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May 20, 2022

Things I learn while testing files in Rust

Last updated at: Aug 18, 2022

Dealing with files while running tests in Rust can be tricky, namely running the tests concurrently and dealing with file clean up when a test fail or panic halfway.

Concurrency

By default, cargo test run your Rust tests in multiple threads. Depending on your tests, if you’re using the same filename in multiple test cases, it might caused unexpected failures.

The easiest way to overcome this is to limit cargo test to run only on a single thread:

cargo test -- --test-threads=1

However, this will caused your cargo test to takes longer time to complete. To overcome this, use a different filename in each of your test cases. It can be done dynamically in a thread safe manner utilizing the current thread information:

filename = format!("file-{:?}", std::thread::current().id());
// file-ThreadId(1)

At the time of writing, the ThreadId can only be typecast to u64 with as_u64() in the nightly version.

File clean up

In general, file clean up can be achieve by calling std::fs::remove_file at the end of each test case:

let _ = std::fs::remove_file(format!("test-{:?}.db", std::thread::current().id()));

However, there will be sometime where you’re working on the correct implementation and running the tests might caused panic and thus not calling the file clean up code as expected.

We can overcome this by setting up a custom panic hook with std::panic::set_hook:

std::panic::set_hook(Box::new(|p| {
  let _ = std::fs::remove_file(format!("test-{:?}.db", std::thread::current().id()));
  println!("{p}");
}));

We are calling println!("{p}") again to mimic the default panic behaviour. I learn about this thanks to this topic in Rust forum.

(230 words)