When writing integration tests for my Rustnish reverse proxy project I have hard-coded port numbers in tests. This is not ideal because it is hard to keep track of which port numbers have already been used and which ones are available when writing a new test. Because Rust's test runner executes test cases in parallel it is important to coordinate which test uses which ports so that there are no clashes that break the tests.

One obvious solution to this problem would be to disable parallel test execution with cargo test -- --test-threads=1. But we want to cover program and test isolation with our test so this is not really an option.

A naive try

The basic idea is to have a function get_free_port() that hands out port numbers incrementally and is called by tests:

pub fn get_free_port() -> u16 {
    static mut PORT_NR: u16 = 9090;
    PORT_NR += 1;

We initialize with the number 9090 here and return an incremented number for each call. The compiler doesn't seem to like it:

error[E0133]: use of mutable static requires unsafe function or block
  --> tests/common/mod.rs:99:5
99 |     PORT_NR += 1;
   |     ^^^^^^^ use of mutable static

The compiler is saving me from a race condition here. Since tests are executed concurrently 2 tests could enter this function at the same time. One increments the port number, but before returning the operating system hands over execution to the second test thread which also increments the port number. Now both calls suddenly would return the same port number, which is exactly what we want to avoid.

We need to isolate the calls to this function or access to the static shared variable. In Java we would use the synchronize keyword on the function definition to ensure that only one thread can enter it at a time. But Rust uses more primitive synchronization constructs.

Protecting static variables with AtomicUsize

The standard library has some good documentation about synchronized atomic access that we can use.

pub fn get_free_port() -> u16 {
    static PORT_NR: AtomicUsize = ATOMIC_USIZE_INIT;

    PORT_NR.compare_and_swap(0, 9090, Ordering::SeqCst);
    PORT_NR.fetch_add(1, Ordering::SeqCst) as u16

This works, but is a bit annoying:

  1. We have to initialize the static variable with ATOMIC_USIZE_INIT instead of our desired value 9090. If you try

    static PORT_NR: AtomicUsize = AtomicUsize::new(9090);

    then the compiler will complain:

    error: const fns are an unstable feature
       --> tests/common/mod.rs:98:35
    98 |     static PORT_NR: AtomicUsize = AtomicUsize::new(9090);
       |                                   ^^^^^^^^^^^^^^^^^^^^^^
       = help: in Nightly builds, add `#![feature(const_fn)]` to the crate attributes to enable

    We don't want to depend on the nightly compiler, so this is not possible right now.

  2. The compare_and_swap() call is only necessary because we could not directly initialize our value to 9090. It is executed on every call to get_free_port() and is just a waste of execution time.

  3. I have no idea what Ordering::SeqCst means. The documentation says that this variant is the most restrictive one but I don't know if this is necessary or ideal in my use case. I'm using it because it is used in the docs example ¯\_(ツ)_/¯

  4. We have to cast to u16 in the end because there is only an AtomicUsize type but no AtomicU16.

Postponing the offset calculation

Thanks to a tip from Steven Fackler we can postpone our offset to the very end:

pub fn get_free_port() -> u16 {
    static PORT_NR: AtomicUsize = ATOMIC_USIZE_INIT;

    PORT_NR.fetch_add(1, Ordering::SeqCst) as u16 + 9090

That way we can remove the initialization condition and always operate on a fixed offset of 9090. This is still not super intuitive because the initial value of our counter is at the very end which makes this function hard to read.


Rust is great at detecting race conditions at compile time and helps you do the right thing with static variables. The solution to synchronize concurrent access with atomics feels a bit clumsy and there might be a better way that I have not discovered yet.