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>

1 files changed, 47 insertions, 7 deletions

diff --git a/src/garage/server.rs b/src/garage/server.rs
index b58ad286..697d3358 100644
--- a/src/garage/server.rs
+++ b/src/garage/server.rs
@@ -2,6 +2,8 @@ use std::path::PathBuf;
 
 use tokio::sync::watch;
 
+use garage_db as db;
+
 use garage_util::background::*;
 use garage_util::config::*;
 use garage_util::error::Error;
@@ -31,13 +33,51 @@ pub async fn run_server(config_file: PathBuf) -> Result<(), Error> {
 
 	info!("Opening database...");
 	let mut db_path = config.metadata_dir.clone();
-	db_path.push("db");
-	let db = sled::Config::default()
-		.path(&db_path)
-		.cache_capacity(config.sled_cache_capacity)
-		.flush_every_ms(Some(config.sled_flush_every_ms))
-		.open()
-		.expect("Unable to open sled DB");
+	std::fs::create_dir_all(&db_path).expect("Unable to create Garage meta data directory");
+	let db = match config.db_engine.as_str() {
+		"sled" => {
+			db_path.push("db");
+			info!("Opening Sled database at: {}", db_path.display());
+			let db = db::sled_adapter::sled::Config::default()
+				.path(&db_path)
+				.cache_capacity(config.sled_cache_capacity)
+				.flush_every_ms(Some(config.sled_flush_every_ms))
+				.open()
+				.expect("Unable to open sled DB");
+			db::sled_adapter::SledDb::init(db)
+		}
+		"sqlite" | "sqlite3" | "rusqlite" => {
+			db_path.push("db.sqlite");
+			info!("Opening Sqlite database at: {}", db_path.display());
+			let db = db::sqlite_adapter::rusqlite::Connection::open(db_path)
+				.expect("Unable to open sqlite DB");
+			db::sqlite_adapter::SqliteDb::init(db)
+		}
+		"lmdb" | "heed" => {
+			db_path.push("db.lmdb");
+			info!("Opening LMDB database at: {}", db_path.display());
+			std::fs::create_dir_all(&db_path).expect("Unable to create LMDB data directory");
+			let map_size = if u32::MAX as usize == usize::MAX {
+				warn!("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 = db::lmdb_adapter::heed::EnvOpenOptions::new()
+				.max_dbs(100)
+				.map_size(map_size)
+				.open(&db_path)
+				.expect("Unable to open LMDB DB");
+			db::lmdb_adapter::LmdbDb::init(db)
+		}
+		e => {
+			return Err(Error::Message(format!(
+				"Unsupported DB engine: {} (options: sled, sqlite, lmdb)",
+				e
+			)));
+		}
+	};
 
 	info!("Initializing background runner...");
 	let watch_cancel = netapp::util::watch_ctrl_c();