Add get_global for Sandbox (#4756)

* Add `get_global` for `Sandbox`

This pr adds `get_global` to retrieve a `global` variable from an
instantiated sandbox wasm blob.

* Bump `spec_version`

* Update primitives/wasm-interface/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* `get_global` -> `get_global_val`

Co-authored-by: Sergei Pepyakin <s.pepyakin@gmail.com>
Co-authored-by: Gavin Wood <github@gavwood.com>

substrate/client/executor/common/src/sandbox.rs

@@ -224,7 +224,8 @@ fn trap(msg: &'static str) -> Trap {
 }
 
 fn deserialize_result(serialized_result: &[u8]) -> std::result::Result<Option<RuntimeValue>, Trap> {
-	use self::sandbox_primitives::{HostError, ReturnValue};
+	use self::sandbox_primitives::HostError;
+	use sp_wasm_interface::ReturnValue;
 	let result_val = std::result::Result::<ReturnValue, HostError>::decode(&mut &serialized_result[..])
 		.map_err(|_| trap("Decoding Result<ReturnValue, HostError> failed!"))?;
 
@@ -260,7 +261,7 @@ impl<'a, FE: SandboxCapabilities + 'a> Externals for GuestExternals<'a, FE> {
 		let invoke_args_data: Vec<u8> = args.as_ref()
 			.iter()
 			.cloned()
-			.map(sandbox_primitives::TypedValue::from)
+			.map(sp_wasm_interface::Value::from)
 			.collect::<Vec<_>>()
 			.encode();
 
@@ -362,6 +363,18 @@ impl<FR> SandboxInstance<FR> {
 			},
 		)
 	}
+
+	/// Get the value from a global with the given `name`.
+	///
+	/// Returns `Some(_)` if the global could be found.
+	pub fn get_global_val(&self, name: &str) -> Option<sp_wasm_interface::Value> {
+		let global = self.instance
+			.export_by_name(name)?
+			.as_global()?
+			.get();
+
+		Some(global.into())
+	}
 }
 
 /// Error occurred during instantiation of a sandboxed module.