Fix WASM executor without instance reuse; cleanups and refactoring (#10313)

* Fix WASM executor without instance reuse; cleanups and refactoring

* Align to review comments

* Move the functions for reading/writing memory to `util.rs`

* Only `#[ignore]` the test in debug builds

* More review comments and minor extra comments

substrate/client/executor/wasmtime/src/tests.rs

@@ -310,3 +310,39 @@ fn test_max_memory_pages() {
 	)
 	.unwrap();
 }
+
+// This test takes quite a while to execute in a debug build (over 6 minutes on a TR 3970x)
+// so it's ignored by default unless it was compiled with `--release`.
+#[cfg_attr(build_type = "debug", ignore)]
+#[test]
+fn test_instances_without_reuse_are_not_leaked() {
+	use sp_wasm_interface::HostFunctions;
+
+	let runtime = crate::create_runtime(
+		RuntimeBlob::uncompress_if_needed(&wasm_binary_unwrap()[..]).unwrap(),
+		crate::Config {
+			heap_pages: 2048,
+			max_memory_size: None,
+			allow_missing_func_imports: true,
+			cache_path: None,
+			semantics: crate::Semantics {
+				fast_instance_reuse: false,
+				deterministic_stack_limit: None,
+				canonicalize_nans: false,
+				parallel_compilation: true,
+			},
+		},
+		sp_io::SubstrateHostFunctions::host_functions(),
+	)
+	.unwrap();
+
+	// As long as the `wasmtime`'s `Store` lives the instances spawned through it
+	// will live indefinitely. Currently it has a maximum limit of 10k instances,
+	// so let's spawn 10k + 1 of them to make sure our code doesn't keep the `Store`
+	// alive longer than it is necessary. (And since we disabled instance reuse
+	// a new instance will be spawned on each call.)
+	let mut instance = runtime.new_instance().unwrap();
+	for _ in 0..10001 {
+		instance.call_export("test_empty_return", &[0]).unwrap();
+	}
+}