# rusty-fork

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[![](http://meritbadge.herokuapp.com/rusty-fork)](https://crates.io/crates/rusty-fork)

Rusty-fork provides a way to "fork" unit tests into separate processes.

There are a number of reasons to want to run some tests in isolated
processes:

- When tests share a process, if any test causes the process to abort,
segfault, overflow the stack, etc., the entire test runner process dies. If
the test is in a subprocess, only the subprocess dies and the test runner
simply fails the test.

- Isolating a test to a subprocess makes it possible to add a timeout to
the test and forcibly terminate it and produce a normal test failure.

- Tests which need to interact with some inherently global property, such
as the current working directory, can do so without interfering with other
tests.

This crate itself provides two things:

- The [`rusty_fork_test!`](macro.rusty_fork_test.html) macro, which is a
simple way to wrap standard Rust tests to be run in subprocesses with
optional timeouts.

- The [`fork`](fn.fork.html) function which can be used as a building block
to make other types of process isolation strategies.

## Quick Start

If you just want to run normal Rust tests in isolated processes, getting
started is pretty quick.

In `Cargo.toml`, add

```toml
[dev-dependencies]
rusty-fork = "0.3.0"
```

Then, you can simply wrap any test(s) to be isolated with the
[`rusty_fork_test!`](macro.rusty_fork_test.html) macro.

```rust
use rusty_fork::rusty_fork_test;

rusty_fork_test! {
    #[test]
    fn my_test() {
        assert_eq!(2, 1 + 1);
    }

    // more tests...
}
```

For more advanced usage, have a look at the [`fork`](fn.fork.html)
function.

## How rusty-fork works

Unix-style process forking isn't really viable within the standard Rust
test environment for a number of reasons.

- While true process forking can be done on Windows, it's neither fast nor
reliable.

- The Rust test environment is multi-threaded, so attempting to do anything
non-trivial after a process fork would result in undefined behaviour.

Rusty-fork instead works by _spawning_ a fresh instance of the current
process, after adjusting the command-line to ensure that only the desired
test is entered. Some additional coordination establishes the parent/child
branches and (not quite seamlessly) integrates the child's output with the
test output capture system.

Coordination between the processes is performed via environment variables,
since there is otherwise no way to pass parameters to a test.

Since it needs to spawn new copies of the test runner executable,
rusty-fork does need to know about the meaning of every flag passed by the
user. If any unknown flags are encountered, forking will fail. Please do
not hesitate to file
[issues](https://github.com/AltSysrq/rusty-fork/issues) if rusty-fork fails
to recognise any valid flags passed to the test runner.

It is possible to inform rusty-fork of new flags without patching by
setting environment variables. For example, if a new `--frob-widgets` flag
were added to the test runner, you could set `RUSTY_FORK_FLAG_FROB_WIDGETS`
to one of the following:

- `pass` — Pass the flag (just the flag) to the child process
- `pass-arg` — Pass the flag and its following argument to the child process
- `drop` — Don't pass the flag to the child process
- `drop-arg` — Don't pass the flag to the child process, and ignore whatever
  argument follows.

In general, arguments that affect which tests are run should be dropped,
and others should be passed.


## Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.