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, 32 insertions, 18 deletions

diff --git a/src/garage/repair.rs b/src/garage/repair.rs
index 830eac71..17e14b8b 100644
--- a/src/garage/repair.rs
+++ b/src/garage/repair.rs
@@ -64,13 +64,23 @@ impl Repair {
 
 	async fn repair_versions(&self, must_exit: &watch::Receiver<bool>) -> Result<(), Error> {
 		let mut pos = vec![];
+		let mut i = 0;
 
-		while let Some((item_key, item_bytes)) =
-			self.garage.version_table.data.store.get_gt(&pos)?
-		{
-			pos = item_key.to_vec();
+		while !*must_exit.borrow() {
+			let item_bytes = match self.garage.version_table.data.store.get_gt(pos)? {
+				Some((k, v)) => {
+					pos = k;
+					v
+				}
+				None => break,
+			};
+
+			i += 1;
+			if i % 1000 == 0 {
+				info!("repair_versions: {}", i);
+			}
 
-			let version = rmp_serde::decode::from_read_ref::<_, Version>(item_bytes.as_ref())?;
+			let version = rmp_serde::decode::from_read_ref::<_, Version>(&item_bytes)?;
 			if version.deleted.get() {
 				continue;
 			}
@@ -98,23 +108,30 @@ impl Repair {
 					))
 					.await?;
 			}
-
-			if *must_exit.borrow() {
-				break;
-			}
 		}
+		info!("repair_versions: finished, done {}", i);
 		Ok(())
 	}
 
 	async fn repair_block_ref(&self, must_exit: &watch::Receiver<bool>) -> Result<(), Error> {
 		let mut pos = vec![];
+		let mut i = 0;
 
-		while let Some((item_key, item_bytes)) =
-			self.garage.block_ref_table.data.store.get_gt(&pos)?
-		{
-			pos = item_key.to_vec();
+		while !*must_exit.borrow() {
+			let item_bytes = match self.garage.block_ref_table.data.store.get_gt(pos)? {
+				Some((k, v)) => {
+					pos = k;
+					v
+				}
+				None => break,
+			};
+
+			i += 1;
+			if i % 1000 == 0 {
+				info!("repair_block_ref: {}", i);
+			}
 
-			let block_ref = rmp_serde::decode::from_read_ref::<_, BlockRef>(item_bytes.as_ref())?;
+			let block_ref = rmp_serde::decode::from_read_ref::<_, BlockRef>(&item_bytes)?;
 			if block_ref.deleted.get() {
 				continue;
 			}
@@ -139,11 +156,8 @@ impl Repair {
 					})
 					.await?;
 			}
-
-			if *must_exit.borrow() {
-				break;
-			}
 		}
+		info!("repair_block_ref: finished, done {}", i);
 		Ok(())
 	}
 }