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.rs

@@ -0,0 +1,120 @@
+use std::cmp::Ordering;
+
+use serde::{Deserialize, Serialize};
+
+use crate::time::now_msec;
+
+use crate::crdt::crdt::*;
+
+/// Last Write Win (LWW)
+///
+/// An LWW CRDT associates a timestamp with a value, in order to implement a
+/// time-based reconciliation rule: the most recent write wins.
+/// For completeness, the LWW reconciliation rule must also be defined for two LWW CRDTs
+/// with the same timestamp but different values.
+///
+/// In our case, we add the constraint that the value that is wrapped inside the LWW CRDT must
+/// itself be a CRDT: in the case when the timestamp does not allow us to decide on which value to
+/// keep, the merge rule of the inner CRDT is applied on the wrapped values.  (Note that all types
+/// that implement the `Ord` trait get a default CRDT implemetnation that keeps the maximum value.
+/// This enables us to use LWW directly with primitive data types such as numbers or strings. It is
+/// generally desirable in this case to never explicitly produce LWW values with the same timestamp
+/// but different inner values, as the rule to keep the maximum value isn't generally the desired
+/// semantics.)
+///
+/// As multiple computers clocks are always desynchronized,
+/// when operations are close enough, it is equivalent to
+/// take one copy and drop the other one.
+///
+/// Given that clocks are not too desynchronized, this assumption
+/// is enough for most cases, as there is few chance that two humans
+/// coordonate themself faster than the time difference between two NTP servers.
+///
+/// As a more concret example, let's suppose you want to upload a file
+/// with the same key (path) in the same bucket at the very same time.
+/// For each request, the file will be timestamped by the receiving server
+/// and may differ from what you observed with your atomic clock!
+///
+/// This scheme is used by AWS S3 or Soundcloud and often without knowing
+/// in enterprise when reconciliating databases with ad-hoc scripts.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct Lww<T> {
+	ts: u64,
+	v: T,
+}
+
+impl<T> Lww<T>
+where
+	T: Crdt,
+{
+	/// Creates a new CRDT
+	///
+	/// CRDT's internal timestamp is set with current node's clock.
+	pub fn new(value: T) -> Self {
+		Self {
+			ts: now_msec(),
+			v: value,
+		}
+	}
+
+	/// Build a new CRDT from a previous non-compatible one
+	///
+	/// Compared to new, the CRDT's timestamp is not set to now
+	/// but must be set to the previous, non-compatible, CRDT's timestamp.
+	pub fn migrate_from_raw(ts: u64, value: T) -> Self {
+		Self { ts, v: value }
+	}
+
+	/// Update the LWW CRDT while keeping some causal ordering.
+	///
+	/// The timestamp of the LWW CRDT is updated to be the current node's clock
+	/// at time of update, or the previous timestamp + 1 if that's bigger,
+	/// so that the new timestamp is always strictly larger than the previous one.
+	/// This ensures that merging the update with the old value will result in keeping
+	/// the updated value.
+	pub fn update(&mut self, new_value: T) {
+		self.ts = std::cmp::max(self.ts + 1, now_msec());
+		self.v = new_value;
+	}
+
+	/// Get the CRDT value
+	pub fn get(&self) -> &T {
+		&self.v
+	}
+
+	/// Get a mutable reference to the CRDT's value
+	///
+	/// This is usefull to mutate the inside value without changing the LWW timestamp.
+	/// When such mutation is done, the merge between two LWW values is done using the inner
+	/// CRDT's merge operation. This is usefull in the case where the inner CRDT is a large
+	/// data type, such as a map, and we only want to change a single item in the map.
+	/// To do this, we can produce a "CRDT delta", i.e. a LWW that contains only the modification.
+	/// This delta consists in a LWW with the same timestamp, and the map
+	/// inside only contains the updated value.
+	/// The advantage of such a delta is that it is much smaller than the whole map.
+	///
+	/// Avoid using this if the inner data type is a primitive type such as a number or a string,
+	/// as you will then rely on the merge function defined on `Ord` types by keeping the maximum
+	/// of both values.
+	pub fn get_mut(&mut self) -> &mut T {
+		&mut self.v
+	}
+}
+
+impl<T> Crdt for Lww<T>
+where
+	T: Clone + Crdt,
+{
+	fn merge(&mut self, other: &Self) {
+		match other.ts.cmp(&self.ts) {
+			Ordering::Greater => {
+				self.ts = other.ts;
+				self.v = other.v.clone();
+			}
+			Ordering::Equal => {
+				self.v.merge(&other.v);
+			}
+			Ordering::Less => (),
+		}
+	}
+}