Rust Testing Deep Dive Injected Fake Called Outside Scope Sporadically

Introduction

In the realm of Rust testing, ensuring the isolation and predictability of test cases is paramount. One common technique to achieve this is through the use of dependency injection and mocking. However, when using mocking frameworks like injectorpp-for-rust, unexpected behavior can sometimes arise, leading to test failures and confusion. This article delves into a specific scenario where an injected fake is called outside of its intended scope, sporadically causing tests to fail. We'll explore the code structure, expected behavior, actual behavior, and potential causes of this issue, offering a deep dive into the intricacies of Rust testing and dependency injection.

Test Scenario

To illustrate the problem, consider the following Rust project structure:

.
├── main.rs
└── test_module
    ├── first.rs
    ├── mod.rs
    └── second.rs

This structure represents a simplified module organization where we have two modules, first and second, within a test_module directory. Each module contains a struct and associated methods, along with its own test suite. Let's examine the code within first.rs:

first.rs

pub struct First {
    a: i32,
}

impl First {
    pub fn new() -> Self {
        First { a: 0 }
    }

    pub fn get(&self) -> i32 {
        self.a
    }
}

#[cfg(test)]
mod tests {
    use injectorpp::interface::injector::*;
    use crate::test_module::first::First;

    #[test]
    fn get_call_mock() {
        let mut injector = InjectorPP::new();
        injector
            .when_called(injectorpp::func!(fn (First::get)(&First) -> i32))
            .will_execute(injectorpp::fake!(func_type: fn(_i: &First)-> i32, returns: 7));

        assert_eq!(First::new().get(), 7);
    }
}

In first.rs, we define a struct named First with a single field a of type i32. The First struct has two methods: new(), which initializes a to 0, and get(), which returns the value of a. The test module within first.rs demonstrates the use of injectorpp to mock the get() method. Specifically, the get_call_mock test creates an InjectorPP instance, configures it to intercept calls to First::get(), and injects a fake implementation that always returns 7. The assertion then verifies that calling First::new().get() indeed returns the mocked value of 7.

Now, let's examine the code within second.rs:

second.rs

use super::first::First;

pub struct Second {
    tm: First,
}

impl Second {
    pub fn new() -> Self {
        Second { tm: First::new() }
    }

    pub fn get(&self) -> i32 {
        self.tm.get()
    }
}

#[cfg(test)]
mod tests {
    use super::Second;

    #[test]
    fn get_do_not_call_mock() {
        assert_eq!(Second::new().get(), 0);
    }
}

In second.rs, we define a struct named Second that contains a field tm of type First. The Second struct also has two methods: new(), which creates a new Second instance with tm initialized to a new First instance, and get(), which calls the get() method on its internal First instance. The test module within second.rs includes a single test, get_do_not_call_mock, which creates a Second instance and asserts that calling get() on it returns 0. The expectation here is that the mock injected in first.rs should not affect the behavior of Second::get(), as it should only be scoped to the tests within first.rs.

Expected Behavior

The core expectation is that the .get() method in the test of second.rs should consistently return 0. This is because the new() method initializes the internal First instance with a set to 0, and the injected fake for First::get() is intended to be valid only within the test scope of first.rs. The test in second.rs should therefore operate with the original, unmocked behavior of First::get().

Actual Behavior

However, the actual behavior deviates from this expectation. When running cargo test repeatedly using a loop like:

for i in $(seq 0 300); do cargo test; done

a sporadic test failure occurs in second.rs. The output shows:

running 2 tests
test test_module::first::tests::get_call_mock ... ok
test test_module::second::tests::get_do_not_call_mock ... FAILED

failures:

---- test_module::second::tests::get_do_not_call_mock stdout ----
thread 'test_module::second::tests::get_do_not_call_mock' panicked at src/test_module/second.rs:23:9:
assertion `left == right` failed
  left: 7
 right: 0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test_module::second::tests::get_do_not_call_mock

test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

This output reveals that the get_do_not_call_mock test in second.rs fails because it unexpectedly receives a value of 7 from Second::new().get(), which contradicts the expected value of 0. This indicates that the injected fake for First::get() in first.rs is somehow being applied in the context of second.rs, leading to the test failure. The sporadic nature of the failure suggests a potential issue with test isolation or shared state between tests.

Analysis of the Issue

The core problem lies in the fact that the injected fake for First::get() is called outside the scope (test) it was defined in. This violates the principle of test isolation, where each test should operate independently without interference from other tests. The sporadic nature of the failure suggests that it's not a consistent, deterministic issue but rather one that depends on factors like test execution order or shared mutable state.

Several potential factors could contribute to this behavior:

1. InjectorPP's Scope Management: The mocking framework injectorpp might have a scoping mechanism that isn't strictly limited to individual tests. If the injector instance or the injected fakes are not properly isolated between tests, a mock defined in one test could inadvertently affect other tests.
2. Shared Global State: While Rust promotes memory safety and prevents data races, there might be shared global state within injectorpp or the underlying testing framework that's not being properly reset between tests. This could lead to a mock being inadvertently persisted across test boundaries.
3. Test Execution Order: The order in which tests are executed can sometimes influence the outcome, especially if there are side effects or shared resources involved. If the first.rs tests are executed before second.rs tests, the mock might linger and affect the subsequent tests.
4. Concurrency Issues: If the tests are run in parallel, there could be race conditions or other concurrency issues within injectorpp that lead to mocks being applied in the wrong context.

Potential Solutions and Mitigation Strategies

To address this issue and ensure proper test isolation, several strategies can be employed:

1. Scoped Injector Instances: Ensure that each test creates its own isolated instance of InjectorPP. This prevents mocks defined in one test from affecting others. Instead of using a global or shared injector, create a new injector within each test function.
2. Explicit Mock Cleanup: After each test, explicitly clear or reset the mocks defined within injectorpp. This can help prevent mocks from leaking into subsequent tests. If injectorpp provides a mechanism for clearing mocks, use it at the end of each test.
3. Test Ordering Control: If test execution order is suspected to be a factor, consider using mechanisms to control the order in which tests are run. Some testing frameworks allow you to specify dependencies or ordering constraints between tests. However, relying on test order should be a last resort, as it can make tests more brittle and harder to maintain.
4. Review InjectorPP's Documentation and Implementation: Carefully review the documentation and implementation of injectorpp to understand its scoping rules and mock management mechanisms. Look for any configuration options or best practices that promote test isolation. If necessary, consider contributing to the project or reporting a bug if you identify an issue in the framework itself.
5. Disable Parallel Test Execution: If concurrency is suspected to be a factor, try running tests sequentially to see if the issue disappears. If sequential execution resolves the problem, it suggests a potential race condition or other concurrency issue within the mocking framework or the test setup.
6. Dependency Injection Best Practices: Adhere to dependency injection best practices to make your code more testable and maintainable. Design your code so that dependencies are explicitly passed in rather than being hardcoded or accessed through global state. This makes it easier to mock dependencies and isolate units of code for testing.

Conclusion

The issue of injected fakes being called outside their intended scope highlights the importance of test isolation and the potential pitfalls of mocking frameworks. By understanding the code structure, expected behavior, and actual behavior, we can identify potential causes and implement appropriate solutions. Ensuring proper scoping of mocks, managing shared state, and adhering to dependency injection best practices are crucial for writing reliable and maintainable tests in Rust. This deep dive into a specific testing scenario provides valuable insights into the complexities of Rust testing and the importance of careful design and implementation when using mocking frameworks.