Fix some weirdness in offchain_worker (paritytech#7541)

We call `offchain_worker` with the state of the imported block and pass
the header of this block. However in the runtime we call all
`offchain_worker` functions with the number of the parent block. Besides
that we also pass all digests and not only the pre runtime digests. In
the context where the offchain worker is executed we have all digests, so
there is no real reason to only pass pre runtime digests. Another fix is
that we also insert the hash of the current header into the block hash map.

frame/executive/src/lib.rs

@@ -117,7 +117,7 @@
 
 use sp_std::{prelude::*, marker::PhantomData};
 use frame_support::{
-	storage::StorageValue, weights::{GetDispatchInfo, DispatchInfo, DispatchClass},
+	StorageValue, StorageMap, weights::{GetDispatchInfo, DispatchInfo, DispatchClass},
 	traits::{OnInitialize, OnFinalize, OnRuntimeUpgrade, OffchainWorker},
 	dispatch::PostDispatchInfo,
 };
@@ -453,7 +453,7 @@ where
 		// We need to keep events available for offchain workers,
 		// hence we initialize the block manually.
 		// OffchainWorker RuntimeApi should skip initialization.
-		let digests = Self::extract_pre_digest(header);
+		let digests = header.digest().clone();
 
 		<frame_system::Module<System>>::initialize(
 			header.number(),
@@ -463,15 +463,16 @@ where
 			frame_system::InitKind::Inspection,
 		);
 
+		// Frame system only inserts the parent hash into the block hashes as normally we don't know
+		// the hash for the header before. However, here we are aware of the hash and we can add it
+		// as well.
+		frame_system::BlockHash::<System>::insert(header.number(), header.hash());
+
 		// Initialize logger, so the log messages are visible
 		// also when running WASM.
 		frame_support::debug::RuntimeLogger::init();
 
-		<AllModules as OffchainWorker<System::BlockNumber>>::offchain_worker(
-			// to maintain backward compatibility we call module offchain workers
-			// with parent block number.
-			header.number().saturating_sub(1u32.into())
-		)
+		<AllModules as OffchainWorker<System::BlockNumber>>::offchain_worker(*header.number())
 	}
 }
 
@@ -481,7 +482,7 @@ mod tests {
 	use super::*;
 	use sp_core::H256;
 	use sp_runtime::{
-		generic::Era, Perbill, DispatchError, testing::{Digest, Header, Block},
+		generic::{Era, DigestItem}, Perbill, DispatchError, testing::{Digest, Header, Block},
 		traits::{Header as HeaderT, BlakeTwo256, IdentityLookup},
 		transaction_validity::{
 			InvalidTransaction, ValidTransaction, TransactionValidityError, UnknownTransaction
@@ -547,6 +548,10 @@ mod tests {
 					sp_io::storage::set(super::TEST_KEY, "module".as_bytes());
 					200
 				}
+
+				fn offchain_worker(n: T::BlockNumber) {
+					assert_eq!(T::BlockNumber::from(1u32), n);
+				}
 			}
 		}
 
@@ -1115,4 +1120,27 @@ mod tests {
 			);
 		});
 	}
+
+	#[test]
+	fn offchain_worker_works_as_expected() {
+		new_test_ext(1).execute_with(|| {
+			let parent_hash = sp_core::H256::from([69u8; 32]);
+			let mut digest = Digest::default();
+			digest.push(DigestItem::Seal([1, 2, 3, 4], vec![5, 6, 7, 8]));
+
+			let header = Header::new(
+				1,
+				H256::default(),
+				H256::default(),
+				parent_hash,
+				digest.clone(),
+			);
+
+			Executive::offchain_worker(&header);
+
+			assert_eq!(digest, System::digest());
+			assert_eq!(parent_hash, System::block_hash(0));
+			assert_eq!(header.hash(), System::block_hash(1));
+		});
+	}
 }