validate-block: Fix TrieCache implementation (paritytech#2214)

The trie cache implementation was ignoring the `storage_root` when
setting up the value cache. The problem with this is that the value
cache works using `storage_keys` and these keys are not unique across
different tries. A block can actually have different tries (main trie
and multiple child tries). This pull request fixes the issue by not
ignoring the `storage_root` and returning an unique `value_cache` per
`storage_root`. It also adds a test for the seen bug and improves
documentation that this doesn't happen again.

substrate/primitives/state-machine/src/trie_backend.rs

@@ -52,7 +52,10 @@ pub trait TrieCacheProvider<H: Hasher> {
 
 	/// Return a [`trie_db::TrieDB`] compatible cache.
 	///
-	/// The `storage_root` parameter should be the storage root of the used trie.
+	/// The `storage_root` parameter *must* be the storage root of the trie this cache is used for.
+	///
+	/// NOTE: Implementors should use the `storage_root` to differentiate between storage keys that
+	/// may belong to different tries.
 	fn as_trie_db_cache(&self, storage_root: H::Out) -> Self::Cache<'_>;
 
 	/// Returns a cache that can be used with a [`trie_db::TrieDBMut`].

substrate/test-utils/client/src/client_ext.rs

@@ -20,9 +20,11 @@
 use sc_client_api::{backend::Finalizer, client::BlockBackend};
 use sc_consensus::{BlockImport, BlockImportParams, ForkChoiceStrategy};
 use sc_service::client::Client;
-use sp_consensus::{BlockOrigin, Error as ConsensusError};
+use sp_consensus::Error as ConsensusError;
 use sp_runtime::{traits::Block as BlockT, Justification, Justifications};
 
+pub use sp_consensus::BlockOrigin;
+
 /// Extension trait for a test client.
 pub trait ClientExt<Block: BlockT>: Sized {
 	/// Finalize a block.

substrate/test-utils/client/src/lib.rs

@@ -21,7 +21,7 @@
 
 pub mod client_ext;
 
-pub use self::client_ext::{ClientBlockImportExt, ClientExt};
+pub use self::client_ext::{BlockOrigin, ClientBlockImportExt, ClientExt};
 pub use sc_client_api::{execution_extensions::ExecutionExtensions, BadBlocks, ForkBlocks};
 pub use sc_client_db::{self, Backend, BlocksPruning};
 pub use sc_executor::{self, NativeElseWasmExecutor, WasmExecutionMethod, WasmExecutor};