Ensure referenda TracksInfo is sorted (#3325)

Changes:
- Add `integrity_check` to trait `TracksInfo` to assert the sorting 
- Use the check in `integrity_test`
- Rely upon sorted property in the `info` for speedup

Note that the pallet already relies upon this (so far) untested
assumption
[here](https://github.com/paritytech/polkadot-sdk/blob/44c7a5eb8c375d77f685abf585961421e5f8b228/substrate/frame/referenda/src/lib.rs#L1280).

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>

prdoc/pr_3308.prdoc

@@ -1,4 +1,4 @@
-title: Parachains-Aura: Only produce once per slot
+title: "Parachains-Aura: Only produce once per slot"
 
 doc:
   - audience: Node Dev

prdoc/pr_3325.prdoc

@@ -0,0 +1,10 @@
+title: "Ensure `TracksInfo` tracks are sorted by ID."
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Add a `integrity_check` function to trait `TracksInfo` and explicitly state that tracks must
+      always be sorted by ID. The referenda pallet now also uses this check in its `integrity_test`.
+
+crates:
+  - name: pallet-referenda

substrate/frame/referenda/src/lib.rs

@@ -433,6 +433,11 @@ pub mod pallet {
 			Self::do_try_state()?;
 			Ok(())
 		}
+
+		#[cfg(any(feature = "std", test))]
+		fn integrity_test() {
+			T::Tracks::check_integrity().expect("Static tracks configuration is valid.");
+		}
 	}
 
 	#[pallet::call]

substrate/frame/referenda/src/types.rs

@@ -144,7 +144,10 @@ pub trait TracksInfo<Balance, Moment> {
 	/// The origin type from which a track is implied.
 	type RuntimeOrigin;
 
-	/// Return the array of known tracks and their information.
+	/// Sorted array of known tracks and their information.
+	///
+	/// The array MUST be sorted by `Id`. Consumers of this trait are advised to assert
+	/// [`Self::check_integrity`] prior to any use.
 	fn tracks() -> &'static [(Self::Id, TrackInfo<Balance, Moment>)];
 
 	/// Determine the voting track for the given `origin`.
@@ -152,7 +155,23 @@ pub trait TracksInfo<Balance, Moment> {
 
 	/// Return the track info for track `id`, by default this just looks it up in `Self::tracks()`.
 	fn info(id: Self::Id) -> Option<&'static TrackInfo<Balance, Moment>> {
-		Self::tracks().iter().find(|x| x.0 == id).map(|x| &x.1)
+		let tracks = Self::tracks();
+		let maybe_index = tracks.binary_search_by_key(&id, |t| t.0).ok()?;
+
+		tracks.get(maybe_index).map(|(_, info)| info)
+	}
+
+	/// Check assumptions about the static data that this trait provides.
+	fn check_integrity() -> Result<(), &'static str>
+	where
+		Balance: 'static,
+		Moment: 'static,
+	{
+		if Self::tracks().windows(2).all(|w| w[0].0 < w[1].0) {
+			Ok(())
+		} else {
+			Err("The tracks that were returned by `tracks` were not sorted by `Id`")
+		}
 	}
 }
 
@@ -670,4 +689,72 @@ mod tests {
 		assert_eq!(c.delay(pc(30).less_epsilon()), pc(100));
 		assert_eq!(c.delay(pc(0)), pc(100));
 	}
+
+	#[test]
+	fn tracks_integrity_check_detects_unsorted() {
+		use crate::mock::RuntimeOrigin;
+
+		pub struct BadTracksInfo;
+		impl TracksInfo<u64, u64> for BadTracksInfo {
+			type Id = u8;
+			type RuntimeOrigin = <RuntimeOrigin as OriginTrait>::PalletsOrigin;
+			fn tracks() -> &'static [(Self::Id, TrackInfo<u64, u64>)] {
+				static DATA: [(u8, TrackInfo<u64, u64>); 2] = [
+					(
+						1u8,
+						TrackInfo {
+							name: "root",
+							max_deciding: 1,
+							decision_deposit: 10,
+							prepare_period: 4,
+							decision_period: 4,
+							confirm_period: 2,
+							min_enactment_period: 4,
+							min_approval: Curve::LinearDecreasing {
+								length: Perbill::from_percent(100),
+								floor: Perbill::from_percent(50),
+								ceil: Perbill::from_percent(100),
+							},
+							min_support: Curve::LinearDecreasing {
+								length: Perbill::from_percent(100),
+								floor: Perbill::from_percent(0),
+								ceil: Perbill::from_percent(100),
+							},
+						},
+					),
+					(
+						0u8,
+						TrackInfo {
+							name: "none",
+							max_deciding: 3,
+							decision_deposit: 1,
+							prepare_period: 2,
+							decision_period: 2,
+							confirm_period: 1,
+							min_enactment_period: 2,
+							min_approval: Curve::LinearDecreasing {
+								length: Perbill::from_percent(100),
+								floor: Perbill::from_percent(95),
+								ceil: Perbill::from_percent(100),
+							},
+							min_support: Curve::LinearDecreasing {
+								length: Perbill::from_percent(100),
+								floor: Perbill::from_percent(90),
+								ceil: Perbill::from_percent(100),
+							},
+						},
+					),
+				];
+				&DATA[..]
+			}
+			fn track_for(_: &Self::RuntimeOrigin) -> Result<Self::Id, ()> {
+				unimplemented!()
+			}
+		}
+
+		assert_eq!(
+			BadTracksInfo::check_integrity(),
+			Err("The tracks that were returned by `tracks` were not sorted by `Id`")
+		);
+	}
 }