feat/offchain: add remove interface method

client/db/src/offchain.rs

@@ -65,6 +65,14 @@ impl sp_core::offchain::OffchainStorage for LocalStorage {
 		self.db.commit(tx);
 	}
 
+	fn remove(&mut self, prefix: &[u8], key: &[u8]) {
+		let key: Vec<u8> = prefix.iter().chain(key).cloned().collect();
+		let mut tx = Transaction::new();
+		tx.remove(columns::OFFCHAIN, &key);
+
+		self.db.commit(tx);
+	}
+
 	fn get(&self, prefix: &[u8], key: &[u8]) -> Option<Vec<u8>> {
 		let key: Vec<u8> = prefix.iter().chain(key).cloned().collect();
 		self.db.get(columns::OFFCHAIN, &key)

client/offchain/src/api.rs

@@ -100,6 +100,13 @@ impl<Storage: OffchainStorage> OffchainExt for Api<Storage> {
 		}
 	}
 
+	fn local_storage_remove(&mut self, kind: StorageKind, key: &[u8]) {
+		match kind {
+			StorageKind::PERSISTENT => self.db.remove(STORAGE_PREFIX, key),
+			StorageKind::LOCAL => unavailable_yet(LOCAL_DB),
+		}
+	}
+
 	fn local_storage_compare_and_set(
 		&mut self,
 		kind: StorageKind,

primitives/core/src/offchain/mod.rs

@@ -36,6 +36,9 @@ pub trait OffchainStorage: Clone + Send + Sync {
 	/// Persist a value in storage under given key and prefix.
 	fn set(&mut self, prefix: &[u8], key: &[u8], value: &[u8]);
 
+	/// Remove a perviously pin storage ersisted value under given key and prefix.
+	fn remove(&mut self, prefix: &[u8], key: &[u8]);
+
 	/// Retrieve a value from storage under given key and prefix.
 	fn get(&self, prefix: &[u8], key: &[u8]) -> Option<Vec<u8>>;
 
@@ -347,6 +350,13 @@ pub trait Externalities: Send {
 	/// Note this storage is not part of the consensus, it's only accessible by
 	/// offchain worker tasks running on the same machine. It IS persisted between runs.
 	fn local_storage_set(&mut self, kind: StorageKind, key: &[u8], value: &[u8]);
+
+	/// Removes a value in the local storage.
+	///
+	/// Note this storage is not part of the consensus, it's only accessible by
+	/// offchain worker tasks running on the same machine. It IS persisted between runs.
+	fn local_storage_remove(&mut self, kind: StorageKind, key: &[u8]);
+
 	/// Sets a value in the local storage if it matches current value.
 	///
 	/// Since multiple offchain workers may be running concurrently, to prevent
@@ -511,6 +521,10 @@ impl<T: Externalities + ?Sized> Externalities for Box<T> {
 		(&mut **self).local_storage_set(kind, key, value)
 	}
 
+	fn local_storage_remove(&mut self, kind: StorageKind, key: &[u8]) {
+		(&mut **self).local_storage_remove(kind, key)
+	}
+
 	fn local_storage_compare_and_set(
 		&mut self,
 		kind: StorageKind,
@@ -616,6 +630,11 @@ impl<T: Externalities> Externalities for LimitedExternalities<T> {
 		self.externalities.local_storage_set(kind, key, value)
 	}
 
+	fn local_storage_remove(&mut self, kind: StorageKind, key: &[u8]) {
+		self.check(Capability::OffchainWorkerDbWrite, "local_storage_remove");
+		self.externalities.local_storage_remove(kind, key)
+	}
+
 	fn local_storage_compare_and_set(
 		&mut self,
 		kind: StorageKind,

primitives/core/src/offchain/storage.rs

@@ -50,6 +50,11 @@ impl OffchainStorage for InMemOffchainStorage {
 		self.storage.insert(key, value.to_vec());
 	}
 
+	fn remove(&mut self, prefix: &[u8], key: &[u8]) {
+		let key: Vec<u8> = prefix.iter().chain(key).cloned().collect();
+		self.storage.remove(&key);
+	}
+
 	fn get(&self, prefix: &[u8], key: &[u8]) -> Option<Vec<u8>> {
 		let key: Vec<u8> = prefix.iter().chain(key).cloned().collect();
 		self.storage.get(&key).cloned()

primitives/core/src/offchain/testing.rs

@@ -178,6 +178,14 @@ impl offchain::Externalities for TestOffchainExt {
 		}.set(b"", key, value);
 	}
 
+	fn local_storage_remove(&mut self, kind: StorageKind, key: &[u8]) {
+		let mut state = self.0.write();
+		match kind {
+			StorageKind::LOCAL => &mut state.local_storage,
+			StorageKind::PERSISTENT => &mut state.persistent_storage,
+		}.remove(b"", key);
+	}
+
 	fn local_storage_compare_and_set(
 		&mut self,
 		kind: StorageKind,

primitives/io/src/lib.rs

@@ -711,6 +711,16 @@ pub trait Offchain {
 			.local_storage_set(kind, key, value)
 	}
 
+	/// Remove a value from the local storage.
+	///
+	/// Note this storage is not part of the consensus, it's only accessible by
+	/// offchain worker tasks running on the same machine. It IS persisted between runs.
+	fn local_storage_remove(&mut self, kind: StorageKind, key: &[u8]) {
+		self.extension::<OffchainExt>()
+			.expect("local_storage_remove can be called only in the offchain worker context")
+			.local_storage_remove(kind, key)
+	}
+
 	/// Sets a value in the local storage if it matches current value.
 	///
 	/// Since multiple offchain workers may be running concurrently, to prevent

primitives/session/src/offchain_hashdb.rs

@@ -90,17 +90,20 @@ L: TrieLayout + Send,
             let tree_prefix = tree_prefix.as_slice();
 
             self.get_index(session_index)
-            .into_iter()
-            .filter_map(move |(derived_key, prefix)| {
-                if tree_prefix == prefix.as_slice() {
-                    // @todo FIXME erase
-                    offchain::local_storage_set(StorageKind::PERSISTENT, derived_key.as_slice(), &[]);
-                    None
-                } else {
-                    Some((derived_key, prefix))
-                }
-            })
-            .collect::<BTreeSet<_>>()
+                .into_iter()
+                .filter_map(move |(derived_key, prefix)| {
+                    if tree_prefix == prefix.as_slice() {
+                        // @todo FIXME erase
+                        offchain::local_storage_set(
+                            StorageKind::PERSISTENT,
+                            derived_key.as_slice(),
+                            &[]);
+                        None
+                    } else {
+                        Some((derived_key, prefix))
+                    }
+                })
+                .collect::<BTreeSet<_>>()
 
         } else {