+++
title = "Testing"
weight = 4
path = "testing"
date = 2019-04-27

[extra]
chapter = "Bare Bones"
+++

This post explores unit and integration testing in `no_std` executables. We will use Rust's support for custom test frameworks to execute test functions inside our kernel. To report the results out of QEMU, we will use different features of QEMU and the `bootimage` tool.

<!-- more -->

This blog is openly developed on [GitHub]. If you have any problems or questions, please open an issue there. You can also leave comments [at the bottom]. The complete source code for this post can be found in the [`post-04`][post branch] branch.

[GitHub]: https://github.com/phil-opp/blog_os
[at the bottom]: #comments
[post branch]: https://github.com/phil-opp/blog_os/tree/post-04

<!-- toc -->

## Requirements

This post replaces the (now deprecated) [_Unit Testing_] and [_Integration Tests_] posts. It assumes that you have followed the [_A Minimal Rust Kernel_] post after 2019-04-27. Mainly, it requires that you have a `.cargo/config` file that [sets a default target] and [defines a runner executable].

[_Unit Testing_]: @/second-edition/posts/deprecated/04-unit-testing/index.md
[_Integration Tests_]: @/second-edition/posts/deprecated/05-integration-tests/index.md
[_A Minimal Rust Kernel_]: @/second-edition/posts/02-minimal-rust-kernel/index.md
[sets a default target]: @/second-edition/posts/02-minimal-rust-kernel/index.md#set-a-default-target
[defines a runner executable]: @/second-edition/posts/02-minimal-rust-kernel/index.md#using-cargo-run

## Testing in Rust

Rust has a [built-in test framework] that is capable of running unit tests without the need to set anything up. Just create a function that checks some results through assertions and add the `#[test]` attribute to the function header. Then `cargo test` will automatically find and execute all test functions of your crate.

[built-in test framework]: https://doc.rust-lang.org/book/second-edition/ch11-00-testing.html

Unfortunately it's a bit more complicated for `no_std` applications such as our kernel. The problem is that Rust's test framework implicitly uses the built-in [`test`] library, which depends on the standard library. This means that we can't use the default test framework for our `#[no_std]` kernel.

[`test`]: https://doc.rust-lang.org/test/index.html

We can see this when we try to run `cargo xtest` in our project:

```
> cargo xtest
   Compiling blog_os v0.1.0 (/…/blog_os)
error[E0463]: can't find crate for `test`
```

Since the `test` crate depends on the standard library, it is not available for our bare metal target. While porting the `test` crate to a `#[no_std]` context [is possible][utest], it is highly unstable and requires some hacks such as redefining the `panic` macro.

[utest]: https://github.com/japaric/utest

### Custom Test Frameworks

Fortunately, Rust supports replacing the default test framework through the unstable [`custom_test_frameworks`] feature. This feature requires no external libraries and thus also works in `#[no_std]` environments. It works by collecting all functions annotated with a `#[test_case]` attribute and then invoking a user-specified runner function with the list of tests as argument. Thus it gives the implementation maximal control over the test process.

[`custom_test_frameworks`]: https://doc.rust-lang.org/unstable-book/language-features/custom-test-frameworks.html

The disadvantage compared to the default test framework is that many advanced features such as [`should_panic` tests] are not available. Instead, it is up to the implementation to provide such features itself if needed. This is ideal for us since we have a very special execution environment where the default implementations of such advanced features probably wouldn't work anyway. For example, the `#[should_panic]` attribute relies on stack unwinding to catch the panics, which we disabled for our kernel.

[`should_panic` tests]: https://doc.rust-lang.org/book/ch11-01-writing-tests.html#checking-for-panics-with-should_panic

To implement a custom test framework for our kernel, we add the following to our `main.rs`:

```rust
// in src/main.rs

#![feature(custom_test_frameworks)]
#![test_runner(crate::test_runner)]

#[cfg(test)]
fn test_runner(tests: &[&dyn Fn()]) {
    println!("Running {} tests", tests.len());
    for test in tests {
        test();
    }
}
```

Our runner just prints a short debug message and then calls each test function in the list. The argument type `&[&dyn Fn()]` is a [_slice_] of [_trait object_] references of the [_Fn()_] trait. It is basically a list of references to types that can be called like a function. Since the function is useless for non-test runs, we use the `#[cfg(test)]` attribute to include it only for tests.

[_slice_]: https://doc.rust-lang.org/std/primitive.slice.html
[_trait object_]: https://doc.rust-lang.org/1.30.0/book/first-edition/trait-objects.html
[_Fn()_]: https://doc.rust-lang.org/std/ops/trait.Fn.html

When we run `cargo xtest` now, we see that it now succeeds. However, we still see our "Hello World" instead of the message from our `test_runner`. The reason is that our `_start` function is still used as entry point. The custom test frameworks feature generates a `main` function that calls `test_runner`, but this function is ignored because we use the `#[no_main]` attribute and provide our own entry point.

To fix this, we first need to change the name of the generated function to something different than `main` through the `reexport_test_harness_main` attribute. Then we can call the renamed function from our `_start` function:

```rust
// in src/main.rs

#![reexport_test_harness_main = "test_main"]

#[no_mangle]
pub extern "C" fn _start() -> ! {
    println!("Hello World{}", "!");

    #[cfg(test)]
    test_main();

    loop {}
}
```

We set the name of the test framework entry function to `test_main` and call it from our `_start` entry point. We use [conditional compilation] to add the call to `test_main` only in test contexts because the function is not generated on a normal run.

When we now execute `cargo xtest`, we see the "Running 0 tests" message from our `test_runner` on the screen. We are now ready to create our first test function:

```rust
// in src/main.rs

#[test_case]
fn trivial_assertion() {
    print!("trivial assertion... ");
    assert_eq!(1, 1);
    println!("[ok]");
}
```

When we run `cargo xtest` now, we see the following output:

![QEMU printing "Hello World!", "Running 1 tests", and "trivial assertion... [ok]"](qemu-test-runner-output.png)

The `tests` slice passed to our `test_runner` function now contains a reference to the `trivial_assertion` function. From the `trivial assertion... [ok]` output on the screen we see that the test was called and that it succeeded.

After executing the tests, our `test_runner` returns to the `test_main` function, which in turn returns to our `_start` entry point function. At the end of `_start`, we enter an endless loop because the entry point function is not allowed to return. This is a problem, because we want `cargo xtest` to exit after running all tests.

## Exiting QEMU

Right now we have an endless loop at the end of our `_start` function and need to close QEMU manually on each execution of `cargo xtest`. This is unfortunate because we also want to run `cargo xtest` in scripts without user interaction. The clean solution to this would be to implement a proper way to shutdown our OS. Unfortunately this is relatively complex, because it requires implementing support for either the [APM] or [ACPI] power management standard.

[APM]: https://wiki.osdev.org/APM
[ACPI]: https://wiki.osdev.org/ACPI

Luckily, there is an escape hatch: QEMU supports a special `isa-debug-exit` device, which provides an easy way to exit QEMU from the guest system. To enable it, we need to pass a `-device` argument to QEMU. We can do so by adding a `package.metadata.bootimage.test-args` configuration key in our `Cargo.toml`:

```toml
# in Cargo.toml

[package.metadata.bootimage]
test-args = ["-device", "isa-debug-exit,iobase=0xf4,iosize=0x04"]
```

The `bootimage runner` appends the `test-args` to the default QEMU command for all test executables. For a normal `cargo xrun`, the arguments are ignored.

Together with the device name (`isa-debug-exit`), we pass the two parameters `iobase` and `iosize` that specify the _I/O port_ through which the device can be reached from our kernel.

### I/O Ports

There are two different approaches for communicating between the CPU and peripheral hardware on x86, **memory-mapped I/O** and **port-mapped I/O**. We already used memory-mapped I/O for accessing the [VGA text buffer] through the memory address `0xb8000`. This address is not mapped to RAM, but to some memory on the VGA device.

[VGA text buffer]: @/second-edition/posts/03-vga-text-buffer/index.md

In contrast, port-mapped I/O uses a separate I/O bus for communication. Each connected peripheral has one or more port numbers. To communicate with such an I/O port there are special CPU instructions called `in` and `out`, which take a port number and a data byte (there are also variations of these commands that allow sending an `u16` or `u32`).

The `isa-debug-exit` devices uses port-mapped I/O. The `iobase` parameter specifies on which port address the device should live (`0xf4` is a [generally unused][list of x86 I/O ports] port on the x86's IO bus) and the `iosize` specifies the port size (`0x04` means four bytes).

[list of x86 I/O ports]: https://wiki.osdev.org/I/O_Ports#The_list

### Using the Exit Device

The functionality of the `isa-debug-exit` device is very simple. When a `value` is written to the I/O port specified by `iobase`, it causes QEMU to exit with [exit status] `(value << 1) | 1`. So when we write `0` to the port QEMU will exit with exit status `(0 << 1) | 1 = 1` and when we write `1` to the port it will exit with exit status `(1 << 1) | 1 = 3`.

[exit status]: https://en.wikipedia.org/wiki/Exit_status

Instead of manually invoking the `in` and `out` assembly instructions, we use the abstractions provided by the [`x86_64`] crate. To add a dependency on that crate, we add it to the `dependencies` section in our `Cargo.toml`:

[`x86_64`]: https://docs.rs/x86_64/0.9.6/x86_64/

```toml
# in Cargo.toml

[dependencies]
x86_64 = "0.9.5"
```

Now we can use the [`Port`] type provided by the crate to create an `exit_qemu` function:

[`Port`]: https://docs.rs/x86_64/0.9.6/x86_64/instructions/port/struct.Port.html

```rust
// in src/main.rs

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u32)]
pub enum QemuExitCode {
    Success = 0x10,
    Failed = 0x11,
}

pub fn exit_qemu(exit_code: QemuExitCode) {
    use x86_64::instructions::port::Port;

    unsafe {
        let mut port = Port::new(0xf4);
        port.write(exit_code as u32);
    }
}
```

The function creates a new [`Port`] at `0xf4`, which is the `iobase` of the `isa-debug-exit` device. Then it writes the passed exit code to the port. We use `u32` because we specified the `iosize` of the `isa-debug-exit` device as 4 bytes. Both operations are unsafe, because writing to an I/O port can generally result in arbitrary behavior.

For specifying the exit status, we create a `QemuExitCode` enum. The idea is to exit with the success exit code if all tests succeeded and with the failure exit code otherwise. The enum is marked as `#[repr(u32)]` to represent each variant by an `u32` integer. We use exit code `0x10` for success and `0x11` for failure. The actual exit codes do not matter much, as long as they don't clash with the default exit codes of QEMU. For example, using exit code `0` for success is not a good idea because it becomes `(0 << 1) | 1 = 1` after the transformation, which is the default exit code when QEMU failed to run. So we could not differentiate a QEMU error from a successful test run.

We can now update our `test_runner` to exit QEMU after all tests ran:

```rust
fn test_runner(tests: &[&dyn Fn()]) {
    println!("Running {} tests", tests.len());
    for test in tests {
        test();
    }
    /// new
    exit_qemu(QemuExitCode::Success);
}
```

When we run `cargo xtest` now, we see that QEMU immediately closes after executing the tests. The problem is that `cargo test` interprets the test as failed even though we passed our `Success` exit code:

```
> cargo xtest
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running target/x86_64-blog_os/debug/deps/blog_os-5804fc7d2dd4c9be
Building bootloader
   Compiling bootloader v0.5.3 (/home/philipp/Documents/bootloader)
    Finished release [optimized + debuginfo] target(s) in 1.07s
Running: `qemu-system-x86_64 -drive format=raw,file=/…/target/x86_64-blog_os/debug/
    deps/bootimage-blog_os-5804fc7d2dd4c9be.bin -device isa-debug-exit,iobase=0xf4,
    iosize=0x04`
error: test failed, to rerun pass '--bin blog_os'
```

The problem is that `cargo test` considers all error codes other than `0` as failure.

### Success Exit Code

To work around this, `bootimage` provides a `test-success-exit-code` configuration key that maps a specified exit code to the exit code `0`:

```toml
[package.metadata.bootimage]
test-args = […]
test-success-exit-code = 33         # (0x10 << 1) | 1
```

With this configuration, `bootimage` maps our success exit code to exit code 0, so that `cargo xtest` correctly recognizes the success case and does no count the test as failed.

Our test runner now automatically closes QEMU and correctly reports the test results out. We still see the QEMU window open for a very short time, but it does not suffice to read the results. It would be nice if we could print the test results to the console instead, so that we can still see them after QEMU exited.

## Printing to the Console

To see the test output on the console, we need to send the data from our kernel to the host system somehow. There are various ways to achieve this, for example by sending the data over a TCP network interface. However, setting up a networking stack is a quite complex task, so we will choose a simpler solution instead.

### Serial Port

A simple way to send the data is to use the [serial port], an old interface standard which is no longer found in modern computers. It is easy to program and QEMU can redirect the bytes sent over serial to the host's standard output or a file.

[serial port]: https://en.wikipedia.org/wiki/Serial_port

The chips implementing a serial interface are called [UARTs]. There are [lots of UART models] on x86, but fortunately the only differences between them are some advanced features we don't need. The common UARTs today are all compatible to the [16550 UART], so we will use that model for our testing framework.

[UARTs]: https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter
[lots of UART models]: https://en.wikipedia.org/wiki/Universal_asynchronous_receiver-transmitter#UART_models
[16550 UART]: https://en.wikipedia.org/wiki/16550_UART

We will use the [`uart_16550`] crate to initialize the UART and send data over the serial port. To add it as a dependency, we update our `Cargo.toml` and `main.rs`:

[`uart_16550`]: https://docs.rs/uart_16550

```toml
# in Cargo.toml

[dependencies]
uart_16550 = "0.2.0"
```

The `uart_16550` crate contains a `SerialPort` struct that represents the UART registers, but we still need to construct an instance of it ourselves. For that we create a new `serial` module with the following content:

```rust
// in src/main.rs

mod serial;
```

```rust
// in src/serial.rs

use uart_16550::SerialPort;
use spin::Mutex;
use lazy_static::lazy_static;

lazy_static! {
    pub static ref SERIAL1: Mutex<SerialPort> = {
        let mut serial_port = unsafe { SerialPort::new(0x3F8) };
        serial_port.init();
        Mutex::new(serial_port)
    };
}
```

Like with the [VGA text buffer][vga lazy-static], we use `lazy_static` and a spinlock to create a `static` writer instance. By using `lazy_static` we can ensure that the `init` method is called exactly once on its first use.

Like the `isa-debug-exit` device, the UART is programmed using port I/O. Since the UART is more complex, it uses multiple I/O ports for programming different device registers. The unsafe `SerialPort::new` function expects the address of the first I/O port of the UART as argument, from which it can calculate the addresses of all needed ports. We're passing the port address `0x3F8`, which is the standard port number for the first serial interface.

[vga lazy-static]: @/second-edition/posts/03-vga-text-buffer/index.md#lazy-statics

To make the serial port easily usable, we add `serial_print!` and `serial_println!` macros:

```rust
#[doc(hidden)]
pub fn _print(args: ::core::fmt::Arguments) {
    use core::fmt::Write;
    SERIAL1.lock().write_fmt(args).expect("Printing to serial failed");
}

/// Prints to the host through the serial interface.
#[macro_export]
macro_rules! serial_print {
    ($($arg:tt)*) => {
        $crate::serial::_print(format_args!($($arg)*));
    };
}

/// Prints to the host through the serial interface, appending a newline.
#[macro_export]
macro_rules! serial_println {
    () => ($crate::serial_print!("\n"));
    ($fmt:expr) => ($crate::serial_print!(concat!($fmt, "\n")));
    ($fmt:expr, $($arg:tt)*) => ($crate::serial_print!(
        concat!($fmt, "\n"), $($arg)*));
}
```

The implementation is very similar to the implementation of our `print` and `println` macros. Since the `SerialPort` type already implements the [`fmt::Write`] trait, we don't need to provide our own implementation.

[`fmt::Write`]: https://doc.rust-lang.org/nightly/core/fmt/trait.Write.html

Now we can print to the serial interface instead of the VGA text buffer in our test code:

```rust
// in src/main.rs

#[cfg(test)]
fn test_runner(tests: &[&dyn Fn()]) {
    serial_println!("Running {} tests", tests.len());
    […]
}

#[test_case]
fn trivial_assertion() {
    serial_print!("trivial assertion... ");
    assert_eq!(1, 1);
    serial_println!("[ok]");
}
```

Note that the `serial_println` macro lives directly under the root namespace because we used the `#[macro_export]` attribute, so importing it through `use crate::serial::serial_println` will not work.

### QEMU Arguments

To see the serial output from QEMU, we need use the `-serial` argument to redirect the output to stdout:

```toml
# in Cargo.toml

[package.metadata.bootimage]
test-args = [
    "-device", "isa-debug-exit,iobase=0xf4,iosize=0x04", "-serial", "stdio"
]
```

When we run `cargo xtest` now, we see the test output directly in the console:

```
> cargo xtest
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running target/x86_64-blog_os/debug/deps/blog_os-7b7c37b4ad62551a
Building bootloader
    Finished release [optimized + debuginfo] target(s) in 0.02s
Running: `qemu-system-x86_64 -drive format=raw,file=/…/target/x86_64-blog_os/debug/
    deps/bootimage-blog_os-7b7c37b4ad62551a.bin -device
    isa-debug-exit,iobase=0xf4,iosize=0x04 -serial stdio`
Running 1 tests
trivial assertion... [ok]
```

However, when a test fails we still see the output inside QEMU because our panic handler still uses `println`. To simulate this, we can change the assertion in our `trivial_assertion` test to `assert_eq!(0, 1)`:

![QEMU printing "Hello World!" and "panicked at 'assertion failed: `(left == right)`
    left: `0`, right: `1`', src/main.rs:55:5](qemu-failed-test.png)

We see that the panic message is still printed to the VGA buffer, while the other test output is printed to the serial port. The panic message is quite useful, so it would be useful to see it in the console too.

### Print an Error Message on Panic

To exit QEMU with an error message on a panic, we can use [conditional compilation] to use a different panic handler in testing mode:

[conditional compilation]: https://doc.rust-lang.org/1.30.0/book/first-edition/conditional-compilation.html

```rust
// our existing panic handler
#[cfg(not(test))] // new attribute
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    println!("{}", info);
    loop {}
}

// our panic handler in test mode
#[cfg(test)]
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    serial_println!("[failed]\n");
    serial_println!("Error: {}\n", info);
    exit_qemu(QemuExitCode::Failed);
    loop {}
}
```

For our test panic handler, we use `serial_println` instead of `println` and then exit QEMU with a failure exit code. Note that we still need an endless `loop` after the `exit_qemu` call because the compiler does not know that the `isa-debug-exit` device causes a program exit.

Now QEMU also exits for failed tests and prints a useful error message on the console:

```
> cargo xtest
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running target/x86_64-blog_os/debug/deps/blog_os-7b7c37b4ad62551a
Building bootloader
    Finished release [optimized + debuginfo] target(s) in 0.02s
Running: `qemu-system-x86_64 -drive format=raw,file=/…/target/x86_64-blog_os/debug/
    deps/bootimage-blog_os-7b7c37b4ad62551a.bin -device
    isa-debug-exit,iobase=0xf4,iosize=0x04 -serial stdio`
Running 1 tests
trivial assertion... [failed]

Error: panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `1`', src/main.rs:65:5
```

Since we see all test output on the console now, we no longer need the QEMU window that pops up for a short time. So we can hide it completely.

### Hiding QEMU

Since we report out the complete test results using the `isa-debug-exit` device and the serial port, we don't need the QEMU window anymore. We can hide it by passing the `-display none` argument to QEMU:

```toml
# in Cargo.toml

[package.metadata.bootimage]
test-args = [
    "-device", "isa-debug-exit,iobase=0xf4,iosize=0x04", "-serial", "stdio",
    "-display", "none"
]
```

Now QEMU runs completely in the background and no window is opened anymore. This is not only less annoying, but also allows our test framework to run in environments without a graphical user interface, such as CI services or [SSH] connections.

[SSH]: https://en.wikipedia.org/wiki/Secure_Shell

### Timeouts

Since `cargo xtest` waits until the test runner exits, a test that never returns can block the test runner forever. That's unfortunate, but not a big problem in practice since it's normally easy to avoid endless loops. In our case, however, endless loops can occur in various situations:

- The bootloader fails to load our kernel, which causes the system to reboot endlessly.
- The BIOS/UEFI firmware fails to load the bootloader, which causes the same endless rebooting.
- The CPU enters a `loop {}` statement at the end of some of our functions, for example because the QEMU exit device doesn't work properly.
- The hardware causes a system reset, for example when a CPU exception is not caught (explained in a future post).

Since endless loops can occur in so many situations, the `bootimage` tool sets a timeout of 5 minutes for each test executable by default. If the test does not finish in this time, it is marked as failed and a "Timed Out" error is printed to the console. This feature ensures that tests that are stuck in an endless loop don't block `cargo xtest` forever.

You can try it yourself by adding a `loop {}` statement in the `trivial_assertion` test. When you run `cargo xtest`, you see that the test is marked as timed out after 5 minutes. The timeout duration is [configurable][bootimage config] through a `test-timeout` key in the Cargo.toml:

[bootimage config]: https://github.com/rust-osdev/bootimage#configuration

```toml
# in Cargo.toml

[package.metadata.bootimage]
test-timeout = 300          # (in seconds)
```

If you don't want to wait 5 minutes for the `trivial_assertion` test to time out, you can temporarily decrease the above value.

After this, we no longer need the `trivial_assertion` test, so we can delete it.

## Testing the VGA Buffer

Now that we have a working test framework, we can create a few tests for our VGA buffer implementation. First, we create a very simple test to verify that `println` works without panicking:

```rust
// in src/vga_buffer.rs

#[cfg(test)]
use crate::{serial_print, serial_println};

#[test_case]
fn test_println_simple() {
    serial_print!("test_println... ");
    println!("test_println_simple output");
    serial_println!("[ok]");
}
```

The test just prints something to the VGA buffer. If it finishes without panicking, it means that the `println` invocation did not panic either. Since we only need the `serial_println` import in test mode, we add the `cfg(test)` attribute to avoid the unused import warning for a normal `cargo xbuild`.

To ensure that no panic occurs even if many lines are printed and lines are shifted off the screen, we can create another test:

```rust
// in src/vga_buffer.rs

#[test_case]
fn test_println_many() {
    serial_print!("test_println_many... ");
    for _ in 0..200 {
        println!("test_println_many output");
    }
    serial_println!("[ok]");
}
```

We can also create a test function to verify that the printed lines really appear on the screen:

```rust
// in src/vga_buffer.rs

#[test_case]
fn test_println_output() {
    serial_print!("test_println_output... ");

    let s = "Some test string that fits on a single line";
    println!("{}", s);
    for (i, c) in s.chars().enumerate() {
        let screen_char = WRITER.lock().buffer.chars[BUFFER_HEIGHT - 2][i].read();
        assert_eq!(char::from(screen_char.ascii_character), c);
    }

    serial_println!("[ok]");
}
```

The function defines a test string, prints it using `println`, and then iterates over the screen characters of the static `WRITER`, which represents the vga text buffer. Since `println` prints to the last screen line and then immediately appends a newline, the string should appear on line `BUFFER_HEIGHT - 2`.

By using [`enumerate`], we count the number of iterations in the variable `i`, which we then use for loading the screen character corresponding to `c`. By comparing the `ascii_character` of the screen character with `c`, we ensure that each character of the string really appears in the vga text buffer.

[`enumerate`]: https://doc.rust-lang.org/core/iter/trait.Iterator.html#method.enumerate

As you can imagine, we could create many more test functions, for example a function that tests that no panic occurs when printing very long lines and that they're wrapped correctly. Or a function for testing that newlines, non-printable characters, and non-unicode characters are handled correctly.

For the rest of this post, however, we will explain how to create _integration tests_ to test the interaction of different components together.

## Integration Tests

The convention for [integration tests] in Rust is to put them into a `tests` directory in the project root (i.e. next to the `src` directory). Both the default test framework and custom test frameworks will automatically pick up and execute all tests in that directory.

[integration tests]: https://doc.rust-lang.org/book/ch11-03-test-organization.html#integration-tests

All integration tests are their own executables and completely separate from our `main.rs`. This means that each test needs to define its own entry point function. Let's create an example integration test named `basic_boot` to see how it works in detail:

```rust
// in tests/basic_boot.rs

#![no_std]
#![no_main]
#![feature(custom_test_frameworks)]
#![test_runner(crate::test_runner)]
#![reexport_test_harness_main = "test_main"]

use core::panic::PanicInfo;

#[no_mangle] // don't mangle the name of this function
pub extern "C" fn _start() -> ! {
    test_main();

    loop {}
}

fn test_runner(tests: &[&dyn Fn()]) {
    unimplemented!();
}

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    loop {}
}
```

Since integration tests are separate executables, we need to provide all the crate attributes (`no_std`, `no_main`, `test_runner`, etc.) again. We also need to create a new entry point function `_start`, which calls the test entry point function `test_main`. We don't need any `cfg(test)` attributes because integration test executables are never built in non-test mode.

We use the [`unimplemented`] macro that always panics as a placeholder for the `test_runner` function and just `loop` in the `panic` handler for now. Ideally, we want to implement these functions exactly as we did in our `main.rs` using the `serial_println` macro and the `exit_qemu` function. The problem is that we don't have access to these functions since tests are built completely separately of our `main.rs` executable.

[`unimplemented`]: https://doc.rust-lang.org/core/macro.unimplemented.html

If you run `cargo xtest` at this stage, you will get an endless loop because the panic handler loops endlessly. You need to use the `Ctrl+c` keyboard shortcut for exiting QEMU.

### Create a Library

To make the required functions available to our integration test, we need to split off a library from our `main.rs`, which can be included by other crates and integration test executables. To do this, we create a new `src/lib.rs` file:

```rust
// src/lib.rs

#![no_std]
```

Like the `main.rs`, the `lib.rs` is a special file that is automatically recognized by cargo. The library is a separate compilation unit, so we need to specify the `#![no_std]` attribute again.

To make our library work with `cargo xtest`, we need to also add the test functions and attributes:

```rust
// in src/lib.rs

#![cfg_attr(test, no_main)]
#![feature(custom_test_frameworks)]
#![test_runner(crate::test_runner)]
#![reexport_test_harness_main = "test_main"]

use core::panic::PanicInfo;

pub fn test_runner(tests: &[&dyn Fn()]) {
    serial_println!("Running {} tests", tests.len());
    for test in tests {
        test();
    }
    exit_qemu(QemuExitCode::Success);
}

pub fn test_panic_handler(info: &PanicInfo) -> ! {
    serial_println!("[failed]\n");
    serial_println!("Error: {}\n", info);
    exit_qemu(QemuExitCode::Failed);
    loop {}
}

/// Entry point for `cargo xtest`
#[cfg(test)]
#[no_mangle]
pub extern "C" fn _start() -> ! {
    test_main();
    loop {}
}

#[cfg(test)]
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    test_panic_handler(info)
}
```

To make our `test_runner` available to executables and integration tests, we don't apply the `cfg(test)` attribute to it and make it public. We also factor out the implementation of our panic handler into a public `test_panic_handler` function, so that it is available for executables too.

Since our `lib.rs` is tested independently of our `main.rs`, we need to add a `_start` entry point and a panic handler when the library is compiled in test mode. By using the [`cfg_attr`] crate attribute, we conditionally enable the `no_main` attribute in this case.

[`cfg_attr`]: https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg_attr-attribute

We also move over the `QemuExitCode` enum and the `exit_qemu` function and make them public:

```rust
// in src/lib.rs

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u32)]
pub enum QemuExitCode {
    Success = 0x10,
    Failed = 0x11,
}

pub fn exit_qemu(exit_code: QemuExitCode) {
    use x86_64::instructions::port::Port;

    unsafe {
        let mut port = Port::new(0xf4);
        port.write(exit_code as u32);
    }
}
```

Now executables and integration tests can import these functions from the library and don't need to define their own implementations. To also make `println` and `serial_println` available, we move the module declarations too:

```rust
// in src/lib.rs

pub mod serial;
pub mod vga_buffer;
```

We make the modules public to make them usable from outside of our library. This is also required for making our `println` and `serial_println` macros usable, since they use the `_print` functions of the modules.

Now we can update our `main.rs` to use the library:

```rust
// src/main.rs

#![no_std]
#![no_main]
#![feature(custom_test_frameworks)]
#![test_runner(blog_os::test_runner)]
#![reexport_test_harness_main = "test_main"]

use core::panic::PanicInfo;
use blog_os::println;

#[no_mangle]
pub extern "C" fn _start() -> ! {
    println!("Hello World{}", "!");

    #[cfg(test)]
    test_main();

    loop {}
}

/// This function is called on panic.
#[cfg(not(test))]
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    println!("{}", info);
    loop {}
}

#[cfg(test)]
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    blog_os::test_panic_handler(info)
}
```

The library is usable like a normal external crate. It is called like our crate, which is `blog_os` in our case. The above code uses the `blog_os::test_runner` function in the `test_runner` attribute and the `blog_os::test_panic_handler` function in our `cfg(test)` panic handler. It also imports the `println` macro to make it available to our `_start` and `panic` functions.

At this point, `cargo xrun` and `cargo xtest` should work again. Of course, `cargo xtest` still loops endlessly (you can exit with `ctrl+c`). Let's fix this by using the required library functions in our integration test.

### Completing the Integration Test

Like our `src/main.rs`, our `tests/basic_boot.rs` executable can import types from our new library. This allows us to import the missing components to complete our test.

```rust
// in tests/basic_boot.rs

#![test_runner(blog_os::test_runner)]

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    blog_os::test_panic_handler(info)
}
```

Instead of reimplementing the test runner, we use the `test_runner` function from our library. For our `panic` handler, we call the `blog_os::test_panic_handler` function like we did in our `main.rs`.

Now `cargo xtest` exits normally again. When you run it, you see that it builds and runs the tests for our `lib.rs`, `main.rs`, and `basic_boot.rs` separately after each other. For the `main.rs` and the `basic_boot` integration test, it reports "Running 0 tests" since these files don't have any functions annotated with `#[test_case]`.

We can now add tests to our `basic_boot.rs`. For example, we can test that `println` works without panicking, like we did in the vga buffer tests:

```rust
// in tests/basic_boot.rs

use blog_os::{println, serial_print, serial_println};

#[test_case]
fn test_println() {
    serial_print!("test_println... ");
    println!("test_println output");
    serial_println!("[ok]");
}
```

When we run `cargo xtest` now, we see that it finds and executes the test function.

The test might seem a bit useless right now since it's almost identical to one of the VGA buffer tests. However, in the future the `_start` functions of our `main.rs` and `lib.rs` might grow and call various initialization routines before running the `test_main` function, so that the two tests are executed in very different environments.

By testing `println` in a `basic_boot` environment without calling any initialization routines in `_start`, we can ensure that `println` works right after booting. This is important because we rely on it e.g. for printing panic messages.

### Future Tests

The power of integration tests is that they're treated as completely separate executables. This gives them complete control over the environment, which makes it possible to test that the code interacts correctly with the CPU or hardware devices.

Our `basic_boot` test is a very simple example for an integration test. In the future, our kernel will become much more featureful and interact with the hardware in various ways. By adding integration tests, we can ensure that these interactions work (and keep working) as expected. Some ideas for possible future tests are:

- **CPU Exceptions**: When the code performs invalid operations (e.g. divides by zero), the CPU throws an exception. The kernel can register handler functions for such exceptions. An integration test could verify that the correct exception handler is called when a CPU exception occurs or that the execution continues correctly after resolvable exceptions.
- **Page Tables**: Page tables define which memory regions are valid and accessible. By modifying the page tables, it is possible to allocate new memory regions, for example when launching programs. An integration test could perform some modifications of the page tables in the `_start` function and then verify that the modifications have the desired effects in `#[test_case]` functions.
- **Userspace Programs**: Userspace programs are programs with limited access to the system's resources. For example, they don't have access to kernel data structures or to the memory of other programs. An integration test could launch userspace programs that perform forbidden operations and verify that the kernel prevents them all.

As you can imagine, many more tests are possible. By adding such tests, we can ensure that we don't break them accidentally when we add new features to our kernel or refactor our code. This is especially important when our kernel becomes larger and more complex.

### Tests that Should Panic

The test framework of the standard library supports a [`#[should_panic]` attribute][should_panic] that allows to construct tests that should fail. This is useful for example to verify that a function fails when an invalid argument is passed. Unfortunately this attribute isn't supported in `#[no_std]` crates since it requires support from the standard library.

[should_panic]: https://doc.rust-lang.org/rust-by-example/testing/unit_testing.html#testing-panics

While we can't use the `#[should_panic]` attribute in our kernel, we can get similar behavior by creating an integration test that exits with a success error code from the panic handler. Let's start creating such a test with the name `should_panic`:

```rust
// in tests/should_panic.rs

#![no_std]
#![no_main]

use core::panic::PanicInfo;
use blog_os::{QemuExitCode, exit_qemu, serial_println};

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    serial_println!("[ok]");
    exit_qemu(QemuExitCode::Success);
    loop {}
}
```

This test is still incomplete as it doesn't define a `_start` function or any of the custom test runner attributes yet. Let's add the missing parts:

```rust
// in tests/should_panic.rs

#![feature(custom_test_frameworks)]
#![test_runner(test_runner)]
#![reexport_test_harness_main = "test_main"]

#[no_mangle]
pub extern "C" fn _start() -> ! {
    test_main();

    loop {}
}

pub fn test_runner(tests: &[&dyn Fn()]) {
    serial_println!("Running {} tests", tests.len());
    for test in tests {
        test();
        serial_println!("[test did not panic]");
        exit_qemu(QemuExitCode::Failed);
    }
    exit_qemu(QemuExitCode::Success);
}
```

Instead of reusing the `test_runner` from our `lib.rs`, the test defines its own `test_runner` function that exits with a failure exit code when a test returns without panicking (we want our tests to panic). If no test function is defined, the runner exits with a success error code. Since the runner always exits after running a single test, it does not make sense to define more than one `#[test_case]` function.

Now we can create a test that should fail:

```rust
// in tests/should_panic.rs

use blog_os::serial_print;

#[test_case]
fn should_fail() {
    serial_print!("should_fail... ");
    assert_eq!(0, 1);
}
```

The test uses the `assert_eq` to assert that `0` and `1` are equal. This of course fails, so that our test panics as desired.

When we run the test through `cargo xtest --test should_panic` we see that it is successful because the test panicked as expected. When we comment out the assertion and run the test again, we see that it indeed fails with the _"test did not panic"_ message.

A significant drawback of this approach is that it only works for a single test function. With multiple `#[test_case]` functions, only the first function is executed because the execution cannot continue after the panic handler has been called. I currently don't know of a good way to solve this problem, so let me know if you have an idea!

### No Harness Tests

For integration tests that only have a single test function (like our `should_panic` test), the test runner isn't really needed. For cases like this, we can disable the test runner completely and run our test directly in the `_start` function.

The key to this is disable the `harness` flag for the test in the `Cargo.toml`, which defines whether a test runner is used for an integration test. When it's set to `false`, both the default test runner and the custom test runner feature are disabled, so that the test is treated like a normal executable.

Let's disable the `harness` flag for our `should_panic` test:

```toml
# in Cargo.toml

[[test]]
name = "should_panic"
harness = false
```

Now we vastly simplify our `should_panic` test by removing the test runner related code. The result looks like this:

```rust
// in tests/should_panic.rs

#![no_std]
#![no_main]

use core::panic::PanicInfo;
use blog_os::{exit_qemu, serial_print, serial_println, QemuExitCode};

#[no_mangle]
pub extern "C" fn _start() -> ! {
    should_fail();
    serial_println!("[test did not panic]");
    exit_qemu(QemuExitCode::Failed);
    loop{}
}

fn should_fail() {
    serial_print!("should_fail... ");
    assert_eq!(0, 1);
}

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    serial_println!("[ok]");
    exit_qemu(QemuExitCode::Success);
    loop {}
}
```

We now call the `should_fail` function directly from our `_start` function and exit with a failure exit code if it returns. When we run `cargo xtest --test should_panic` now, we see that the test behaves exactly as before.

Apart from creating `should_panic` tests, disabling the `harness` attribute can also be useful for complex integration tests, for example when the individual test functions have side effects and need to be run in a specified order.

## Summary

Testing is a very useful technique to ensure that certain components have a desired behavior. Even if they cannot show the absence of bugs, they're still an useful tool for finding them and especially for avoiding regressions.

This post explained how to set up a test framework for our Rust kernel. We used the custom test frameworks feature of Rust to implement support for a simple `#[test_case]` attribute in our bare-metal environment. By using the `isa-debug-exit` device of QEMU, our test runner can exit QEMU after running the tests and report the test status out. To print error messages to the console instead of the VGA buffer, we created a basic driver for the serial port.

After creating some tests for our `println` macro, we explored integration tests in the second half of the post. We learned that they live in the `tests` directory and are treated as completely separate executables. To give them access to the `exit_qemu` function and the `serial_println` macro, we moved most of our code into a library that can be imported by all executables and integration tests. Since integration tests run in their own separate environment, they make it possible to test interactions with the hardware or to create tests that should panic.

We now have a test framework that runs in a realistic environment inside QEMU. By creating more tests in future posts, we can keep our kernel maintainable when it becomes more complex.

## What's next?

In the next post, we will explore _CPU exceptions_. These exceptions are thrown by the CPU when something illegal happens, such as a division by zero or an access to an unmapped memory page (a so-called “page fault”). Being able to catch and examine these exceptions is very important for debugging future errors. Exception handling is also very similar to the handling of hardware interrupts, which is required for keyboard support.
