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, 29 insertions, 16 deletions

diff --git a/src/garage/admin.rs b/src/garage/admin.rs
index bc1f494a..c662aa00 100644
--- a/src/garage/admin.rs
+++ b/src/garage/admin.rs
@@ -660,11 +660,11 @@ impl AdminRpcHandler {
 			}
 			Ok(AdminRpc::Ok(ret))
 		} else {
-			Ok(AdminRpc::Ok(self.gather_stats_local(opt)))
+			Ok(AdminRpc::Ok(self.gather_stats_local(opt)?))
 		}
 	}
 
-	fn gather_stats_local(&self, opt: StatsOpt) -> String {
+	fn gather_stats_local(&self, opt: StatsOpt) -> Result<String, Error> {
 		let mut ret = String::new();
 		writeln!(
 			&mut ret,
@@ -672,6 +672,7 @@ impl AdminRpcHandler {
 			self.garage.system.garage_version(),
 		)
 		.unwrap();
+		writeln!(&mut ret, "\nDatabase engine: {}", self.garage.db.engine()).unwrap();
 
 		// Gather ring statistics
 		let ring = self.garage.system.ring.borrow().clone();
@@ -689,59 +690,71 @@ impl AdminRpcHandler {
 			writeln!(&mut ret, "  {:?} {}", n, c).unwrap();
 		}
 
-		self.gather_table_stats(&mut ret, &self.garage.bucket_table, &opt);
-		self.gather_table_stats(&mut ret, &self.garage.key_table, &opt);
-		self.gather_table_stats(&mut ret, &self.garage.object_table, &opt);
-		self.gather_table_stats(&mut ret, &self.garage.version_table, &opt);
-		self.gather_table_stats(&mut ret, &self.garage.block_ref_table, &opt);
+		self.gather_table_stats(&mut ret, &self.garage.bucket_table, &opt)?;
+		self.gather_table_stats(&mut ret, &self.garage.key_table, &opt)?;
+		self.gather_table_stats(&mut ret, &self.garage.object_table, &opt)?;
+		self.gather_table_stats(&mut ret, &self.garage.version_table, &opt)?;
+		self.gather_table_stats(&mut ret, &self.garage.block_ref_table, &opt)?;
 
 		writeln!(&mut ret, "\nBlock manager stats:").unwrap();
 		if opt.detailed {
 			writeln!(
 				&mut ret,
 				"  number of RC entries (~= number of blocks): {}",
-				self.garage.block_manager.rc_len()
+				self.garage.block_manager.rc_len()?
 			)
 			.unwrap();
 		}
 		writeln!(
 			&mut ret,
 			"  resync queue length: {}",
-			self.garage.block_manager.resync_queue_len()
+			self.garage.block_manager.resync_queue_len()?
 		)
 		.unwrap();
 		writeln!(
 			&mut ret,
 			"  blocks with resync errors: {}",
-			self.garage.block_manager.resync_errors_len()
+			self.garage.block_manager.resync_errors_len()?
 		)
 		.unwrap();
 
-		ret
+		Ok(ret)
 	}
 
-	fn gather_table_stats<F, R>(&self, to: &mut String, t: &Arc<Table<F, R>>, opt: &StatsOpt)
+	fn gather_table_stats<F, R>(
+		&self,
+		to: &mut String,
+		t: &Arc<Table<F, R>>,
+		opt: &StatsOpt,
+	) -> Result<(), Error>
 	where
 		F: TableSchema + 'static,
 		R: TableReplication + 'static,
 	{
 		writeln!(to, "\nTable stats for {}", F::TABLE_NAME).unwrap();
 		if opt.detailed {
-			writeln!(to, "  number of items: {}", t.data.store.len()).unwrap();
+			writeln!(
+				to,
+				"  number of items: {}",
+				t.data.store.len().map_err(GarageError::from)?
+			)
+			.unwrap();
 			writeln!(
 				to,
 				"  Merkle tree size: {}",
-				t.merkle_updater.merkle_tree_len()
+				t.merkle_updater.merkle_tree_len()?
 			)
 			.unwrap();
 		}
 		writeln!(
 			to,
 			"  Merkle updater todo queue length: {}",
-			t.merkle_updater.todo_len()
+			t.merkle_updater.todo_len()?
 		)
 		.unwrap();
-		writeln!(to, "  GC todo queue length: {}", t.data.gc_todo_len()).unwrap();
+		writeln!(to, "  GC todo queue length: {}", t.data.gc_todo_len()?).unwrap();
+
+		Ok(())
 	}
 }