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, 10 insertions, 11 deletions

diff --git a/src/table/metrics.rs b/src/table/metrics.rs
index 752a2a6d..3a1783e0 100644
--- a/src/table/metrics.rs
+++ b/src/table/metrics.rs
@@ -1,6 +1,7 @@
 use opentelemetry::{global, metrics::*, KeyValue};
 
-use garage_util::sled_counter::SledCountedTree;
+use garage_db as db;
+use garage_db::counted_tree_hack::CountedTree;
 
 /// TableMetrics reference all counter used for metrics
 pub struct TableMetrics {
@@ -19,21 +20,19 @@ pub struct TableMetrics {
 	pub(crate) sync_items_received: Counter<u64>,
 }
 impl TableMetrics {
-	pub fn new(
-		table_name: &'static str,
-		merkle_todo: sled::Tree,
-		gc_todo: SledCountedTree,
-	) -> Self {
+	pub fn new(table_name: &'static str, merkle_todo: db::Tree, gc_todo: CountedTree) -> Self {
 		let meter = global::meter(table_name);
 		TableMetrics {
 			_merkle_todo_len: meter
 				.u64_value_observer(
 					"table.merkle_updater_todo_queue_length",
 					move |observer| {
-						observer.observe(
-							merkle_todo.len() as u64,
-							&[KeyValue::new("table_name", table_name)],
-						)
+						if let Ok(v) = merkle_todo.len() {
+							observer.observe(
+								v as u64,
+								&[KeyValue::new("table_name", table_name)],
+							);
+						}
 					},
 				)
 				.with_description("Merkle tree updater TODO queue length")
@@ -45,7 +44,7 @@ impl TableMetrics {
 						observer.observe(
 							gc_todo.len() as u64,
 							&[KeyValue::new("table_name", table_name)],
-						)
+						);
 					},
 				)
 				.with_description("Table garbage collector TODO queue length")