)
+	}
+
+	fn remove(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let mut tx = self.db.write_txn()?;
+		let old_val = tree.get(&tx, key)?.map(Vec::from);
+		tree.delete(&mut tx, key)?;
+		tx.commit()?;
+		Ok(old_val)
+	}
+
+	fn iter(&self, tree: usize) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		TxAndIterator::make(tx, |tx| Ok(tree.iter(tx)?))
+	}
+
+	fn iter_rev(&self, tree: usize) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		TxAndIterator::make(tx, |tx| Ok(tree.rev_iter(tx)?))
+	}
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		TxAndIterator::make(tx, |tx| Ok(tree.range(tx, &(low, high))?))
+	}
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		let tx = self.db.read_txn()?;
+		TxAndIterator::make(tx, |tx| Ok(tree.rev_range(tx, &(low, high))?))
+	}
+
+	// ----
+
+	fn transaction(&self, f: &dyn ITxFn) -> TxResult<(), ()> {
+		let trees = self.trees.read().unwrap();
+		let mut tx = LmdbTx {
+			trees: &trees.0[..],
+			tx: self
+				.db
+				.write_txn()
+				.map_err(Error::from)
+				.map_err(TxError::Db)?,
+		};
+
+		let res = f.try_on(&mut tx);
+		match res {
+			TxFnResult::Ok => {
+				tx.tx.commit().map_err(Error::from).map_err(TxError::Db)?;
+				Ok(())
+			}
+			TxFnResult::Abort => {
+				tx.tx.abort().map_err(Error::from).map_err(TxError::Db)?;
+				Err(TxError::Abort(()))
+			}
+			TxFnResult::DbErr => {
+				tx.tx.abort().map_err(Error::from).map_err(TxError::Db)?;
+				Err(TxError::Db(Error(
+					"(this message will be discarded)".into(),
+				)))
+			}
+		}
+	}
+}
+
+// ----
+
+struct LmdbTx<'a> {
+	trees: &'a [Database],
+	tx: RwTxn<'a, 'a>,
+}
+
+impl<'a> LmdbTx<'a> {
+	fn get_tree(&self, i: usize) -> TxOpResult<&Database> {
+		self.trees.get(i).ok_or_else(|| {
+			TxOpError(Error(
+				"invalid tree id (it might have been openned after the transaction started)".into(),
+			))
+		})
+	}
+}
+
+impl<'a> ITx for LmdbTx<'a> {
+	fn get(&self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		match tree.get(&self.tx, key)? {
+			Some(v) => Ok(Some(v.to_vec())),
+			None => Ok(None),
+		}
+	}
+	fn len(&self, _tree: usize) -> TxOpResult<usize> {
+		unimplemented!(".len() in transaction not supported with LMDB backend")
+	}
+
+	fn insert(&mut self, tree: usize, key: &[u8], value: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = *self.get_tree(tree)?;
+		let old_val = tree.get(&self.tx, key)?.map(Vec::from);
+		tree.put(&mut self.tx, key, value)?;
+		Ok(old_val)
+	}
+	fn remove(&mut self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = *self.get_tree(tree)?;
+		let old_val = tree.get(&self.tx, key)?.map(Vec::from);
+		tree.delete(&mut self.tx, key)?;
+		Ok(old_val)
+	}
+
+	fn iter(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with LMDB backend");
+	}
+	fn iter_rev(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with LMDB backend");
+	}
+
+	fn range<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with LMDB backend");
+	}
+	fn range_rev<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with LMDB backend");
+	}
+}
+
+// ----
+
+type IteratorItem<'a> = heed::Result<(
+	<ByteSlice as BytesDecode<'a>>::DItem,
+	<ByteSlice as BytesDecode<'a>>::DItem,
+)>;
+
+struct TxAndIterator<'a, I>
+where
+	I: Iterator<Item = IteratorItem<'a>> + 'a,
+{
+	tx: RoTxn<'a>,
+	iter: Option<I>,
+}
+
+impl<'a, I> TxAndIterator<'a, I>
+where
+	I: Iterator<Item = IteratorItem<'a>> + 'a,
+{
+	fn make<F>(tx: RoTxn<'a>, iterfun: F) -> Result<ValueIter<'a>>
+	where
+		F: FnOnce(&'a RoTxn<'a>) -> Result<I>,
+	{
+		let mut res = TxAndIterator { tx, iter: None };
+
+		let tx = unsafe { NonNull::from(&res.tx).as_ref() };
+		res.iter = Some(iterfun(tx)?);
+
+		Ok(Box::new(res))
+	}
+}
+
+impl<'a, I> Drop for TxAndIterator<'a, I>
+where
+	I: Iterator<Item = IteratorItem<'a>> + 'a,
+{
+	fn drop(&mut self) {
+		drop(self.iter.take());
+	}
+}
+
+impl<'a, I> Iterator for TxAndIterator<'a, I>
+where
+	I: Iterator<Item = IteratorItem<'a>> + 'a,
+{
+	type Item = Result<(Value, Value)>;
+
+	fn next(&mut self) -> Option<Self::Item> {
+		match self.iter.as_mut().unwrap().next() {
+			None => None,
+			Some(Err(e)) => Some(Err(e.into())),
+			Some(Ok((k, v))) => Some(Ok((k.to_vec(), v.to_vec()))),
+		}
+	}
+}

src/db/sled_adapter.rs

@@ -0,0 +1,260 @@
+use core::ops::Bound;
+
+use std::cell::Cell;
+use std::collections::HashMap;
+use std::sync::{Arc, RwLock};
+
+use sled::transaction::{
+	ConflictableTransactionError, TransactionError, Transactional, TransactionalTree,
+	UnabortableTransactionError,
+};
+
+use crate::{
+	Db, Error, IDb, ITx, ITxFn, Result, TxError, TxFnResult, TxOpError, TxOpResult, TxResult,
+	TxValueIter, Value, ValueIter,
+};
+
+pub use sled;
+
+// -- err
+
+impl From<sled::Error> for Error {
+	fn from(e: sled::Error) -> Error {
+		Error(format!("Sled: {}", e).into())
+	}
+}
+
+impl From<sled::Error> for TxOpError {
+	fn from(e: sled::Error) -> TxOpError {
+		TxOpError(e.into())
+	}
+}
+
+// -- db
+
+pub struct SledDb {
+	db: sled::Db,
+	trees: RwLock<(Vec<sled::Tree>, HashMap<String, usize>)>,
+}
+
+impl SledDb {
+	pub fn init(db: sled::Db) -> Db {
+		let s = Self {
+			db,
+			trees: RwLock::new((Vec::new(), HashMap::new())),
+		};
+		Db(Arc::new(s))
+	}
+
+	fn get_tree(&self, i: usize) -> Result<sled::Tree> {
+		self.trees
+			.read()
+			.unwrap()
+			.0
+			.get(i)
+			.cloned()
+			.ok_or_else(|| Error("invalid tree id".into()))
+	}
+}
+
+impl IDb for SledDb {
+	fn engine(&self) -> String {
+		"Sled".into()
+	}
+
+	fn open_tree(&self, name: &str) -> Result<usize> {
+		let mut trees = self.trees.write().unwrap();
+		if let Some(i) = trees.1.get(name) {
+			Ok(*i)
+		} else {
+			let tree = self.db.open_tree(name)?;
+			let i = trees.0.len();
+			trees.0.push(tree);
+			trees.1.insert(name.to_string(), i);
+			Ok(i)
+		}
+	}
+
+	fn list_trees(&self) -> Result<Vec<String>> {
+		let mut trees = vec![];
+		for name in self.db.tree_names() {
+			let name = std::str::from_utf8(&name)
+				.map_err(|e| Error(format!("{}", e).into()))?
+				.to_string();
+			if name != "__sled__default" {
+				trees.push(name);
+			}
+		}
+		Ok(trees)
+	}
+
+	// ----
+
+	fn get(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let val = tree.get(key)?;
+		Ok(val.map(|x| x.to_vec()))
+	}
+
+	fn len(&self, tree: usize) -> Result<usize> {
+		let tree = self.get_tree(tree)?;
+		Ok(tree.len())
+	}
+
+	fn insert(&self, tree: usize, key: &[u8], value: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = tree.insert(key, value)?;
+		Ok(old_val.map(|x| x.to_vec()))
+	}
+
+	fn remove(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = tree.remove(key)?;
+		Ok(old_val.map(|x| x.to_vec()))
+	}
+
+	fn iter(&self, tree: usize) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		Ok(Box::new(tree.iter().map(|v| {
+			v.map(|(x, y)| (x.to_vec(), y.to_vec())).map_err(Into::into)
+		})))
+	}
+
+	fn iter_rev(&self, tree: usize) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		Ok(Box::new(tree.iter().rev().map(|v| {
+			v.map(|(x, y)| (x.to_vec(), y.to_vec())).map_err(Into::into)
+		})))
+	}
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		Ok(Box::new(tree.range::<&'r [u8], _>((low, high)).map(|v| {
+			v.map(|(x, y)| (x.to_vec(), y.to_vec())).map_err(Into::into)
+		})))
+	}
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		let tree = self.get_tree(tree)?;
+		Ok(Box::new(tree.range::<&'r [u8], _>((low, high)).rev().map(
+			|v| v.map(|(x, y)| (x.to_vec(), y.to_vec())).map_err(Into::into),
+		)))
+	}
+
+	// ----
+
+	fn transaction(&self, f: &dyn ITxFn) -> TxResult<(), ()> {
+		let trees = self.trees.read().unwrap();
+		let res = trees.0.transaction(|txtrees| {
+			let mut tx = SledTx {
+				trees: txtrees,
+				err: Cell::new(None),
+			};
+			match f.try_on(&mut tx) {
+				TxFnResult::Ok => {
+					assert!(tx.err.into_inner().is_none());
+					Ok(())
+				}
+				TxFnResult::Abort => {
+					assert!(tx.err.into_inner().is_none());
+					Err(ConflictableTransactionError::Abort(()))
+				}
+				TxFnResult::DbErr => {
+					let e = tx.err.into_inner().expect("No DB error");
+					Err(e.into())
+				}
+			}
+		});
+		match res {
+			Ok(()) => Ok(()),
+			Err(TransactionError::Abort(())) => Err(TxError::Abort(())),
+			Err(TransactionError::Storage(s)) => Err(TxError::Db(s.into())),
+		}
+	}
+}
+
+// ----
+
+struct SledTx<'a> {
+	trees: &'a [TransactionalTree],
+	err: Cell<Option<UnabortableTransactionError>>,
+}
+
+impl<'a> SledTx<'a> {
+	fn get_tree(&self, i: usize) -> TxOpResult<&TransactionalTree> {
+		self.trees.get(i).ok_or_else(|| {
+			TxOpError(Error(
+				"invalid tree id (it might have been openned after the transaction started)".into(),
+			))
+		})
+	}
+
+	fn save_error<R>(
+		&self,
+		v: std::result::Result<R, UnabortableTransactionError>,
+	) -> TxOpResult<R> {
+		match v {
+			Ok(x) => Ok(x),
+			Err(e) => {
+				let txt = format!("{}", e);
+				self.err.set(Some(e));
+				Err(TxOpError(Error(txt.into())))
+			}
+		}
+	}
+}
+
+impl<'a> ITx for SledTx<'a> {
+	fn get(&self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let tmp = self.save_error(tree.get(key))?;
+		Ok(tmp.map(|x| x.to_vec()))
+	}
+	fn len(&self, _tree: usize) -> TxOpResult<usize> {
+		unimplemented!(".len() in transaction not supported with Sled backend")
+	}
+
+	fn insert(&mut self, tree: usize, key: &[u8], value: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = self.save_error(tree.insert(key, value))?;
+		Ok(old_val.map(|x| x.to_vec()))
+	}
+	fn remove(&mut self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = self.save_error(tree.remove(key))?;
+		Ok(old_val.map(|x| x.to_vec()))
+	}
+
+	fn iter(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with Sled backend");
+	}
+	fn iter_rev(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with Sled backend");
+	}
+
+	fn range<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with Sled backend");
+	}
+	fn range_rev<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!("Iterators in transactions not supported with Sled backend");
+	}
+}

src/db/sqlite_adapter.rs

@@ -0,0 +1,500 @@
+use core::ops::Bound;
+
+use std::borrow::BorrowMut;
+use std::marker::PhantomPinned;
+use std::pin::Pin;
+use std::ptr::NonNull;
+use std::sync::{Arc, Mutex, MutexGuard};
+
+use log::trace;
+
+use rusqlite::{params, Connection, Rows, Statement, Transaction};
+
+use crate::{
+	Db, Error, IDb, ITx, ITxFn, Result, TxError, TxFnResult, TxOpError, TxOpResult, TxResult,
+	TxValueIter, Value, ValueIter,
+};
+
+pub use rusqlite;
+
+// --- err
+
+impl From<rusqlite::Error> for Error {
+	fn from(e: rusqlite::Error) -> Error {
+		Error(format!("Sqlite: {}", e).into())
+	}
+}
+
+impl From<rusqlite::Error> for TxOpError {
+	fn from(e: rusqlite::Error) -> TxOpError {
+		TxOpError(e.into())
+	}
+}
+
+// -- db
+
+pub struct SqliteDb(Mutex<SqliteDbInner>);
+
+struct SqliteDbInner {
+	db: Connection,
+	trees: Vec<String>,
+}
+
+impl SqliteDb {
+	pub fn init(db: rusqlite::Connection) -> Db {
+		let s = Self(Mutex::new(SqliteDbInner {
+			db,
+			trees: Vec::new(),
+		}));
+		Db(Arc::new(s))
+	}
+}
+
+impl SqliteDbInner {
+	fn get_tree(&self, i: usize) -> Result<&'_ str> {
+		self.trees
+			.get(i)
+			.map(String::as_str)
+			.ok_or_else(|| Error("invalid tree id".into()))
+	}
+
+	fn internal_get(&self, tree: &str, key: &[u8]) -> Result<Option<Value>> {
+		let mut stmt = self
+			.db
+			.prepare(&format!("SELECT v FROM {} WHERE k = ?1", tree))?;
+		let mut res_iter = stmt.query([key])?;
+		match res_iter.next()? {
+			None => Ok(None),
+			Some(v) => Ok(Some(v.get::<_, Vec<u8>>(0)?)),
+		}
+	}
+}
+
+impl IDb for SqliteDb {
+	fn engine(&self) -> String {
+		format!("sqlite3 v{} (using rusqlite crate)", rusqlite::version())
+	}
+
+	fn open_tree(&self, name: &str) -> Result<usize> {
+		let name = format!("tree_{}", name.replace(':', "_COLON_"));
+		let mut this = self.0.lock().unwrap();
+
+		if let Some(i) = this.trees.iter().position(|x| x == &name) {
+			Ok(i)
+		} else {
+			trace!("create table {}", name);
+			this.db.execute(
+				&format!(
+					"CREATE TABLE IF NOT EXISTS {} (
+						k BLOB PRIMARY KEY,
+						v BLOB
+					)",
+					name
+				),
+				[],
+			)?;
+			trace!("table created: {}, unlocking", name);
+
+			let i = this.trees.len();
+			this.trees.push(name.to_string());
+			Ok(i)
+		}
+	}
+
+	fn list_trees(&self) -> Result<Vec<String>> {
+		let mut trees = vec![];
+
+		trace!("list_trees: lock db");
+		let this = self.0.lock().unwrap();
+		trace!("list_trees: lock acquired");
+
+		let mut stmt = this.db.prepare(
+			"SELECT name FROM sqlite_schema WHERE type = 'table' AND name LIKE 'tree_%'",
+		)?;
+		let mut rows = stmt.query([])?;
+		while let Some(row) = rows.next()? {
+			let name = row.get::<_, String>(0)?;
+			let name = name.replace("_COLON_", ":");
+			let name = name.strip_prefix("tree_").unwrap().to_string();
+			trees.push(name);
+		}
+		Ok(trees)
+	}
+
+	// ----
+
+	fn get(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		trace!("get {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("get {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		this.internal_get(tree, key)
+	}
+
+	fn len(&self, tree: usize) -> Result<usize> {
+		trace!("len {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("len {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let mut stmt = this.db.prepare(&format!("SELECT COUNT(*) FROM {}", tree))?;
+		let mut res_iter = stmt.query([])?;
+		match res_iter.next()? {
+			None => Ok(0),
+			Some(v) => Ok(v.get::<_, usize>(0)?),
+		}
+	}
+
+	fn insert(&self, tree: usize, key: &[u8], value: &[u8]) -> Result<Option<Value>> {
+		trace!("insert {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("insert {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let old_val = this.internal_get(tree, key)?;
+
+		let sql = match &old_val {
+			Some(_) => format!("UPDATE {} SET v = ?2 WHERE k = ?1", tree),
+			None => format!("INSERT INTO {} (k, v) VALUES (?1, ?2)", tree),
+		};
+		let n = this.db.execute(&sql, params![key, value])?;
+		assert_eq!(n, 1);
+
+		Ok(old_val)
+	}
+
+	fn remove(&self, tree: usize, key: &[u8]) -> Result<Option<Value>> {
+		trace!("remove {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("remove {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let old_val = this.internal_get(tree, key)?;
+
+		if old_val.is_some() {
+			let n = this
+				.db
+				.execute(&format!("DELETE FROM {} WHERE k = ?1", tree), params![key])?;
+			assert_eq!(n, 1);
+		}
+
+		Ok(old_val)
+	}
+
+	fn iter(&self, tree: usize) -> Result<ValueIter<'_>> {
+		trace!("iter {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("iter {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let sql = format!("SELECT k, v FROM {} ORDER BY k ASC", tree);
+		DbValueIterator::make(this, &sql, [])
+	}
+
+	fn iter_rev(&self, tree: usize) -> Result<ValueIter<'_>> {
+		trace!("iter_rev {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("iter_rev {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+		let sql = format!("SELECT k, v FROM {} ORDER BY k DESC", tree);
+		DbValueIterator::make(this, &sql, [])
+	}
+
+	fn range<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		trace!("range {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("range {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+
+		let (bounds_sql, params) = bounds_sql(low, high);
+		let sql = format!("SELECT k, v FROM {} {} ORDER BY k ASC", tree, bounds_sql);
+
+		let params = params
+			.iter()
+			.map(|x| x as &dyn rusqlite::ToSql)
+			.collect::<Vec<_>>();
+
+		DbValueIterator::make::<&[&dyn rusqlite::ToSql]>(this, &sql, params.as_ref())
+	}
+	fn range_rev<'r>(
+		&self,
+		tree: usize,
+		low: Bound<&'r [u8]>,
+		high: Bound<&'r [u8]>,
+	) -> Result<ValueIter<'_>> {
+		trace!("range_rev {}: lock db", tree);
+		let this = self.0.lock().unwrap();
+		trace!("range_rev {}: lock acquired", tree);
+
+		let tree = this.get_tree(tree)?;
+
+		let (bounds_sql, params) = bounds_sql(low, high);
+		let sql = format!("SELECT k, v FROM {} {} ORDER BY k DESC", tree, bounds_sql);
+
+		let params = params
+			.iter()
+			.map(|x| x as &dyn rusqlite::ToSql)
+			.collect::<Vec<_>>();
+
+		DbValueIterator::make::<&[&dyn rusqlite::ToSql]>(this, &sql, params.as_ref())
+	}
+
+	// ----
+
+	fn transaction(&self, f: &dyn ITxFn) -> TxResult<(), ()> {
+		trace!("transaction: lock db");
+		let mut this = self.0.lock().unwrap();
+		trace!("transaction: lock acquired");
+
+		let this_mut_ref: &mut SqliteDbInner = this.borrow_mut();
+
+		let mut tx = SqliteTx {
+			tx: this_mut_ref
+				.db
+				.transaction()
+				.map_err(Error::from)
+				.map_err(TxError::Db)?,
+			trees: &this_mut_ref.trees,
+		};
+		let res = match f.try_on(&mut tx) {
+			TxFnResult::Ok => {
+				tx.tx.commit().map_err(Error::from).map_err(TxError::Db)?;
+				Ok(())
+			}
+			TxFnResult::Abort => {
+				tx.tx.rollback().map_err(Error::from).map_err(TxError::Db)?;
+				Err(TxError::Abort(()))
+			}
+			TxFnResult::DbErr => {
+				tx.tx.rollback().map_err(Error::from).map_err(TxError::Db)?;
+				Err(TxError::Db(Error(
+					"(this message will be discarded)".into(),
+				)))
+			}
+		};
+
+		trace!("transaction done");
+		res
+	}
+}
+
+// ----
+
+struct SqliteTx<'a> {
+	tx: Transaction<'a>,
+	trees: &'a [String],
+}
+
+impl<'a> SqliteTx<'a> {
+	fn get_tree(&self, i: usize) -> TxOpResult<&'_ str> {
+		self.trees.get(i).map(String::as_ref).ok_or_else(|| {
+			TxOpError(Error(
+				"invalid tree id (it might have been openned after the transaction started)".into(),
+			))
+		})
+	}
+
+	fn internal_get(&self, tree: &str, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let mut stmt = self
+			.tx
+			.prepare(&format!("SELECT v FROM {} WHERE k = ?1", tree))?;
+		let mut res_iter = stmt.query([key])?;
+		match res_iter.next()? {
+			None => Ok(None),
+			Some(v) => Ok(Some(v.get::<_, Vec<u8>>(0)?)),
+		}
+	}
+}
+
+impl<'a> ITx for SqliteTx<'a> {
+	fn get(&self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		self.internal_get(tree, key)
+	}
+	fn len(&self, tree: usize) -> TxOpResult<usize> {
+		let tree = self.get_tree(tree)?;
+		let mut stmt = self.tx.prepare(&format!("SELECT COUNT(*) FROM {}", tree))?;
+		let mut res_iter = stmt.query([])?;
+		match res_iter.next()? {
+			None => Ok(0),
+			Some(v) => Ok(v.get::<_, usize>(0)?),
+		}
+	}
+
+	fn insert(&mut self, tree: usize, key: &[u8], value: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = self.internal_get(tree, key)?;
+
+		let sql = match &old_val {
+			Some(_) => format!("UPDATE {} SET v = ?2 WHERE k = ?1", tree),
+			None => format!("INSERT INTO {} (k, v) VALUES (?1, ?2)", tree),
+		};
+		let n = self.tx.execute(&sql, params![key, value])?;
+		assert_eq!(n, 1);
+
+		Ok(old_val)
+	}
+	fn remove(&mut self, tree: usize, key: &[u8]) -> TxOpResult<Option<Value>> {
+		let tree = self.get_tree(tree)?;
+		let old_val = self.internal_get(tree, key)?;
+
+		if old_val.is_some() {
+			let n = self
+				.tx
+				.execute(&format!("DELETE FROM {} WHERE k = ?1", tree), params![key])?;
+			assert_eq!(n, 1);
+		}
+
+		Ok(old_val)
+	}
+
+	fn iter(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!();
+	}
+	fn iter_rev(&self, _tree: usize) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!();
+	}
+
+	fn range<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!();
+	}
+	fn range_rev<'r>(
+		&self,
+		_tree: usize,
+		_low: Bound<&'r [u8]>,
+		_high: Bound<&'r [u8]>,
+	) -> TxOpResult<TxValueIter<'_>> {
+		unimplemented!();
+	}
+}
+
+// ----
+
+struct DbValueIterator<'a> {
+	db: MutexGuard<'a, SqliteDbInner>,
+	stmt: Option<Statement<'a>>,
+	iter: Option<Rows<'a>>,
+	_pin: PhantomPinned,
+}
+
+impl<'a> DbValueIterator<'a> {
+	fn make<P: rusqlite::Params>(
+		db: MutexGuard<'a, SqliteDbInner>,
+		sql: &str,
+		args: P,
+	) -> Result<ValueIter<'a>> {
+		let res = DbValueIterator {
+			db,
+			stmt: None,
+			iter: None,
+			_pin: PhantomPinned,
+		};
+		let mut boxed = Box::pin(res);
+		trace!("make iterator with sql: {}", sql);
+
+		unsafe {
+			let db = NonNull::from(&boxed.db);
+			let stmt = db.as_ref().db.prepare(sql)?;
+
+			let mut_ref: Pin<&mut DbValueIterator<'a>> = Pin::as_mut(&mut boxed);
+			Pin::get_unchecked_mut(mut_ref).stmt = Some(stmt);
+
+			let mut stmt = NonNull::from(&boxed.stmt);
+			let iter = stmt.as_mut().as_mut().unwrap().query(args)?;
+
+			let mut_ref: Pin<&mut DbValueIterator<'a>> = Pin::as_mut(&mut boxed);
+			Pin::get_unchecked_mut(mut_ref).iter = Some(iter);
+		}
+
+		Ok(Box::new(DbValueIteratorPin(boxed)))
+	}
+}
+
+impl<'a> Drop for DbValueIterator<'a> {
+	fn drop(&mut self) {
+		trace!("drop iter");
+		drop(self.iter.take());
+		drop(self.stmt.take());
+	}
+}
+
+struct DbValueIteratorPin<'a>(Pin<Box<DbValueIterator<'a>>>);
+
+impl<'a> Iterator for DbValueIteratorPin<'a> {
+	type Item = Result<(Value, Value)>;
+
+	fn next(&mut self) -> Option<Self::Item> {
+		let next = unsafe {
+			let mut_ref: Pin<&mut DbValueIterator<'a>> = Pin::as_mut(&mut self.0);
+			Pin::get_unchecked_mut(mut_ref).iter.as_mut()?.next()
+		};
+		let row = match next {
+			Err(e) => return Some(Err(e.into())),
+			Ok(None) => return None,
+			Ok(Some(r)) => r,
+		};
+		let k = match row.get::<_, Vec<u8>>(0) {
+			Err(e) => return Some(Err(e.into())),
+			Ok(x) => x,
+		};
+		let v = match row.get::<_, Vec<u8>>(1) {
+			Err(e) => return Some(Err(e.into())),
+			Ok(y) => y,
+		};
+		Some(Ok((k, v)))
+	}
+}
+
+// ----
+
+fn bounds_sql<'r>(low: Bound<&'r [u8]>, high: Bound<&'r [u8]>) -> (String, Vec<Vec<u8>>) {
+	let mut sql = String::new();
+	let mut params: Vec<Vec<u8>> = vec![];
+
+	match low {
+		Bound::Included(b) => {
+			sql.push_str(" WHERE k >= ?1");
+			params.push(b.to_vec());
+		}
+		Bound::Excluded(b) => {
+			sql.push_str(" WHERE k > ?1");
+			params.push(b.to_vec());
+		}
+		Bound::Unbounded => (),
+	};
+
+	match high {
+		Bound::Included(b) => {
+			if !params.is_empty() {
+				sql.push_str(" AND k <= ?2");
+			} else {
+				sql.push_str(" WHERE k <= ?1");
+			}
+			params.push(b.to_vec());
+		}
+		Bound::Excluded(b) => {
+			if !params.is_empty() {
+				sql.push_str(" AND k < ?2");
+			} else {
+				sql.push_str(" WHERE k < ?1");
+			}
+			params.push(b.to_vec());
+		}
+		Bound::Unbounded => (),
+	}
+
+	(sql, params)
+}

src/db/test.rs

@@ -0,0 +1,106 @@
+use crate::*;
+
+use crate::lmdb_adapter::LmdbDb;
+use crate::sled_adapter::SledDb;
+use crate::sqlite_adapter::SqliteDb;
+
+fn test_suite(db: Db) {
+	let tree = db.open_tree("tree").unwrap();
+
+	let ka: &[u8] = &b"test"[..];
+	let kb: &[u8] = &b"zwello"[..];
+	let kint: &[u8] = &b"tz"[..];
+	let va: &[u8] = &b"plop"[..];
+	let vb: &[u8] = &b"plip"[..];
+	let vc: &[u8] = &b"plup"[..];
+
+	assert!(tree.insert(ka, va).unwrap().is_none());
+	assert_eq!(tree.get(ka).unwrap().unwrap(), va);
+
+	let res = db.transaction::<_, (), _>(|mut tx| {
+		assert_eq!(tx.get(&tree, ka).unwrap().unwrap(), va);
+
+		assert_eq!(tx.insert(&tree, ka, vb).unwrap().unwrap(), va);
+
+		assert_eq!(tx.get(&tree, ka).unwrap().unwrap(), vb);
+
+		tx.commit(12)
+	});
+	assert!(matches!(res, Ok(12)));
+	assert_eq!(tree.get(ka).unwrap().unwrap(), vb);
+
+	let res = db.transaction::<(), _, _>(|mut tx| {
+		assert_eq!(tx.get(&tree, ka).unwrap().unwrap(), vb);
+
+		assert_eq!(tx.insert(&tree, ka, vc).unwrap().unwrap(), vb);
+
+		assert_eq!(tx.get(&tree, ka).unwrap().unwrap(), vc);
+
+		tx.abort(42)
+	});
+	assert!(matches!(res, Err(TxError::Abort(42))));
+	assert_eq!(tree.get(ka).unwrap().unwrap(), vb);
+
+	let mut iter = tree.iter().unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (ka, vb));
+	assert!(iter.next().is_none());
+	drop(iter);
+
+	assert!(tree.insert(kb, vc).unwrap().is_none());
+	assert_eq!(tree.get(kb).unwrap().unwrap(), vc);
+
+	let mut iter = tree.iter().unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (ka, vb));
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (kb, vc));
+	assert!(iter.next().is_none());
+	drop(iter);
+
+	let mut iter = tree.range(kint..).unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (kb, vc));
+	assert!(iter.next().is_none());
+	drop(iter);
+
+	let mut iter = tree.range_rev(..kint).unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (ka, vb));
+	assert!(iter.next().is_none());
+	drop(iter);
+
+	let mut iter = tree.iter_rev().unwrap();
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (kb, vc));
+	let next = iter.next().unwrap().unwrap();
+	assert_eq!((next.0.as_ref(), next.1.as_ref()), (ka, vb));
+	assert!(iter.next().is_none());
+	drop(iter);
+}
+
+#[test]
+fn test_lmdb_db() {
+	let path = mktemp::Temp::new_dir().unwrap();
+	let db = heed::EnvOpenOptions::new()
+		.max_dbs(100)
+		.open(&path)
+		.unwrap();
+	let db = LmdbDb::init(db);
+	test_suite(db);
+	drop(path);
+}
+
+#[test]
+fn test_sled_db() {
+	let path = mktemp::Temp::new_dir().unwrap();
+	let db = SledDb::init(sled::open(path.to_path_buf()).unwrap());
+	test_suite(db);
+	drop(path);
+}
+
+#[test]
+fn test_sqlite_db() {
+	let db = SqliteDb::init(rusqlite::Connection::open_in_memory().unwrap());
+	test_suite(db);
+}