expose offchain worker storage prefix (#3977)

* expose offchain worker storage prefix

* add docs

* move STORAGE_PREFIX to primitives

substrate/core/offchain/primitives/src/lib.rs

@@ -22,6 +22,9 @@
 use client::decl_runtime_apis;
 use sr_primitives::traits::NumberFor;
 
+/// Local Storage Prefix used by the Offchain Worker API to
+pub const STORAGE_PREFIX: &[u8] = b"storage";
+
 decl_runtime_apis! {
 	/// The offchain worker api.
 	pub trait OffchainWorkerApi {

substrate/core/offchain/src/api.rs

@@ -30,6 +30,7 @@ use primitives::offchain::{
 	Externalities as OffchainExt, HttpRequestId, Timestamp, HttpRequestStatus, HttpError,
 	OpaqueNetworkState, OpaquePeerId, OpaqueMultiaddr, StorageKind,
 };
+pub use offchain_primitives::STORAGE_PREFIX;
 use sr_primitives::{generic::BlockId, traits::{self, Extrinsic}};
 use transaction_pool::txpool::{Pool, ChainApi};
 
@@ -71,7 +72,6 @@ fn unavailable_yet<R: Default>(name: &str) -> R {
 }
 
 const LOCAL_DB: &str = "LOCAL (fork-aware) DB";
-const STORAGE_PREFIX: &[u8] = b"storage";
 
 impl<Storage, Block> OffchainExt for Api<Storage, Block>
 where

substrate/core/offchain/src/lib.rs

@@ -53,7 +53,7 @@ mod api;
 
 pub mod testing;
 
-pub use offchain_primitives::OffchainWorkerApi;
+pub use offchain_primitives::{OffchainWorkerApi, STORAGE_PREFIX};
 
 /// An offchain workers manager.
 pub struct OffchainWorkers<Client, Storage, Block: traits::Block> {