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, 106 insertions, 0 deletions

diff --git a/src/db/test.rs b/src/db/test.rs
new file mode 100644
index 00000000..cfcee643
--- /dev/null
+++ b/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);
+}