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/block/rc.rs

@@ -1,5 +1,7 @@
 use std::convert::TryInto;
 
+use garage_db as db;
+
 use garage_util::data::*;
 use garage_util::error::*;
 use garage_util::time::*;
@@ -7,31 +9,41 @@ use garage_util::time::*;
 use crate::manager::BLOCK_GC_DELAY;
 
 pub struct BlockRc {
-	pub(crate) rc: sled::Tree,
+	pub(crate) rc: db::Tree,
 }
 
 impl BlockRc {
-	pub(crate) fn new(rc: sled::Tree) -> Self {
+	pub(crate) fn new(rc: db::Tree) -> Self {
 		Self { rc }
 	}
 
 	/// Increment the reference counter associated to a hash.
 	/// Returns true if the RC goes from zero to nonzero.
-	pub(crate) fn block_incref(&self, hash: &Hash) -> Result<bool, Error> {
-		let old_rc = self
-			.rc
-			.fetch_and_update(&hash, |old| RcEntry::parse_opt(old).increment().serialize())?;
-		let old_rc = RcEntry::parse_opt(old_rc);
+	pub(crate) fn block_incref(
+		&self,
+		tx: &mut db::Transaction,
+		hash: &Hash,
+	) -> db::TxOpResult<bool> {
+		let old_rc = RcEntry::parse_opt(tx.get(&self.rc, &hash)?);
+		match old_rc.increment().serialize() {
+			Some(x) => tx.insert(&self.rc, &hash, x)?,
+			None => unreachable!(),
+		};
 		Ok(old_rc.is_zero())
 	}
 
 	/// Decrement the reference counter associated to a hash.
 	/// Returns true if the RC is now zero.
-	pub(crate) fn block_decref(&self, hash: &Hash) -> Result<bool, Error> {
-		let new_rc = self
-			.rc
-			.update_and_fetch(&hash, |old| RcEntry::parse_opt(old).decrement().serialize())?;
-		let new_rc = RcEntry::parse_opt(new_rc);
+	pub(crate) fn block_decref(
+		&self,
+		tx: &mut db::Transaction,
+		hash: &Hash,
+	) -> db::TxOpResult<bool> {
+		let new_rc = RcEntry::parse_opt(tx.get(&self.rc, &hash)?).decrement();
+		match new_rc.serialize() {
+			Some(x) => tx.insert(&self.rc, &hash, x)?,
+			None => tx.remove(&self.rc, &hash)?,
+		};
 		Ok(matches!(new_rc, RcEntry::Deletable { .. }))
 	}
 
@@ -44,12 +56,15 @@ impl BlockRc {
 	/// deletion time has passed
 	pub(crate) fn clear_deleted_block_rc(&self, hash: &Hash) -> Result<(), Error> {
 		let now = now_msec();
-		self.rc.update_and_fetch(&hash, |rcval| {
-			let updated = match RcEntry::parse_opt(rcval) {
-				RcEntry::Deletable { at_time } if now > at_time => RcEntry::Absent,
-				v => v,
+		self.rc.db().transaction(|mut tx| {
+			let rcval = RcEntry::parse_opt(tx.get(&self.rc, &hash)?);
+			match rcval {
+				RcEntry::Deletable { at_time } if now > at_time => {
+					tx.remove(&self.rc, &hash)?;
+				}
+				_ => (),
 			};
-			updated.serialize()
+			tx.commit(())
 		})?;
 		Ok(())
 	}