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, 92 insertions, 35 deletions

diff --git a/src/block/manager.rs b/src/block/manager.rs
index 9b2d9cad..32ba0431 100644
--- a/src/block/manager.rs
+++ b/src/block/manager.rs
@@ -1,3 +1,5 @@
+use core::ops::Bound;
+
 use std::convert::TryInto;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
@@ -17,10 +19,12 @@ use opentelemetry::{
 	Context, KeyValue,
 };
 
+use garage_db as db;
+use garage_db::counted_tree_hack::CountedTree;
+
 use garage_util::data::*;
 use garage_util::error::*;
 use garage_util::metrics::RecordDuration;
-use garage_util::sled_counter::SledCountedTree;
 use garage_util::time::*;
 use garage_util::tranquilizer::Tranquilizer;
 
@@ -91,9 +95,9 @@ pub struct BlockManager {
 
 	rc: BlockRc,
 
-	resync_queue: SledCountedTree,
+	resync_queue: CountedTree,
 	resync_notify: Notify,
-	resync_errors: SledCountedTree,
+	resync_errors: CountedTree,
 
 	system: Arc<System>,
 	endpoint: Arc<Endpoint<BlockRpc, Self>>,
@@ -108,7 +112,7 @@ struct BlockManagerLocked();
 
 impl BlockManager {
 	pub fn new(
-		db: &sled::Db,
+		db: &db::Db,
 		data_dir: PathBuf,
 		compression_level: Option<i32>,
 		background_tranquility: u32,
@@ -123,12 +127,14 @@ impl BlockManager {
 		let resync_queue = db
 			.open_tree("block_local_resync_queue")
 			.expect("Unable to open block_local_resync_queue tree");
-		let resync_queue = SledCountedTree::new(resync_queue);
+		let resync_queue =
+			CountedTree::new(resync_queue).expect("Could not count block_local_resync_queue");
 
 		let resync_errors = db
 			.open_tree("block_local_resync_errors")
 			.expect("Unable to open block_local_resync_errors tree");
-		let resync_errors = SledCountedTree::new(resync_errors);
+		let resync_errors =
+			CountedTree::new(resync_errors).expect("Could not count block_local_resync_errors");
 
 		let endpoint = system
 			.netapp
@@ -219,11 +225,44 @@ impl BlockManager {
 	/// to fix any mismatch between the two.
 	pub async fn repair_data_store(&self, must_exit: &watch::Receiver<bool>) -> Result<(), Error> {
 		// 1. Repair blocks from RC table.
-		for (i, entry) in self.rc.rc.iter().enumerate() {
-			let (hash, _) = entry?;
-			let hash = Hash::try_from(&hash[..]).unwrap();
-			self.put_to_resync(&hash, Duration::from_secs(0))?;
-			if i & 0xFF == 0 && *must_exit.borrow() {
+		let mut next_start: Option<Hash> = None;
+		loop {
+			// We have to do this complicated two-step process where we first read a bunch
+			// of hashes from the RC table, and then insert them in the to-resync queue,
+			// because of SQLite. Basically, as long as we have an iterator on a DB table,
+			// we can't do anything else on the DB. The naive approach (which we had previously)
+			// of just iterating on the RC table and inserting items one to one in the resync
+			// queue can't work here, it would just provoke a deadlock in the SQLite adapter code.
+			// This is mostly because the Rust bindings for SQLite assume a worst-case scenario
+			// where SQLite is not compiled in thread-safe mode, so we have to wrap everything
+			// in a mutex (see db/sqlite_adapter.rs and discussion in PR #322).
+			let mut batch_of_hashes = vec![];
+			let start_bound = match next_start.as_ref() {
+				None => Bound::Unbounded,
+				Some(x) => Bound::Excluded(x.as_slice()),
+			};
+			for entry in self
+				.rc
+				.rc
+				.range::<&[u8], _>((start_bound, Bound::Unbounded))?
+			{
+				let (hash, _) = entry?;
+				let hash = Hash::try_from(&hash[..]).unwrap();
+				batch_of_hashes.push(hash);
+				if batch_of_hashes.len() >= 1000 {
+					break;
+				}
+			}
+			if batch_of_hashes.is_empty() {
+				break;
+			}
+
+			for hash in batch_of_hashes.into_iter() {
+				self.put_to_resync(&hash, Duration::from_secs(0))?;
+				next_start = Some(hash)
+			}
+
+			if *must_exit.borrow() {
 				return Ok(());
 			}
 		}
@@ -264,46 +303,69 @@ impl BlockManager {
 	}
 
 	/// Get lenght of resync queue
-	pub fn resync_queue_len(&self) -> usize {
-		self.resync_queue.len()
+	pub fn resync_queue_len(&self) -> Result<usize, Error> {
+		// This currently can't return an error because the CountedTree hack
+		// doesn't error on .len(), but this will change when we remove the hack
+		// (hopefully someday!)
+		Ok(self.resync_queue.len())
 	}
 
 	/// Get number of blocks that have an error
-	pub fn resync_errors_len(&self) -> usize {
-		self.resync_errors.len()
+	pub fn resync_errors_len(&self) -> Result<usize, Error> {
+		// (see resync_queue_len comment)
+		Ok(self.resync_errors.len())
 	}
 
 	/// Get number of items in the refcount table
-	pub fn rc_len(&self) -> usize {
-		self.rc.rc.len()
+	pub fn rc_len(&self) -> Result<usize, Error> {
+		Ok(self.rc.rc.len()?)
 	}
 
 	//// ----- Managing the reference counter ----
 
 	/// Increment the number of time a block is used, putting it to resynchronization if it is
 	/// required, but not known
-	pub fn block_incref(&self, hash: &Hash) -> Result<(), Error> {
-		if self.rc.block_incref(hash)? {
+	pub fn block_incref(
+		self: &Arc<Self>,
+		tx: &mut db::Transaction,
+		hash: Hash,
+	) -> db::TxOpResult<()> {
+		if self.rc.block_incref(tx, &hash)? {
 			// When the reference counter is incremented, there is
 			// normally a node that is responsible for sending us the
 			// data of the block. However that operation may fail,
 			// so in all cases we add the block here to the todo list
 			// to check later that it arrived correctly, and if not
 			// we will fecth it from someone.
-			self.put_to_resync(hash, 2 * BLOCK_RW_TIMEOUT)?;
+			let this = self.clone();
+			tokio::spawn(async move {
+				if let Err(e) = this.put_to_resync(&hash, 2 * BLOCK_RW_TIMEOUT) {
+					error!("Block {:?} could not be put in resync queue: {}.", hash, e);
+				}
+			});
 		}
 		Ok(())
 	}
 
 	/// Decrement the number of time a block is used
-	pub fn block_decref(&self, hash: &Hash) -> Result<(), Error> {
-		if self.rc.block_decref(hash)? {
+	pub fn block_decref(
+		self: &Arc<Self>,
+		tx: &mut db::Transaction,
+		hash: Hash,
+	) -> db::TxOpResult<()> {
+		if self.rc.block_decref(tx, &hash)? {
 			// When the RC is decremented, it might drop to zero,
 			// indicating that we don't need the block.
 			// There is a delay before we garbage collect it;
 			// make sure that it is handled in the resync loop
 			// after that delay has passed.
-			self.put_to_resync(hash, BLOCK_GC_DELAY + Duration::from_secs(10))?;
+			let this = self.clone();
+			tokio::spawn(async move {
+				if let Err(e) = this.put_to_resync(&hash, BLOCK_GC_DELAY + Duration::from_secs(10))
+				{
+					error!("Block {:?} could not be put in resync queue: {}.", hash, e);
+				}
+			});
 		}
 		Ok(())
 	}
@@ -503,12 +565,12 @@ impl BlockManager {
 		});
 	}
 
-	fn put_to_resync(&self, hash: &Hash, delay: Duration) -> Result<(), sled::Error> {
+	fn put_to_resync(&self, hash: &Hash, delay: Duration) -> db::Result<()> {
 		let when = now_msec() + delay.as_millis() as u64;
 		self.put_to_resync_at(hash, when)
 	}
 
-	fn put_to_resync_at(&self, hash: &Hash, when: u64) -> Result<(), sled::Error> {
+	fn put_to_resync_at(&self, hash: &Hash, when: u64) -> db::Result<()> {
 		trace!("Put resync_queue: {} {:?}", when, hash);
 		let mut key = u64::to_be_bytes(when).to_vec();
 		key.extend(hash.as_ref());
@@ -547,13 +609,8 @@ impl BlockManager {
 	// - Ok(true) -> a block was processed (successfully or not)
 	// - Ok(false) -> no block was processed, but we are ready for the next iteration
 	// - Err(_) -> a Sled error occurred when reading/writing from resync_queue/resync_errors
-	async fn resync_iter(
-		&self,
-		must_exit: &mut watch::Receiver<bool>,
-	) -> Result<bool, sled::Error> {
-		if let Some(first_pair_res) = self.resync_queue.iter().next() {
-			let (time_bytes, hash_bytes) = first_pair_res?;
-
+	async fn resync_iter(&self, must_exit: &mut watch::Receiver<bool>) -> Result<bool, db::Error> {
+		if let Some((time_bytes, hash_bytes)) = self.resync_queue.first()? {
 			let time_msec = u64::from_be_bytes(time_bytes[0..8].try_into().unwrap());
 			let now = now_msec();
 
@@ -561,7 +618,7 @@ impl BlockManager {
 				let hash = Hash::try_from(&hash_bytes[..]).unwrap();
 
 				if let Some(ec) = self.resync_errors.get(hash.as_slice())? {
-					let ec = ErrorCounter::decode(ec);
+					let ec = ErrorCounter::decode(&ec);
 					if now < ec.next_try() {
 						// if next retry after an error is not yet,
 						// don't do resync and return early, but still
@@ -602,7 +659,7 @@ impl BlockManager {
 					warn!("Error when resyncing {:?}: {}", hash, e);
 
 					let err_counter = match self.resync_errors.get(hash.as_slice())? {
-						Some(ec) => ErrorCounter::decode(ec).add1(now + 1),
+						Some(ec) => ErrorCounter::decode(&ec).add1(now + 1),
 						None => ErrorCounter::new(now + 1),
 					};
 
@@ -966,7 +1023,7 @@ impl ErrorCounter {
 		}
 	}
 
-	fn decode(data: sled::IVec) -> Self {
+	fn decode(data: &[u8]) -> Self {
 		Self {
 			errors: u64::from_be_bytes(data[0..8].try_into().unwrap()),
 			last_try: u64::from_be_bytes(data[8..16].try_into().unwrap()),