kunit: Expose 'static stub' API to redirect functions

Add a simple way of redirecting calls to functions by including a
special prologue in the "real" function which checks to see if the
replacement function should be called (and, if so, calls it).

To redirect calls to a function, make the first (non-declaration) line
of the function:

	KUNIT_STATIC_STUB_REDIRECT(function_name, [function arguments]);

(This will compile away to nothing if KUnit is not enabled, otherwise it
will check if a redirection is active, call the replacement function,
and return. This check is protected by a static branch, so has very
little overhead when there are no KUnit tests running.)

Calls to the real function can be redirected to a replacement using:

	kunit_activate_static_stub(test, real_fn, replacement_fn);

The redirection will only affect calls made from within the kthread of
the current test, and will be automatically disabled when the test
completes. It can also be manually disabled with
kunit_deactivate_static_stub().

The 'example' KUnit test suite has a more complete example.

Co-developed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

1 files changed, 38 insertions, 0 deletions

diff --git a/lib/kunit/kunit-example-test.c b/lib/kunit/kunit-example-test.c
index 66cc4e2365ec..cd8b7e51d02b 100644
--- a/lib/kunit/kunit-example-test.c
+++ b/lib/kunit/kunit-example-test.c
@@ -7,6 +7,7 @@
  */
 
 #include <kunit/test.h>
+#include <kunit/static_stub.h>
 
 /*
  * This is the most fundamental element of KUnit, the test case. A test case
@@ -130,6 +131,42 @@ static void example_all_expect_macros_test(struct kunit *test)
 	KUNIT_ASSERT_GT_MSG(test, sizeof(int), 0, "Your ints are 0-bit?!");
 }
 
+/* This is a function we'll replace with static stubs. */
+static int add_one(int i)
+{
+	/* This will trigger the stub if active. */
+	KUNIT_STATIC_STUB_REDIRECT(add_one, i);
+
+	return i + 1;
+}
+
+/* This is used as a replacement for the above function. */
+static int subtract_one(int i)
+{
+	/* We don't need to trigger the stub from the replacement. */
+
+	return i - 1;
+}
+
+/*
+ * This test shows the use of static stubs.
+ */
+static void example_static_stub_test(struct kunit *test)
+{
+	/* By default, function is not stubbed. */
+	KUNIT_EXPECT_EQ(test, add_one(1), 2);
+
+	/* Replace add_one() with subtract_one(). */
+	kunit_activate_static_stub(test, add_one, subtract_one);
+
+	/* add_one() is now replaced. */
+	KUNIT_EXPECT_EQ(test, add_one(1), 0);
+
+	/* Return add_one() to normal. */
+	kunit_deactivate_static_stub(test, add_one);
+	KUNIT_EXPECT_EQ(test, add_one(1), 2);
+}
+
 /*
  * Here we make a list of all the test cases we want to add to the test suite
  * below.
@@ -145,6 +182,7 @@ static struct kunit_case example_test_cases[] = {
 	KUNIT_CASE(example_skip_test),
 	KUNIT_CASE(example_mark_skipped_test),
 	KUNIT_CASE(example_all_expect_macros_test),
+	KUNIT_CASE(example_static_stub_test),
 	{}
 };