[stable2509] Backport #10311 (#10527)

Backport #10311 into `stable2509` from Ank4n.

See the
[documentation](https://github.com/paritytech/polkadot-sdk/blob/master/docs/BACKPORT.md)
on how to use this bot.

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---------

Co-authored-by: Ankan <[email protected]>
Co-authored-by: Ankan <[email protected]>
Co-authored-by: Egor_P <[email protected]>

Author: 3 people

cumulus/parachains/runtimes/assets/asset-hub-westend/src/staking.rs

@@ -295,8 +295,7 @@ impl pallet_staking_async::Config for Runtime {
 	type EventListeners = (NominationPools, DelegatedStaking);
 	type WeightInfo = weights::pallet_staking_async::WeightInfo<Runtime>;
 	type MaxInvulnerables = frame_support::traits::ConstU32<20>;
-	type PlanningEraOffset =
-		pallet_staking_async::PlanningEraOffsetOf<Runtime, RelaySessionDuration, ConstU32<5>>;
+	type PlanningEraOffset = ConstU32<6>;
 	type RcClientInterface = StakingRcClient;
 	type MaxEraDuration = MaxEraDuration;
 	type MaxPruningItems = MaxPruningItems;
@@ -307,6 +306,8 @@ impl pallet_staking_async_rc_client::Config for Runtime {
 	type AHStakingInterface = Staking;
 	type SendToRelayChain = StakingXcmToRelayChain;
 	type MaxValidatorSetRetries = ConstU32<64>;
+	// export validator session at end of session 4 within an era.
+	type ValidatorSetExportSession = ConstU32<4>;
 }
 
 #[derive(Encode, Decode)]

prdoc/pr_10311.prdoc

@@ -0,0 +1,23 @@
+title: '[Staking] Async Elections Post AHM'
+doc:
+- audience: Runtime Dev
+  description: |-
+    Staking elections can be configured so they run as soon as the era starts, but their results are buffered and
+    only sent to RC after the end of session `n - 2` and before start of session `n - 1`, assuming the new validator set
+    must activate at end of session `n`.
+
+    This helps us avoid difficulty of timing the election precisely as elections are long running and can sometimes
+    take longer than usual.
+
+    See documentation for `pallet_staking_async::Config::PlanningEraOffset` and
+    `pallet_staking_async_rc_client::Config::ValidatorSetExportSession` to learn more.
+crates:
+- name: pallet-staking-async
+  bump: minor
+  validate: false
+- name: pallet-staking-async-rc-client
+  bump: minor
+  validate: false
+- name: asset-hub-westend-runtime
+  bump: minor
+  validate: false