rust: kunit: allow to know if we are in a test

In some cases, you need to call test-only code from outside the test
case, for example, to mock a function or a module.

In order to check whether we are in a test or not, we need to test if
`CONFIG_KUNIT` is set.
Unfortunately, we cannot rely only on this condition because some
distros compile KUnit in production kernels, so checking at runtime
that `current->kunit_test != NULL` is required.

Note that the C function `kunit_get_current_test()` can not be used
because it is not present in the current Rust tree yet. Once it is
available we might want to change our Rust wrapper to use it.

This patch adds a function to know whether we are in a KUnit test or
not and examples showing how to mock a function and a module.

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>

rust/kernel/kunit.rs

@@ -6,6 +6,8 @@
 //!
 //! Reference: <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>
 
+use crate::task::Task;
+use core::ops::Deref;
 use macros::kunit_tests;
 
 /// Asserts that a boolean expression is `true` at runtime.
@@ -183,11 +185,86 @@ macro_rules! kunit_unsafe_test_suite {
     };
 }
 
+/// In some cases, you need to call test-only code from outside the test case, for example, to
+/// create a function mock. This function can be invoked to know whether we are currently running a
+/// KUnit test or not.
+///
+/// # Examples
+///
+/// This example shows how a function can be mocked to return a well-known value while testing:
+///
+/// ```
+/// # use kernel::kunit::in_kunit_test;
+/// #
+/// fn fn_mock_example(n: i32) -> i32 {
+///     if in_kunit_test() {
+///         100
+///     } else {
+///         n + 1
+///     }
+/// }
+///
+/// let mock_res = fn_mock_example(5);
+/// assert_eq!(mock_res, 100);
+/// ```
+///
+/// Sometimes, you don't control the code that needs to be mocked. This example shows how the
+/// `bindings` module can be mocked:
+///
+/// ```
+/// // Import our mock naming it as the real module.
+/// #[cfg(CONFIG_KUNIT)]
+/// use bindings_mock_example as bindings;
+///
+/// // This module mocks `bindings`.
+/// mod bindings_mock_example {
+///     use kernel::kunit::in_kunit_test;
+///     use kernel::bindings::u64_;
+///
+///     // Make the other binding functions available.
+///     pub(crate) use kernel::bindings::*;
+///
+///     // Mock `ktime_get_boot_fast_ns` to return a well-known value when running a KUnit test.
+///     pub(crate) unsafe fn ktime_get_boot_fast_ns() -> u64_ {
+///         if in_kunit_test() {
+///             1234
+///         } else {
+///             unsafe { kernel::bindings::ktime_get_boot_fast_ns() }
+///         }
+///     }
+/// }
+///
+/// // This is the function we want to test. Since `bindings` has been mocked, we can use its
+/// // functions seamlessly.
+/// fn get_boot_ns() -> u64 {
+///     unsafe { bindings::ktime_get_boot_fast_ns() }
+/// }
+///
+/// let time = get_boot_ns();
+/// assert_eq!(time, 1234);
+/// ```
+pub fn in_kunit_test() -> bool {
+    if cfg!(CONFIG_KUNIT) {
+        let test = unsafe { (*Task::current().deref().0.get()).kunit_test };
+        !test.is_null()
+    } else {
+        false
+    }
+}
+
 #[kunit_tests(rust_kernel_kunit)]
 mod tests {
+    use super::*;
+
     #[test]
     fn rust_test_kunit_kunit_tests() {
         let running = true;
         assert_eq!(running, true);
     }
+
+    #[test]
+    fn rust_test_kunit_in_kunit_test() {
+        let in_kunit = in_kunit_test();
+        assert_eq!(in_kunit, true);
+    }
 }