Result<Option<>> rather than Option<Option<>> (#9119)

* Clearer API to code against.

frame/election-provider-multi-phase/src/unsigned.rs

@@ -29,7 +29,10 @@ use sp_npos_elections::{
 	CompactSolution, ElectionResult, assignment_ratio_to_staked_normalized,
 	assignment_staked_to_ratio_normalized, is_score_better, seq_phragmen,
 };
-use sp_runtime::{offchain::storage::StorageValueRef, traits::TrailingZeroInput, SaturatedConversion};
+use sp_runtime::{
+	offchain::storage::{MutateStorageError, StorageValueRef},
+	traits::TrailingZeroInput, SaturatedConversion
+};
 use sp_std::{cmp::Ordering, convert::TryFrom, vec::Vec};
 
 /// Storage key used to store the last block number at which offchain worker ran.
@@ -98,9 +101,9 @@ fn save_solution<T: Config>(call: &Call<T>) -> Result<(), MinerError> {
 	log!(debug, "saving a call to the offchain storage.");
 	let storage = StorageValueRef::persistent(&OFFCHAIN_CACHED_CALL);
 	match storage.mutate::<_, (), _>(|_| Ok(call.clone())) {
-		Ok(Ok(_)) => Ok(()),
-		Ok(Err(_)) => Err(MinerError::FailedToStoreSolution),
-		Err(_) => {
+		Ok(_) => Ok(()),
+		Err(MutateStorageError::ConcurrentModification(_)) => Err(MinerError::FailedToStoreSolution),
+		Err(MutateStorageError::ValueFunctionFailed(_)) => {
 			// this branch should be unreachable according to the definition of
 			// `StorageValueRef::mutate`: that function should only ever `Err` if the closure we
 			// pass it returns an error. however, for safety in case the definition changes, we do
@@ -114,6 +117,7 @@ fn save_solution<T: Config>(call: &Call<T>) -> Result<(), MinerError> {
 fn restore_solution<T: Config>() -> Result<Call<T>, MinerError> {
 	StorageValueRef::persistent(&OFFCHAIN_CACHED_CALL)
 		.get()
+		.ok()
 		.flatten()
 		.ok_or(MinerError::NoStoredSolution)
 }
@@ -135,12 +139,9 @@ fn clear_offchain_repeat_frequency() {
 }
 
 /// `true` when OCW storage contains a solution
-///
-/// More precise than `restore_solution::<T>().is_ok()`; that invocation will return `false`
-/// if a solution exists but cannot be decoded, whereas this just checks whether an item is present.
 #[cfg(test)]
 fn ocw_solution_exists<T: Config>() -> bool {
-	StorageValueRef::persistent(&OFFCHAIN_CACHED_CALL).get::<Call<T>>().is_some()
+	matches!(StorageValueRef::persistent(&OFFCHAIN_CACHED_CALL).get::<Call<T>>(), Ok(Some(_)))
 }
 
 impl<T: Config> Pallet<T> {
@@ -584,13 +585,13 @@ impl<T: Config> Pallet<T> {
 		let last_block = StorageValueRef::persistent(&OFFCHAIN_LAST_BLOCK);
 
 		let mutate_stat = last_block.mutate::<_, &'static str, _>(
-			|maybe_head: Option<Option<T::BlockNumber>>| {
+			|maybe_head: Result<Option<T::BlockNumber>, _>| {
 				match maybe_head {
-					Some(Some(head)) if now < head => Err("fork."),
-					Some(Some(head)) if now >= head && now <= head + threshold => {
+					Ok(Some(head)) if now < head => Err("fork."),
+					Ok(Some(head)) if now >= head && now <= head + threshold => {
 						Err("recently executed.")
 					}
-					Some(Some(head)) if now > head + threshold => {
+					Ok(Some(head)) if now > head + threshold => {
 						// we can run again now. Write the new head.
 						Ok(now)
 					}
@@ -604,11 +605,12 @@ impl<T: Config> Pallet<T> {
 
 		match mutate_stat {
 			// all good
-			Ok(Ok(_)) => Ok(()),
+			Ok(_) => Ok(()),
 			// failed to write.
-			Ok(Err(_)) => Err(MinerError::Lock("failed to write to offchain db.")),
+			Err(MutateStorageError::ConcurrentModification(_)) =>
+				Err(MinerError::Lock("failed to write to offchain db (concurrent modification).")),
 			// fork etc.
-			Err(why) => Err(MinerError::Lock(why)),
+			Err(MutateStorageError::ValueFunctionFailed(why)) => Err(MinerError::Lock(why)),
 		}
 	}
 
@@ -1117,15 +1119,15 @@ mod tests {
 			assert!(MultiPhase::current_phase().is_unsigned());
 
 			// initially, the lock is not set.
-			assert!(guard.get::<bool>().is_none());
+			assert!(guard.get::<bool>().unwrap().is_none());
 
 			// a successful a-z execution.
 			MultiPhase::offchain_worker(25);
 			assert_eq!(pool.read().transactions.len(), 1);
 
 			// afterwards, the lock is not set either..
-			assert!(guard.get::<bool>().is_none());
-			assert_eq!(last_block.get::<BlockNumber>().unwrap().unwrap(), 25);
+			assert!(guard.get::<bool>().unwrap().is_none());
+			assert_eq!(last_block.get::<BlockNumber>().unwrap(), Some(25));
 		});
 	}
 
@@ -1280,7 +1282,7 @@ mod tests {
 			// this ensures that when the resubmit window rolls around, we're ready to regenerate
 			// from scratch if necessary
 			let mut call_cache = StorageValueRef::persistent(&OFFCHAIN_CACHED_CALL);
-			assert!(matches!(call_cache.get::<Call<Runtime>>(), Some(Some(_call))));
+			assert!(matches!(call_cache.get::<Call<Runtime>>(), Ok(Some(_call))));
 			call_cache.clear();
 
 			// attempts to resubmit the tx after the threshold has expired

frame/example-offchain-worker/src/lib.rs

@@ -53,7 +53,7 @@ use frame_support::traits::Get;
 use sp_core::crypto::KeyTypeId;
 use sp_runtime::{
 	RuntimeDebug,
-	offchain::{http, Duration, storage::StorageValueRef},
+	offchain::{http, Duration, storage::{MutateStorageError, StorageRetrievalError, StorageValueRef}},
 	traits::Zero,
 	transaction_validity::{InvalidTransaction, ValidTransaction, TransactionValidity},
 };
@@ -366,15 +366,11 @@ impl<T: Config> Pallet<T> {
 		// low-level method of local storage API, which means that only one worker
 		// will be able to "acquire a lock" and send a transaction if multiple workers
 		// happen to be executed concurrently.
-		let res = val.mutate(|last_send: Option<Option<T::BlockNumber>>| {
-			// We match on the value decoded from the storage. The first `Option`
-			// indicates if the value was present in the storage at all,
-			// the second (inner) `Option` indicates if the value was succesfuly
-			// decoded to expected type (`T::BlockNumber` in our case).
+		let res = val.mutate(|last_send: Result<Option<T::BlockNumber>, StorageRetrievalError>| {
 			match last_send {
 				// If we already have a value in storage and the block number is recent enough
 				// we avoid sending another transaction at this time.
-				Some(Some(block)) if block_number < block + T::GracePeriod::get() => {
+				Ok(Some(block)) if block_number < block + T::GracePeriod::get() => {
 					Err(RECENTLY_SENT)
 				},
 				// In every other case we attempt to acquire the lock and send a transaction.
@@ -390,7 +386,7 @@ impl<T: Config> Pallet<T> {
 		// written to in the meantime.
 		match res {
 			// The value has been set correctly, which means we can safely send a transaction now.
-			Ok(Ok(block_number)) => {
+			Ok(block_number) => {
 				// Depending if the block is even or odd we will send a `Signed` or `Unsigned`
 				// transaction.
 				// Note that this logic doesn't really guarantee that the transactions will be sent
@@ -406,13 +402,13 @@ impl<T: Config> Pallet<T> {
 				else { TransactionType::Raw }
 			},
 			// We are in the grace period, we should not send a transaction this time.
-			Err(RECENTLY_SENT) => TransactionType::None,
+			Err(MutateStorageError::ValueFunctionFailed(RECENTLY_SENT)) => TransactionType::None,
 			// We wanted to send a transaction, but failed to write the block number (acquire a
 			// lock). This indicates that another offchain worker that was running concurrently
 			// most likely executed the same logic and succeeded at writing to storage.
 			// Thus we don't really want to send the transaction, knowing that the other run
 			// already did.
-			Ok(Err(_)) => TransactionType::None,
+			Err(MutateStorageError::ConcurrentModification(_)) => TransactionType::None,
 		}
 	}
 

frame/im-online/src/lib.rs

@@ -80,7 +80,7 @@ use sp_core::offchain::OpaqueNetworkState;
 use sp_std::prelude::*;
 use sp_std::convert::TryInto;
 use sp_runtime::{
-	offchain::storage::StorageValueRef,
+	offchain::storage::{MutateStorageError, StorageRetrievalError, StorageValueRef},
 	traits::{AtLeast32BitUnsigned, Convert, Saturating, TrailingZeroInput},
 	Perbill, Permill, PerThing, RuntimeDebug, SaturatedConversion,
 };
@@ -719,14 +719,15 @@ impl<T: Config> Pallet<T> {
 			key
 		};
 		let storage = StorageValueRef::persistent(&key);
-		let res = storage.mutate(|status: Option<Option<HeartbeatStatus<T::BlockNumber>>>| {
+		let res = storage.mutate(
+			|status: Result<Option<HeartbeatStatus<T::BlockNumber>>, StorageRetrievalError>| {
 			// Check if there is already a lock for that particular block.
 			// This means that the heartbeat has already been sent, and we are just waiting
 			// for it to be included. However if it doesn't get included for INCLUDE_THRESHOLD
 			// we will re-send it.
 			match status {
 				// we are still waiting for inclusion.
-				Some(Some(status)) if status.is_recent(session_index, now) => {
+				Ok(Some(status)) if status.is_recent(session_index, now) => {
 					Err(OffchainErr::WaitingForInclusion(status.sent_at))
 				},
 				// attempt to set new status
@@ -735,7 +736,10 @@ impl<T: Config> Pallet<T> {
 					sent_at: now,
 				}),
 			}
-		})?;
+		});
+		if let Err(MutateStorageError::ValueFunctionFailed(err)) = res {
+			return Err(err);
+		}
 
 		let mut new_status = res.map_err(|_| OffchainErr::FailedToAcquireLock)?;
 

frame/session/src/historical/offchain.rs

@@ -25,7 +25,10 @@
 //! This is used in conjunction with [`ProvingTrie`](super::ProvingTrie) and
 //! the off-chain indexing API.
 
-use sp_runtime::{offchain::storage::StorageValueRef, KeyTypeId};
+use sp_runtime::{
+	offchain::storage::{MutateStorageError, StorageRetrievalError, StorageValueRef},
+	KeyTypeId
+};
 use sp_session::MembershipProof;
 
 use super::super::{Pallet as SessionModule, SessionIndex};
@@ -49,6 +52,7 @@ impl<T: Config> ValidatorSet<T> {
 		let derived_key = shared::derive_key(shared::PREFIX, session_index);
 		StorageValueRef::persistent(derived_key.as_ref())
 			.get::<Vec<(T::ValidatorId, T::FullIdentification)>>()
+			.ok()
 			.flatten()
 			.map(|validator_set| Self { validator_set })
 	}
@@ -100,19 +104,19 @@ pub fn prove_session_membership<T: Config, D: AsRef<[u8]>>(
 pub fn prune_older_than<T: Config>(first_to_keep: SessionIndex) {
 	let derived_key = shared::LAST_PRUNE.to_vec();
 	let entry = StorageValueRef::persistent(derived_key.as_ref());
-	match entry.mutate(|current: Option<Option<SessionIndex>>| -> Result<_, ()> {
+	match entry.mutate(|current: Result<Option<SessionIndex>, StorageRetrievalError>| -> Result<_, ()> {
 		match current {
-			Some(Some(current)) if current < first_to_keep => Ok(first_to_keep),
+			Ok(Some(current)) if current < first_to_keep => Ok(first_to_keep),
 			// do not move the cursor, if the new one would be behind ours
-			Some(Some(current)) => Ok(current),
-			None => Ok(first_to_keep),
+			Ok(Some(current)) => Ok(current),
+			Ok(None) => Ok(first_to_keep),
 			// if the storage contains undecodable data, overwrite with current anyways
 			// which might leak some entries being never purged, but that is acceptable
 			// in this context
-			Some(None) => Ok(first_to_keep),
+			Err(_) => Ok(first_to_keep),
 		}
 	}) {
-		Ok(Ok(new_value)) => {
+		Ok(new_value) => {
 			// on a re-org this is not necessarily true, with the above they might be equal
 			if new_value < first_to_keep {
 				for session_index in new_value..first_to_keep {
@@ -121,8 +125,8 @@ pub fn prune_older_than<T: Config>(first_to_keep: SessionIndex) {
 				}
 			}
 		}
-		Ok(Err(_)) => {} // failed to store the value calculated with the given closure
-		Err(_) => {}     // failed to calculate the value to store with the given closure
+		Err(MutateStorageError::ConcurrentModification(_)) => {}
+		Err(MutateStorageError::ValueFunctionFailed(_)) => {}
 	}
 }
 

primitives/runtime/src/offchain/storage.rs

@@ -28,6 +28,25 @@ pub struct StorageValueRef<'a> {
 	kind: StorageKind,
 }
 
+/// Reason for not being able to provide the stored value
+#[derive(Debug, PartialEq, Eq)]
+pub enum StorageRetrievalError {
+	/// Value found but undecodable
+	Undecodable,
+}
+
+/// Possible errors when mutating a storage value.
+#[derive(Debug, PartialEq, Eq)]
+pub enum MutateStorageError<T, E> {
+	/// The underlying db failed to update due to a concurrent modification.
+	/// Contains the new value that was not stored.
+	ConcurrentModification(T),
+	/// The function given to us to create the value to be stored failed.
+	/// May be used to signal that having looked at the existing value,
+	/// they don't want to mutate it.
+	ValueFunctionFailed(E)
+}
+
 impl<'a> StorageValueRef<'a> {
 	/// Create a new reference to a value in the persistent local storage.
 	pub fn persistent(key: &'a [u8]) -> Self {
@@ -58,30 +77,40 @@ impl<'a> StorageValueRef<'a> {
 	/// Retrieve & decode the value from storage.
 	///
 	/// Note that if you want to do some checks based on the value
-	/// and write changes after that you should rather be using `mutate`.
+	/// and write changes after that, you should rather be using `mutate`.
 	///
-	/// The function returns `None` if the value was not found in storage,
-	/// otherwise a decoding of the value to requested type.
-	pub fn get<T: codec::Decode>(&self) -> Option<Option<T>> {
+	/// Returns the value if stored.
+	/// Returns an error if the value could not be decoded.
+	pub fn get<T: codec::Decode>(&self) -> Result<Option<T>, StorageRetrievalError> {
 		sp_io::offchain::local_storage_get(self.kind, self.key)
-			.map(|val| T::decode(&mut &*val).ok())
+			.map(|val| T::decode(&mut &*val)
+			.map_err(|_| StorageRetrievalError::Undecodable))
+			.transpose()
 	}
 
-	/// Retrieve & decode the value and set it to a new one atomically.
+	/// Retrieve & decode the current value and set it to a new value atomically.
+	///
+	/// Function `mutate_val` takes as input the current value and should
+	/// return a new value that is attempted to be written to storage.
 	///
-	/// Function `f` should return a new value that we should attempt to write to storage.
 	/// This function returns:
-	/// 1. `Ok(Ok(T))` in case the value has been successfully set.
-	/// 2. `Ok(Err(T))` in case the value was calculated by the passed closure `f`,
-	///    but it could not be stored.
-	/// 3. `Err(_)` in case `f` returns an error.
-	pub fn mutate<T, E, F>(&self, f: F) -> Result<Result<T, T>, E> where
+	/// 1. `Ok(T)` in case the value has been successfully set.
+	/// 2. `Err(MutateStorageError::ConcurrentModification(T))` in case the value was calculated
+	/// by the passed closure `mutate_val`, but it could not be stored.
+	/// 3. `Err(MutateStorageError::ValueFunctionFailed(_))` in case `mutate_val` returns an error.
+	pub fn mutate<T, E, F>(&self, mutate_val: F) -> Result<T, MutateStorageError<T,E>> where
 		T: codec::Codec,
-		F: FnOnce(Option<Option<T>>) -> Result<T, E>
+		F: FnOnce(Result<Option<T>, StorageRetrievalError>) -> Result<T, E>
 	{
 		let value = sp_io::offchain::local_storage_get(self.kind, self.key);
-		let decoded = value.as_deref().map(|mut v| T::decode(&mut v).ok());
-		let val = f(decoded)?;
+		let decoded = value.as_deref()
+			.map(|mut bytes| {
+				T::decode(&mut bytes)
+					.map_err(|_| StorageRetrievalError::Undecodable)
+			}).transpose();
+
+		let val = mutate_val(decoded).map_err(|err| MutateStorageError::ValueFunctionFailed(err))?;
+
 		let set = val.using_encoded(|new_val| {
 			sp_io::offchain::local_storage_compare_and_set(
 				self.kind,
@@ -90,11 +119,10 @@ impl<'a> StorageValueRef<'a> {
 				new_val,
 			)
 		});
-
 		if set {
-			Ok(Ok(val))
+			Ok(val)
 		} else {
-			Ok(Err(val))
+			Err(MutateStorageError::ConcurrentModification(val))
 		}
 	}
 }
@@ -117,12 +145,12 @@ mod tests {
 		t.execute_with(|| {
 			let val = StorageValue::persistent(b"testval");
 
-			assert_eq!(val.get::<u32>(), None);
+			assert_eq!(val.get::<u32>(), Ok(None));
 
 			val.set(&15_u32);
 
-			assert_eq!(val.get::<u32>(), Some(Some(15_u32)));
-			assert_eq!(val.get::<Vec<u8>>(), Some(None));
+			assert_eq!(val.get::<u32>(), Ok(Some(15_u32)));
+			assert_eq!(val.get::<Vec<u8>>(), Err(StorageRetrievalError::Undecodable));
 			assert_eq!(
 				state.read().persistent_storage.get(b"testval"),
 				Some(vec![15_u8, 0, 0, 0])
@@ -140,23 +168,23 @@ mod tests {
 			let val = StorageValue::persistent(b"testval");
 
 			let result = val.mutate::<u32, (), _>(|val| {
-				assert_eq!(val, None);
+				assert_eq!(val, Ok(None));
 
 				Ok(16_u32)
 			});
-			assert_eq!(result, Ok(Ok(16_u32)));
-			assert_eq!(val.get::<u32>(), Some(Some(16_u32)));
+			assert_eq!(result, Ok(16_u32));
+			assert_eq!(val.get::<u32>(), Ok(Some(16_u32)));
 			assert_eq!(
 				state.read().persistent_storage.get(b"testval"),
 				Some(vec![16_u8, 0, 0, 0])
 			);
 
 			// mutate again, but this time early-exit.
 			let res = val.mutate::<u32, (), _>(|val| {
-				assert_eq!(val, Some(Some(16_u32)));
+				assert_eq!(val, Ok(Some(16_u32)));
 				Err(())
 			});
-			assert_eq!(res, Err(()));
+			assert_eq!(res, Err(MutateStorageError::ValueFunctionFailed(())));
 		})
 	}
 }