Safety: never voluntarily delete block in 10min interval after RC reaches zero

src/garage/admin.rs

@@ -446,7 +446,7 @@ impl AdminRpcHandler {
 		if opt.detailed {
 			writeln!(
 				&mut ret,
-				"  number of blocks: {}",
+				"  number of RC entries (~= number of blocks): {}",
 				self.garage.block_manager.rc_len()
 			)
 			.unwrap();

src/garage/cli/structs.rs

@@ -8,8 +8,7 @@ pub enum Command {
 	#[structopt(name = "server")]
 	Server,
 
-	/// Print identifier (public key) of this garage node.
-	/// Generates a new keypair if necessary.
+	/// Print identifier (public key) of this Garage node
 	#[structopt(name = "node-id")]
 	NodeId(NodeIdOpt),
 

src/model/block.rs

@@ -31,10 +31,20 @@ pub const INLINE_THRESHOLD: usize = 3072;
 pub const BACKGROUND_WORKERS: u64 = 1;
 pub const BACKGROUND_TRANQUILITY: u32 = 3;
 
-const BLOCK_RW_TIMEOUT: Duration = Duration::from_secs(42);
-const BLOCK_GC_TIMEOUT: Duration = Duration::from_secs(60);
+// Timeout for RPCs that read and write blocks to remote nodes
+const BLOCK_RW_TIMEOUT: Duration = Duration::from_secs(30);
+// Timeout for RPCs that ask other nodes whether they need a copy
+// of a given block before we delete it locally
 const NEED_BLOCK_QUERY_TIMEOUT: Duration = Duration::from_secs(5);
-const RESYNC_RETRY_TIMEOUT: Duration = Duration::from_secs(10);
+
+// The delay between the time where a resync operation fails
+// and the time when it is retried.
+const RESYNC_RETRY_DELAY: Duration = Duration::from_secs(60);
+
+// The delay between the moment when the reference counter
+// drops to zero, and the moment where we allow ourselves
+// to delete the block locally.
+const BLOCK_GC_DELAY: Duration = Duration::from_secs(600);
 
 /// RPC messages used to share blocks of data between nodes
 #[derive(Debug, Serialize, Deserialize)]
@@ -180,7 +190,7 @@ impl BlockManager {
 	/// that are required because of refcount > 0, and will try
 	/// 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
+		// 1. Repair blocks from RC table.
 		let garage = self.garage.load_full().unwrap();
 		let mut last_hash = None;
 		for (i, entry) in garage.block_ref_table.data.store.iter().enumerate() {
@@ -245,40 +255,51 @@ impl BlockManager {
 	/// 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> {
-		let old_rc = self.rc.fetch_and_update(&hash, |old| {
-			let old_v = old.map(u64_from_be_bytes).unwrap_or(0);
-			Some(u64::to_be_bytes(old_v + 1).to_vec())
-		})?;
-		let old_rc = old_rc.map(u64_from_be_bytes).unwrap_or(0);
-		if old_rc == 0 {
-			self.put_to_resync(hash, BLOCK_RW_TIMEOUT)?;
+		let old_rc = self
+			.rc
+			.fetch_and_update(&hash, |old| RcEntry::parse_opt(old).increment().serialize())?;
+		let old_rc = RcEntry::parse_opt(old_rc);
+		if old_rc.is_zero() {
+			// 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)?;
 		}
 		Ok(())
 	}
 
 	/// Decrement the number of time a block is used
 	pub fn block_decref(&self, hash: &Hash) -> Result<(), Error> {
-		let new_rc = self.rc.update_and_fetch(&hash, |old| {
-			let old_v = old.map(u64_from_be_bytes).unwrap_or(0);
-			if old_v > 1 {
-				Some(u64::to_be_bytes(old_v - 1).to_vec())
-			} else {
-				None
-			}
-		})?;
-		if new_rc.is_none() {
-			self.put_to_resync(hash, BLOCK_GC_TIMEOUT)?;
+		let new_rc = self
+			.rc
+			.update_and_fetch(&hash, |old| RcEntry::parse_opt(old).decrement().serialize())?;
+		let new_rc = RcEntry::parse_opt(new_rc);
+		if let RcEntry::Deletable { .. } = new_rc {
+			self.put_to_resync(hash, BLOCK_GC_DELAY + Duration::from_secs(10))?;
 		}
 		Ok(())
 	}
 
 	/// Read a block's reference count
-	pub fn get_block_rc(&self, hash: &Hash) -> Result<u64, Error> {
-		Ok(self
-			.rc
-			.get(hash.as_ref())?
-			.map(u64_from_be_bytes)
-			.unwrap_or(0))
+	fn get_block_rc(&self, hash: &Hash) -> Result<RcEntry, Error> {
+		Ok(RcEntry::parse_opt(self.rc.get(hash.as_ref())?))
+	}
+
+	/// Delete an entry in the RC table if it is deletable and the
+	/// deletion time has passed
+	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,
+			};
+			updated.serialize()
+		})?;
+		Ok(())
 	}
 
 	// ---- Reading and writing blocks locally ----
@@ -300,7 +321,7 @@ impl BlockManager {
 			Ok(f) => f,
 			Err(e) => {
 				// Not found but maybe we should have had it ??
-				self.put_to_resync(hash, Duration::from_millis(0))?;
+				self.put_to_resync(hash, 2 * BLOCK_RW_TIMEOUT)?;
 				return Err(Into::into(e));
 			}
 		};
@@ -314,6 +335,7 @@ impl BlockManager {
 				.await
 				.move_block_to_corrupted(hash, self)
 				.await?;
+			self.put_to_resync(hash, Duration::from_millis(0))?;
 			return Err(Error::CorruptData(*hash));
 		}
 
@@ -328,7 +350,7 @@ impl BlockManager {
 			.await
 			.check_block_status(hash, self)
 			.await?;
-		Ok(needed && !exists)
+		Ok(needed.is_nonzero() && !exists)
 	}
 
 	/// Utility: gives the path of the directory in which a block should be found
@@ -349,7 +371,7 @@ impl BlockManager {
 	// ---- Resync loop ----
 
 	pub fn spawn_background_worker(self: Arc<Self>) {
-		// Launch 2 simultaneous workers for background resync loop preprocessing
+		// Launch n simultaneous workers for background resync loop preprocessing
 		for i in 0..BACKGROUND_WORKERS {
 			let bm2 = self.clone();
 			let background = self.system.background.clone();
@@ -407,7 +429,7 @@ impl BlockManager {
 				let res = self.resync_block(&hash).await;
 				if let Err(e) = &res {
 					warn!("Error when resyncing {:?}: {}", hash, e);
-					self.put_to_resync(&hash, RESYNC_RETRY_TIMEOUT)?;
+					self.put_to_resync(&hash, RESYNC_RETRY_DELAY)?;
 				}
 				Ok(true)
 			} else {
@@ -437,15 +459,18 @@ impl BlockManager {
 			.check_block_status(hash, self)
 			.await?;
 
-		if exists != needed {
-			info!(
-				"Resync block {:?}: exists {}, needed {}",
-				hash, exists, needed
+		if exists != needed.is_needed() || exists != needed.is_nonzero() {
+			debug!(
+				"Resync block {:?}: exists {}, nonzero rc {}, deletable {}",
+				hash,
+				exists,
+				needed.is_nonzero(),
+				needed.is_deletable(),
 			);
 		}
 
-		if exists && !needed {
-			trace!("Offloading block {:?}", hash);
+		if exists && needed.is_deletable() {
+			info!("Resync block {:?}: offloading and deleting", hash);
 
 			let mut who = self.replication.write_nodes(hash);
 			if who.len() < self.replication.write_quorum() {
@@ -488,7 +513,7 @@ impl BlockManager {
 					need_nodes.len()
 				);
 
-				let put_block_message = self.read_block(hash).await.err_context("PutBlock RPC")?;
+				let put_block_message = self.read_block(hash).await?;
 				self.system
 					.rpc
 					.try_call_many(
@@ -499,10 +524,11 @@ impl BlockManager {
 							.with_quorum(need_nodes.len())
 							.with_timeout(BLOCK_RW_TIMEOUT),
 					)
-					.await?;
+					.await
+					.err_context("PutBlock RPC")?;
 			}
 			info!(
-				"Deleting block {:?}, offload finished ({} / {})",
+				"Deleting unneeded block {:?}, offload finished ({} / {})",
 				hash,
 				need_nodes.len(),
 				who.len()
@@ -513,12 +539,16 @@ impl BlockManager {
 				.await
 				.delete_if_unneeded(hash, self)
 				.await?;
+
+			self.clear_deleted_block_rc(hash)?;
 		}
 
-		if needed && !exists {
-			// TODO find a way to not do this if they are sending it to us
-			// Let's suppose this isn't an issue for now with the BLOCK_RW_TIMEOUT delay
-			// between the RC being incremented and this part being called.
+		if needed.is_nonzero() && !exists {
+			info!(
+				"Resync block {:?}: fetching absent but needed block (refcount > 0)",
+				hash
+			);
+
 			let block_data = self.rpc_get_block(hash).await?;
 			self.write_block(hash, &block_data[..]).await?;
 		}
@@ -526,6 +556,8 @@ impl BlockManager {
 		Ok(())
 	}
 
+	// ---- Utility: iteration on files in the data directory ----
+
 	async fn for_each_file<F, Fut, State>(
 		&self,
 		state: State,
@@ -608,7 +640,7 @@ impl EndpointHandler<BlockRpc> for BlockManager {
 
 struct BlockStatus {
 	exists: bool,
-	needed: bool,
+	needed: RcEntry,
 }
 
 impl BlockManagerLocked {
@@ -620,7 +652,7 @@ impl BlockManagerLocked {
 		let path = mgr.block_path(hash);
 
 		let exists = fs::metadata(&path).await.is_ok();
-		let needed = mgr.get_block_rc(hash)? > 0;
+		let needed = mgr.get_block_rc(hash)?;
 
 		Ok(BlockStatus { exists, needed })
 	}
@@ -659,14 +691,13 @@ impl BlockManagerLocked {
 		let mut path2 = path.clone();
 		path2.set_extension("corrupted");
 		fs::rename(path, path2).await?;
-		mgr.put_to_resync(hash, Duration::from_millis(0))?;
 		Ok(())
 	}
 
 	async fn delete_if_unneeded(&self, hash: &Hash, mgr: &BlockManager) -> Result<(), Error> {
 		let BlockStatus { exists, needed } = self.check_block_status(hash, mgr).await?;
 
-		if exists && !needed {
+		if exists && needed.is_deletable() {
 			let path = mgr.block_path(hash);
 			fs::remove_file(path).await?;
 		}
@@ -680,3 +711,104 @@ fn u64_from_be_bytes<T: AsRef<[u8]>>(bytes: T) -> u64 {
 	x8.copy_from_slice(bytes.as_ref());
 	u64::from_be_bytes(x8)
 }
+
+/// Describes the state of the reference counter for a block
+#[derive(Clone, Copy, Debug)]
+enum RcEntry {
+	/// Present: the block has `count` references, with `count` > 0.
+	///
+	/// This is stored as u64::to_be_bytes(count)
+	Present { count: u64 },
+
+	/// Deletable: the block has zero references, and can be deleted
+	/// once time (returned by now_msec) is larger than at_time
+	/// (in millis since Unix epoch)
+	///
+	/// This is stored as [0u8; 8] followed by u64::to_be_bytes(at_time),
+	/// (this allows for the data format to be backwards compatible with
+	/// previous Garage versions that didn't have this intermediate state)
+	Deletable { at_time: u64 },
+
+	/// Absent: the block has zero references, and can be deleted
+	/// immediately
+	Absent,
+}
+
+impl RcEntry {
+	fn parse(bytes: &[u8]) -> Self {
+		if bytes.len() == 8 {
+			RcEntry::Present {
+				count: u64::from_be_bytes(bytes.try_into().unwrap()),
+			}
+		} else if bytes.len() == 16 {
+			RcEntry::Deletable {
+				at_time: u64::from_be_bytes(bytes[8..16].try_into().unwrap()),
+			}
+		} else {
+			panic!("Invalid RC entry: {:?}, database is corrupted. This is an error Garage is currently unable to recover from. Sorry, and also please report a bug.",
+				bytes
+			)
+		}
+	}
+
+	fn parse_opt<V: AsRef<[u8]>>(bytes: Option<V>) -> Self {
+		bytes
+			.map(|b| Self::parse(b.as_ref()))
+			.unwrap_or(Self::Absent)
+	}
+
+	fn serialize(self) -> Option<Vec<u8>> {
+		match self {
+			RcEntry::Present { count } => Some(u64::to_be_bytes(count).to_vec()),
+			RcEntry::Deletable { at_time } => {
+				Some([u64::to_be_bytes(0), u64::to_be_bytes(at_time)].concat())
+			}
+			RcEntry::Absent => None,
+		}
+	}
+
+	fn increment(self) -> Self {
+		let old_count = match self {
+			RcEntry::Present { count } => count,
+			_ => 0,
+		};
+		RcEntry::Present {
+			count: old_count + 1,
+		}
+	}
+
+	fn decrement(self) -> Self {
+		match self {
+			RcEntry::Present { count } => {
+				if count > 1 {
+					RcEntry::Present { count: count - 1 }
+				} else {
+					RcEntry::Deletable {
+						at_time: now_msec() + BLOCK_GC_DELAY.as_millis() as u64,
+					}
+				}
+			}
+			del => del,
+		}
+	}
+
+	fn is_zero(&self) -> bool {
+		matches!(self, RcEntry::Deletable { .. } | RcEntry::Absent)
+	}
+
+	fn is_nonzero(&self) -> bool {
+		!self.is_zero()
+	}
+
+	fn is_deletable(&self) -> bool {
+		match self {
+			RcEntry::Present { .. } => false,
+			RcEntry::Deletable { at_time } => now_msec() > *at_time,
+			RcEntry::Absent => true,
+		}
+	}
+
+	fn is_needed(&self) -> bool {
+		!self.is_deletable()
+	}
+}