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

* Clearer API to code against.

substrate/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