Minor: Update output validity tests (paritytech#13190)

* Minor: Update output validity tests

Quick follow-up to paritytech#13183.

Mainly, I wanted to double check that the `test_return_max_memory_offset` test doesn't pass just
because the output length is 0.

I also:

- Organized these tests into a module.
- Added a comment explaining why we don't use the `wasm_export_functions` macro.

* Update test based on review comment

client/executor/runtime-test/src/lib.rs

@@ -346,33 +346,50 @@ sp_core::wasm_export_functions! {
 	}
 }
 
-// Returns a huge len. It should result in an error, and not an allocation.
-#[no_mangle]
-#[cfg(not(feature = "std"))]
-pub extern "C" fn test_return_huge_len(_params: *const u8, _len: usize) -> u64 {
-	pack_ptr_and_len(0, u32::MAX)
-}
-
-// Returns an offset right at the edge of the wasm memory boundary. With length 0, it should
-// succeed.
-#[no_mangle]
-#[cfg(not(feature = "std"))]
-pub extern "C" fn test_return_max_memory_offset(_params: *const u8, _len: usize) -> u64 {
-	pack_ptr_and_len((core::arch::wasm32::memory_size(0) * WASM_PAGE_SIZE) as u32, 0)
-}
+// Tests that check output validity. We explicitly return the ptr and len, so we avoid using the
+// `wasm_export_functions` macro.
+mod output_validity {
+	#[cfg(not(feature = "std"))]
+	use super::WASM_PAGE_SIZE;
+
+	#[cfg(not(feature = "std"))]
+	use sp_runtime_interface::pack_ptr_and_len;
+
+	// Returns a huge len. It should result in an error, and not an allocation.
+	#[no_mangle]
+	#[cfg(not(feature = "std"))]
+	pub extern "C" fn test_return_huge_len(_params: *const u8, _len: usize) -> u64 {
+		pack_ptr_and_len(0, u32::MAX)
+	}
+
+	// Returns an offset right before the edge of the wasm memory boundary. It should succeed.
+	#[no_mangle]
+	#[cfg(not(feature = "std"))]
+	pub extern "C" fn test_return_max_memory_offset(_params: *const u8, _len: usize) -> u64 {
+		let output_ptr = (core::arch::wasm32::memory_size(0) * WASM_PAGE_SIZE) as u32 - 1;
+		let ptr = output_ptr as *mut u8;
+		unsafe {
+			ptr.write(u8::MAX);
+		}
+		pack_ptr_and_len(output_ptr, 1)
+	}
 
-// Returns an offset right at the edge of the wasm memory boundary. With length 1, it should fail.
-#[no_mangle]
-#[cfg(not(feature = "std"))]
-pub extern "C" fn test_return_max_memory_offset_plus_one(_params: *const u8, _len: usize) -> u64 {
-	pack_ptr_and_len((core::arch::wasm32::memory_size(0) * WASM_PAGE_SIZE) as u32, 1)
-}
+	// Returns an offset right after the edge of the wasm memory boundary. It should fail.
+	#[no_mangle]
+	#[cfg(not(feature = "std"))]
+	pub extern "C" fn test_return_max_memory_offset_plus_one(
+		_params: *const u8,
+		_len: usize,
+	) -> u64 {
+		pack_ptr_and_len((core::arch::wasm32::memory_size(0) * WASM_PAGE_SIZE) as u32, 1)
+	}
 
-// Returns an output that overflows the u32 range. It should result in an error.
-#[no_mangle]
-#[cfg(not(feature = "std"))]
-pub extern "C" fn test_return_overflow(_params: *const u8, _len: usize) -> u64 {
-	pack_ptr_and_len(u32::MAX, 1)
+	// Returns an output that overflows the u32 range. It should result in an error.
+	#[no_mangle]
+	#[cfg(not(feature = "std"))]
+	pub extern "C" fn test_return_overflow(_params: *const u8, _len: usize) -> u64 {
+		pack_ptr_and_len(u32::MAX, 1)
+	}
 }
 
 /// A macro to define a test entrypoint for each available sandbox executor.

client/executor/src/integration_tests/mod.rs

@@ -917,7 +917,7 @@ fn return_max_memory_offset(wasm_method: WasmExecutionMethod) {
 
 	assert_eq!(
 		call_in_wasm("test_return_max_memory_offset", &[], wasm_method, &mut ext).unwrap(),
-		().encode()
+		(u8::MAX).encode()
 	);
 }