garage/src/table/gc.rs

use std::collections::HashMap;
use std::sync::Arc;
use std::time::Duration;

use async_trait::async_trait;
use serde::{Deserialize, Serialize};
use serde_bytes::ByteBuf;

use futures::future::join_all;
use futures::select;
use futures_util::future::*;
use tokio::sync::watch;

use garage_util::data::*;
use garage_util::error::*;

use garage_rpc::system::System;
use garage_rpc::*;

use crate::data::*;
use crate::replication::*;
use crate::schema::*;

const TABLE_GC_BATCH_SIZE: usize = 1024;
const TABLE_GC_RPC_TIMEOUT: Duration = Duration::from_secs(30);

pub(crate) struct TableGc<F: TableSchema + 'static, R: TableReplication + 'static> {
	system: Arc<System>,
	data: Arc<TableData<F, R>>,

	endpoint: Arc<Endpoint<GcRpc, Self>>,
}

#[derive(Serialize, Deserialize)]
enum GcRpc {
	Update(Vec<ByteBuf>),
	DeleteIfEqualHash(Vec<(ByteBuf, Hash)>),
	Ok,
}

impl Rpc for GcRpc {
	type Response = Result<GcRpc, Error>;
}

impl<F, R> TableGc<F, R>
where
	F: TableSchema + 'static,
	R: TableReplication + 'static,
{
	pub(crate) fn launch(system: Arc<System>, data: Arc<TableData<F, R>>) -> Arc<Self> {
		let endpoint = system
			.netapp
			.endpoint(format!("garage_table/gc.rs/Rpc:{}", data.name));

		let gc = Arc::new(Self {
			system: system.clone(),
			data: data.clone(),
			endpoint,
		});

		gc.endpoint.set_handler(gc.clone());

		let gc1 = gc.clone();
		system.background.spawn_worker(
			format!("GC loop for {}", data.name),
			move |must_exit: watch::Receiver<bool>| gc1.gc_loop(must_exit),
		);

		gc
	}

	async fn gc_loop(self: Arc<Self>, mut must_exit: watch::Receiver<bool>) {
		while !*must_exit.borrow() {
			match self.gc_loop_iter().await {
				Ok(true) => {
					// Stuff was done, loop immediately
					continue;
				}
				Ok(false) => {
					// Nothing was done, sleep for some time (below)
				}
				Err(e) => {
					warn!("({}) Error doing GC: {}", self.data.name, e);
				}
			}
			select! {
				_ = tokio::time::sleep(Duration::from_secs(10)).fuse() => {},
				_ = must_exit.changed().fuse() => {},
			}
		}
	}

	async fn gc_loop_iter(&self) -> Result<bool, Error> {
		let mut entries = vec![];
		let mut excluded = vec![];

		// List entries in the GC todo list
		// These entries are put there when a tombstone is inserted in the table
		// This is detected and done in data.rs in update_entry
		for entry_kv in self.data.gc_todo.iter() {
			let (k, vhash) = entry_kv?;
			let mut todo_entry = GcTodoEntry::parse(&k, &vhash);

			let vhash = Hash::try_from(&vhash[..]).unwrap();

			// Check if the tombstone is still the current value of the entry.
			// If not, we don't actually want to GC it, and we will remove it
			// from the gc_todo table later (below).
			todo_entry.value = self
				.data
				.store
				.get(&k[..])?
				.filter(|v| blake2sum(&v[..]) == vhash)
				.map(|v| v.to_vec());

			if todo_entry.value.is_some() {
				entries.push(todo_entry);
				if entries.len() >= TABLE_GC_BATCH_SIZE {
					break;
				}
			} else {
				excluded.push(todo_entry);
			}
		}

		// Remove from gc_todo entries for tombstones where we have
		// detected that the current value has changed and
		// is no longer a tombstone.
		for entry in excluded {
			entry.remove_if_equal(&self.data.gc_todo)?;
		}

		// Remaining in `entries` is the list of entries we want to GC,
		// and for which they are still currently tombstones in the table.

		if entries.is_empty() {
			// Nothing to do in this iteration
			return Ok(false);
		}

		debug!("({}) GC: doing {} items", self.data.name, entries.len());

		// Split entries to GC by the set of nodes on which they are stored.
		// Here we call them partitions but they are not exactly
		// the same as partitions as defined in the ring: those partitions
		// are defined by the first 8 bits of the hash, but two of these
		// partitions can be stored on the same set of nodes.
		// Here we detect when entries are stored on the same set of nodes:
		// even if they are not in the same 8-bit partition, we can still
		// handle them together.
		let mut partitions = HashMap::new();
		for entry in entries {
			let pkh = Hash::try_from(&entry.key[..32]).unwrap();
			let mut nodes = self.data.replication.write_nodes(&pkh);
			nodes.retain(|x| *x != self.system.id);
			nodes.sort();

			if !partitions.contains_key(&nodes) {
				partitions.insert(nodes.clone(), vec![]);
			}
			partitions.get_mut(&nodes).unwrap().push(entry);
		}

		// For each set of nodes that contains some items,
		// ensure they are aware of the tombstone status, and once they
		// are, instruct them to delete the entries.
		let resps = join_all(
			partitions
				.into_iter()
				.map(|(nodes, items)| self.try_send_and_delete(nodes, items)),
		)
		.await;

		// Collect errors and return a single error value even if several
		// errors occurred.
		let mut errs = vec![];
		for resp in resps {
			if let Err(e) = resp {
				errs.push(e);
			}
		}

		if errs.is_empty() {
			Ok(true)
		} else {
			Err(Error::Message(
				errs.into_iter()
					.map(|x| format!("{}", x))
					.collect::<Vec<_>>()
					.join(", "),
			))
			.err_context("in try_send_and_delete:")
		}
	}

	async fn try_send_and_delete(
		&self,
		nodes: Vec<Uuid>,
		items: Vec<GcTodoEntry>,
	) -> Result<(), Error> {
		let n_items = items.len();

		// Strategy: we first send all of the values to the remote nodes,
		// to ensure that they are aware of the tombstone state.
		// (if they have a newer state that overrides the tombstone, that's fine).
		// Second, once everyone is at least at the tombstone state,
		// we instruct everyone to delete the tombstone IF that is still their current state.
		// If they are now at a different state, it means that that state overrides the
		// tombstone in the CRDT lattice, and it will be propagated back to us at some point
		// (either just a regular update that hasn't reached us yet, or later when the
		// table is synced).
		// Here, we store in updates all of the tombstones to send for step 1,
		// and in deletes the list of keys and hashes of value for step 2.
		let mut updates = vec![];
		let mut deletes = vec![];
		for item in items {
			updates.push(ByteBuf::from(item.value.unwrap()));
			deletes.push((ByteBuf::from(item.key), item.value_hash));
		}

		// Step 1: ensure everyone is at least at tombstone in CRDT lattice
		// Here the quorum is nodes.len(): we cannot tolerate even a single failure,
		// otherwise old values before the tombstone might come back in the data.
		// GC'ing is not a critical function of the system, so it's not a big
		// deal if we can't do it right now.
		self.system
			.rpc
			.try_call_many(
				&self.endpoint,
				&nodes[..],
				GcRpc::Update(updates),
				RequestStrategy::with_priority(PRIO_BACKGROUND)
					.with_quorum(nodes.len())
					.with_timeout(TABLE_GC_RPC_TIMEOUT),
			)
			.await
			.err_context("GC: send tombstones")?;

		info!(
			"({}) GC: {} items successfully pushed, will try to delete.",
			self.data.name, n_items
		);

		// Step 2: delete tombstones everywhere.
		// Here we also fail if even a single node returns a failure:
		// it means that the garbage collection wasn't completed and has
		// to be retried later.
		self.system
			.rpc
			.try_call_many(
				&self.endpoint,
				&nodes[..],
				GcRpc::DeleteIfEqualHash(deletes.clone()),
				RequestStrategy::with_priority(PRIO_BACKGROUND)
					.with_quorum(nodes.len())
					.with_timeout(TABLE_GC_RPC_TIMEOUT),
			)
			.await
			.err_context("GC: remote delete tombstones")?;

		// GC has been successfull for all of these entries.
		// We now remove them all from our local table and from the GC todo list.
		for (k, vhash) in deletes {
			self.data
				.delete_if_equal_hash(&k[..], vhash)
				.err_context("GC: local delete tombstones")?;
			self.todo_remove_if_equal(&k[..], vhash)
				.err_context("GC: remove from todo list after successfull GC")?;
		}

		Ok(())
	}

	fn todo_remove_if_equal(&self, key: &[u8], vhash: Hash) -> Result<(), Error> {
		let _ = self
			.data
			.gc_todo
			.compare_and_swap::<_, _, Vec<u8>>(key, Some(vhash), None)?;
		Ok(())
	}
}

#[async_trait]
impl<F, R> EndpointHandler<GcRpc> for TableGc<F, R>
where
	F: TableSchema + 'static,
	R: TableReplication + 'static,
{
	async fn handle(self: &Arc<Self>, message: &GcRpc, _from: NodeID) -> Result<GcRpc, Error> {
		match message {
			GcRpc::Update(items) => {
				self.data.update_many(items)?;
				Ok(GcRpc::Ok)
			}
			GcRpc::DeleteIfEqualHash(items) => {
				for (key, vhash) in items.iter() {
					self.data.delete_if_equal_hash(&key[..], *vhash)?;
					self.todo_remove_if_equal(&key[..], *vhash)?;
				}
				Ok(GcRpc::Ok)
			}
			_ => Err(Error::Message("Unexpected GC RPC".to_string())),
		}
	}
}

/// An entry stored in the gc_todo Sled tree associated with the table
/// Contains helper function for parsing, saving, and removing
/// such entry in Sled
pub(crate) struct GcTodoEntry {
	key: Vec<u8>,
	value_hash: Hash,
	value: Option<Vec<u8>>,
}

impl GcTodoEntry {
	/// Creates a new GcTodoEntry (not saved in Sled) from its components:
	/// the key of an entry in the table, and the hash of the associated
	/// serialized value
	pub(crate) fn new(key: Vec<u8>, value_hash: Hash) -> Self {
		Self {
			key,
			value_hash,
			value: None,
		}
	}

	/// Parses a GcTodoEntry from a (k, v) pair stored in the gc_todo tree
	pub(crate) fn parse(sled_k: &[u8], sled_v: &[u8]) -> Self {
		Self {
			key: sled_k.to_vec(),
			value_hash: Hash::try_from(sled_v).unwrap(),
			value: None,
		}
	}

	/// Saves the GcTodoEntry in the gc_todo tree
	pub(crate) fn save(&self, gc_todo_tree: &sled::Tree) -> Result<(), Error> {
		gc_todo_tree.insert(&self.key[..], self.value_hash.as_slice())?;
		Ok(())
	}

	/// Removes the GcTodoEntry from the gc_todo tree if the
	/// hash of the serialized value is the same here as in the tree.
	/// This is usefull to remove a todo entry only under the condition
	/// that it has not changed since the time it was read, i.e.
	/// what we have to do is still the same
	pub(crate) fn remove_if_equal(&self, gc_todo_tree: &sled::Tree) -> Result<(), Error> {
		let _ = gc_todo_tree.compare_and_swap::<_, _, Vec<u8>>(
			&self.key[..],
			Some(self.value_hash),
			None,
		)?;
		Ok(())
	}
}