Limited Unbonding Chunks. (#2700)

* Limit number of unlocking chunks.

* Bump.

substrate/srml/staking/src/lib.rs

@@ -214,6 +214,11 @@
 //! removed. Once the `BondingDuration` is over, the [`withdraw_unbonded`](./enum.Call.html#variant.withdraw_unbonded) call can be used
 //! to actually withdraw the funds.
 //!
+//! Note that there is a limitation to the number of fund-chunks that can be scheduled to be unlocked in the future
+//! via [`unbond`](enum.Call.html#variant.unbond).
+//! In case this maximum (`MAX_UNLOCKING_CHUNKS`) is reached, the bonded account _must_ first wait until a successful
+//! call to `withdraw_unbonded` to remove some of the chunks.
+//!
 //! ### Election Algorithm
 //!
 //! The current election algorithm is implemented based on Phragmén.
@@ -266,6 +271,8 @@ const RECENT_OFFLINE_COUNT: usize = 32;
 const DEFAULT_MINIMUM_VALIDATOR_COUNT: u32 = 4;
 const MAX_NOMINATIONS: usize = 16;
 const MAX_UNSTAKE_THRESHOLD: u32 = 10;
+const MAX_UNLOCKING_CHUNKS: usize = 32;
+const STAKING_ID: LockIdentifier = *b"staking ";
 
 /// Indicates the initial status of the staker.
 #[cfg_attr(feature = "std", derive(Debug, Serialize, Deserialize))]
@@ -426,8 +433,6 @@ pub trait Trait: system::Trait + session::Trait {
 	type Reward: OnUnbalanced<PositiveImbalanceOf<Self>>;
 }
 
-const STAKING_ID: LockIdentifier = *b"staking ";
-
 decl_storage! {
 	trait Store for Module<T: Trait> as Staking {
 
@@ -605,12 +610,20 @@ decl_module! {
 		/// Once the unlock period is done, you can call `withdraw_unbonded` to actually move
 		/// the funds out of management ready for transfer.
 		///
+		/// No more than a limited number of unlocking chunks (see `MAX_UNLOCKING_CHUNKS`)
+		/// can co-exists at the same time. In that case, [`Call::withdraw_unbonded`] need
+		/// to be called first to remove some of the chunks (if possible).
+		///
 		/// The dispatch origin for this call must be _Signed_ by the controller, not the stash.
 		///
 		/// See also [`Call::withdraw_unbonded`].
 		fn unbond(origin, #[compact] value: BalanceOf<T>) {
 			let controller = ensure_signed(origin)?;
 			let mut ledger = Self::ledger(&controller).ok_or("not a controller")?;
+			ensure!(
+				ledger.unlocking.len() < MAX_UNLOCKING_CHUNKS,
+				"can not schedule more unlock chunks"
+			);
 
 			let mut value = value.min(ledger.active);