add ActiveLeavesUpdate, remove StartWork, StopWork (#1458)

* add ActiveLeavesUpdate, remove StartWork, StopWork

* replace StartWork, StopWork in subsystem crate tests

* mechanically update OverseerSignal in other modules

* convert overseer to take advantage of new multi-hash update abilities

Note: this does not yet convert the tests; some of the tests now freeze:

test tests::overseer_start_stop_works ... test tests::overseer_start_stop_works has been running for over 60 seconds
test tests::overseer_finalize_works ... test tests::overseer_finalize_works has been running for over 60 seconds

* fix broken overseer tests

* manually impl PartialEq for ActiveLeavesUpdate, rm trait Equivalent

This cleans up the code a bit and makes it easier in the future to
do the right thing when comparing ALUs.

* use target in all network bridge logging

* reduce spamming of  and

polkadot/node/subsystem/src/lib.rs

@@ -30,6 +30,7 @@ use futures::future::BoxFuture;
 
 use polkadot_primitives::v1::Hash;
 use async_trait::async_trait;
+use smallvec::SmallVec;
 
 use crate::messages::AllMessages;
 
@@ -38,13 +39,51 @@ pub mod util;
 #[cfg(any(test, feature = "test-helpers"))]
 pub mod test_helpers;
 
+/// How many slots are stack-reserved for active leaves updates
+///
+/// If there are fewer than this number of slots, then we've wasted some stack space.
+/// If there are greater than this number of slots, then we fall back to a heap vector.
+const ACTIVE_LEAVES_SMALLVEC_CAPACITY: usize = 8;
+
+/// Changes in the set of active leaves: the parachain heads which we care to work on.
+///
+/// Note that the activated and deactivated fields indicate deltas, not complete sets.
+#[derive(Clone, Debug, Default, Eq)]
+pub struct ActiveLeavesUpdate {
+	/// New relay chain block hashes of interest.
+	pub activated: SmallVec<[Hash; ACTIVE_LEAVES_SMALLVEC_CAPACITY]>,
+	/// Relay chain block hashes no longer of interest.
+	pub deactivated: SmallVec<[Hash; ACTIVE_LEAVES_SMALLVEC_CAPACITY]>,
+}
+
+impl ActiveLeavesUpdate {
+	/// Create a ActiveLeavesUpdate with a single activated hash
+	pub fn start_work(hash: Hash) -> Self {
+		Self { activated: [hash].as_ref().into(), ..Default::default() }
+	}
+
+	/// Create a ActiveLeavesUpdate with a single deactivated hash
+	pub fn stop_work(hash: Hash) -> Self {
+		Self { deactivated: [hash].as_ref().into(), ..Default::default() }
+	}
+}
+
+impl PartialEq for ActiveLeavesUpdate {
+	/// Equality for `ActiveLeavesUpdate` doesnt imply bitwise equality.
+	///
+	/// Instead, it means equality when `activated` and `deactivated` are considered as sets.
+	fn eq(&self, other: &Self) -> bool {
+		use std::collections::HashSet;
+		self.activated.iter().collect::<HashSet<_>>() == other.activated.iter().collect::<HashSet<_>>() &&
+			self.deactivated.iter().collect::<HashSet<_>>() == other.deactivated.iter().collect::<HashSet<_>>()
+	}
+}
+
 /// Signals sent by an overseer to a subsystem.
 #[derive(PartialEq, Clone, Debug)]
 pub enum OverseerSignal {
-	/// `Subsystem` should start working on block-based work, given by the relay-chain block hash.
-	StartWork(Hash),
-	/// `Subsystem` should stop working on block-based work specified by the relay-chain block hash.
-	StopWork(Hash),
+	/// Subsystems should adjust their jobs to start and stop work on appropriate block hashes.
+	ActiveLeaves(ActiveLeavesUpdate),
 	/// Conclude the work of the `Overseer` and all `Subsystem`s.
 	Conclude,
 }