Introduce a maximum code size and head data size (#835)

* add a maximum code size and head data size

* get existing tests passing

* add tests for slots logic

* test registrar behavior

* introduce maximums and bump versions

* address review grumbles

* work around publicizing derive

* remove unneeded and wrong doc

polkadot/runtime/common/src/parachains.rs

@@ -129,6 +129,14 @@ pub trait Trait: attestations::Trait {
 
 	/// The way that we are able to register parachains.
 	type Registrar: Registrar<Self::AccountId>;
+
+	/// Maximum code size for parachains, in bytes. Note that this is not
+	/// the entire storage burden of the parachain, as old code is stored for
+	/// `SlashPeriod` blocks.
+	type MaxCodeSize: Get<u32>;
+
+	/// Max head data size.
+	type MaxHeadDataSize: Get<u32>;
 }
 
 /// Origin for the parachains module.
@@ -233,6 +241,8 @@ decl_error! {
 		UntaggedVotes,
 		/// Wrong parent head for parachain receipt.
 		ParentMismatch,
+		/// Head data was too large.
+		HeadDataTooLarge,
 	}
 }
 
@@ -778,6 +788,8 @@ impl<T: Trait> Module<T> {
 		let mut validator_groups = GroupedDutyIter::new(&sorted_validators[..]);
 
 		let mut para_block_hashes = Vec::new();
+
+		let max_head_data_size = T::MaxHeadDataSize::get();
 		for candidate in attested_candidates {
 			let para_id = candidate.parachain_index();
 			let validator_group = validator_groups.group_for(para_id)
@@ -801,6 +813,11 @@ impl<T: Trait> Module<T> {
 				Error::<T>::VotesExceedsAuthorities,
 			);
 
+			ensure!(
+				max_head_data_size >= candidate.candidate().head_data.0.len() as _,
+				Error::<T>::HeadDataTooLarge,
+			);
+
 			let fees = candidate.candidate().fees;
 			T::ParachainCurrency::deduct(para_id, fees)?;
 
@@ -1152,6 +1169,11 @@ mod tests {
 		type MaxRetries = MaxRetries;
 	}
 
+	parameter_types! {
+		pub const MaxHeadDataSize: u32 = 100;
+		pub const MaxCodeSize: u32 = 100;
+	}
+
 	impl Trait for Test {
 		type Origin = Origin;
 		type Call = Call;
@@ -1159,6 +1181,8 @@ mod tests {
 		type Randomness = RandomnessCollectiveFlip;
 		type ActiveParachains = registrar::Module<Test>;
 		type Registrar = registrar::Module<Test>;
+		type MaxCodeSize = MaxCodeSize;
+		type MaxHeadDataSize = MaxHeadDataSize;
 	}
 
 	type Parachains = Module<Test>;