Improve how node roles are assigned in Garage

- change the terminology: the network configuration becomes the role
  table, the configuration of a nodes becomes a node's role
- the modification of the role table takes place in two steps: first,
  changes are staged in a CRDT data structure. Then, once the user is
  happy with the changes, they can commit them all at once (or revert
  them).
- update documentation
- fix tests
- implement smarter partition assignation algorithm

This patch breaks the format of the network configuration: when
migrating, the cluster will be in a state where no roles are assigned.
All roles must be re-assigned and commited at once. This migration
should not pose an issue.

src/util/crdt/lww_map.rs

@@ -0,0 +1,167 @@
+use std::cmp::Ordering;
+
+use serde::{Deserialize, Serialize};
+
+use crate::time::now_msec;
+
+use crate::crdt::crdt::*;
+
+/// Last Write Win Map
+///
+/// This types defines a CRDT for a map from keys to values.
+/// The values have an associated timestamp, such that the last written value
+/// takes precedence over previous ones. As for the simpler `LWW` type, the value
+/// type `V` is also required to implement the CRDT trait.
+/// We do not encourage mutating the values associated with a given key
+/// without updating the timestamp, in fact at the moment we do not provide a `.get_mut()`
+/// method that would allow that.
+///
+/// Internally, the map is stored as a vector of keys and values, sorted by ascending key order.
+/// This is why the key type `K` must implement `Ord` (and also to ensure a unique serialization,
+/// such that two values can be compared for equality based on their hashes). As a consequence,
+/// insertions take `O(n)` time. This means that LWWMap should be used for reasonably small maps.
+/// However, note that even if we were using a more efficient data structure such as a `BTreeMap`,
+/// the serialization cost `O(n)` would still have to be paid at each modification, so we are
+/// actually not losing anything here.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct LwwMap<K, V> {
+	vals: Vec<(K, u64, V)>,
+}
+
+impl<K, V> LwwMap<K, V>
+where
+	K: Ord,
+	V: Crdt,
+{
+	/// Create a new empty map CRDT
+	pub fn new() -> Self {
+		Self { vals: vec![] }
+	}
+	/// Used to migrate from a map defined in an incompatible format. This produces
+	/// a map that contains a single item with the specified timestamp (copied from
+	/// the incompatible format). Do this as many times as you have items to migrate,
+	/// and put them all together using the CRDT merge operator.
+	pub fn migrate_from_raw_item(k: K, ts: u64, v: V) -> Self {
+		Self {
+			vals: vec![(k, ts, v)],
+		}
+	}
+	/// Returns a map that contains a single mapping from the specified key to the specified value.
+	/// This map is a mutator, or a delta-CRDT, such that when it is merged with the original map,
+	/// the previous value will be replaced with the one specified here.
+	/// The timestamp in the provided mutator is set to the maximum of the current system's clock
+	/// and 1 + the previous value's timestamp (if there is one), so that the new value will always
+	/// take precedence (LWW rule).
+	///
+	/// Typically, to update the value associated to a key in the map, you would do the following:
+	///
+	/// ```ignore
+	/// let my_update = my_crdt.update_mutator(key_to_modify, new_value);
+	/// my_crdt.merge(&my_update);
+	/// ```
+	///
+	/// However extracting the mutator on its own and only sending that on the network is very
+	/// interesting as it is much smaller than the whole map.
+	pub fn update_mutator(&self, k: K, new_v: V) -> Self {
+		let new_vals = match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(&k)) {
+			Ok(i) => {
+				let (_, old_ts, _) = self.vals[i];
+				let new_ts = std::cmp::max(old_ts + 1, now_msec());
+				vec![(k, new_ts, new_v)]
+			}
+			Err(_) => vec![(k, now_msec(), new_v)],
+		};
+		Self { vals: new_vals }
+	}
+	/// Takes all of the values of the map and returns them. The current map is reset to the
+	/// empty map. This is very usefull to produce in-place a new map that contains only a delta
+	/// that modifies a certain value:
+	///
+	/// ```ignore
+	/// let mut a = get_my_crdt_value();
+	/// let old_a = a.take_and_clear();
+	/// a.merge(&old_a.update_mutator(key_to_modify, new_value));
+	/// put_my_crdt_value(a);
+	/// ```
+	///
+	/// Of course in this simple example we could have written simply
+	/// `pyt_my_crdt_value(a.update_mutator(key_to_modify, new_value))`,
+	/// but in the case where the map is a field in a struct for instance (as is always the case),
+	/// this becomes very handy:
+	///
+	/// ```ignore
+	/// let mut a = get_my_crdt_value();
+	/// let old_a_map = a.map_field.take_and_clear();
+	/// a.map_field.merge(&old_a_map.update_mutator(key_to_modify, new_value));
+	/// put_my_crdt_value(a);
+	/// ```
+	pub fn take_and_clear(&mut self) -> Self {
+		let vals = std::mem::take(&mut self.vals);
+		Self { vals }
+	}
+	/// Removes all values from the map
+	pub fn clear(&mut self) {
+		self.vals.clear();
+	}
+	/// Get a reference to the value assigned to a key
+	pub fn get(&self, k: &K) -> Option<&V> {
+		match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(k)) {
+			Ok(i) => Some(&self.vals[i].2),
+			Err(_) => None,
+		}
+	}
+	/// Gets a reference to all of the items, as a slice. Usefull to iterate on all map values.
+	/// In most case you will want to ignore the timestamp (second item of the tuple).
+	pub fn items(&self) -> &[(K, u64, V)] {
+		&self.vals[..]
+	}
+
+	/// Returns the number of items in the map
+	pub fn len(&self) -> usize {
+		self.vals.len()
+	}
+
+	/// Returns true if the map is empty
+	pub fn is_empty(&self) -> bool {
+		self.len() == 0
+	}
+}
+
+impl<K, V> Crdt for LwwMap<K, V>
+where
+	K: Clone + Ord,
+	V: Clone + Crdt,
+{
+	fn merge(&mut self, other: &Self) {
+		for (k, ts2, v2) in other.vals.iter() {
+			match self.vals.binary_search_by(|(k2, _, _)| k2.cmp(k)) {
+				Ok(i) => {
+					let (_, ts1, _v1) = &self.vals[i];
+					match ts2.cmp(ts1) {
+						Ordering::Greater => {
+							self.vals[i].1 = *ts2;
+							self.vals[i].2 = v2.clone();
+						}
+						Ordering::Equal => {
+							self.vals[i].2.merge(v2);
+						}
+						Ordering::Less => (),
+					}
+				}
+				Err(i) => {
+					self.vals.insert(i, (k.clone(), *ts2, v2.clone()));
+				}
+			}
+		}
+	}
+}
+
+impl<K, V> Default for LwwMap<K, V>
+where
+	K: Ord,
+	V: Crdt,
+{
+	fn default() -> Self {
+		Self::new()
+	}
+}