Fix multiple cores 1.8 (#3477)

First step into elastic scaling: Having a para be on multiple cores,
will not get it more throughput yet, but it will no longer brick it.

This is a backport PR, as this would benefit the Coretime launch on
Kusama.

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Signed-off-by: alindima <alin@parity.io>
Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>
Co-authored-by: alindima <alin@parity.io>
Co-authored-by: eskimor <eskimor@no-such-url.com>

.gitlab/pipeline/zombienet/polkadot.yml

@@ -158,6 +158,14 @@ zombienet-polkadot-functional-0011-async-backing-6-seconds-rate:
       --local-dir="${LOCAL_DIR}/functional"
       --test="0011-async-backing-6-seconds-rate.zndsl"
 
+zombienet-polkadot-functional-0012-elastic-scaling-mvp:
+  extends:
+    - .zombienet-polkadot-common
+  script:
+    - /home/nonroot/zombie-net/scripts/ci/run-test-local-env-manager.sh
+      --local-dir="${LOCAL_DIR}/functional"
+      --test="0012-elastic-scaling-mvp.zndsl"
+
 zombienet-polkadot-smoke-0001-parachains-smoke-test:
   extends:
     - .zombienet-polkadot-common

polkadot/node/core/backing/Cargo.toml

@@ -22,6 +22,7 @@ bitvec = { version = "1.0.0", default-features = false, features = ["alloc"] }
 gum = { package = "tracing-gum", path = "../../gum" }
 thiserror = { workspace = true }
 fatality = "0.0.6"
+schnellru = "0.2.1"
 
 [dev-dependencies]
 sp-core = { path = "../../../../substrate/primitives/core" }
@@ -31,5 +32,6 @@ sc-keystore = { path = "../../../../substrate/client/keystore" }
 sp-tracing = { path = "../../../../substrate/primitives/tracing" }
 futures = { version = "0.3.21", features = ["thread-pool"] }
 assert_matches = "1.4.0"
+rstest = "0.18.2"
 polkadot-node-subsystem-test-helpers = { path = "../../subsystem-test-helpers" }
 test-helpers = { package = "polkadot-primitives-test-helpers", path = "../../../primitives/test-helpers" }

polkadot/node/core/backing/src/error.rs

@@ -48,6 +48,9 @@ pub enum Error {
 	#[error("Candidate is not found")]
 	CandidateNotFound,
 
+	#[error("CoreIndex cannot be determined for a candidate")]
+	CoreIndexUnavailable,
+
 	#[error("Signature is invalid")]
 	InvalidSignature,