tests: eventfd: blocking read-write test for deadlock bug

Previously, the prototypical use case for `eventfd()` was not
possible in Zephyr due to a deadlock associated with the
`fdtable.c` `k_mutex`.

The prototypical use case for `eventfd()` is to make a
blocking call to `eventfd_read()` from one thread, and then
from another thread, call `eventfd_write()` to "wake up" the
reading thread.

This additional test case ensures that Zephyr supports the
prototypical `eventfd()` use case.

Signed-off-by: Christopher Friedt <[email protected]>

Author: Christopher Friedt

tests/posix/eventfd/src/blocking.c

@@ -70,3 +70,30 @@ ZTEST_F(eventfd, test_unset_poll_event_block)
 {
 	eventfd_poll_unset_common(fixture->fd);
 }
+
+K_THREAD_STACK_DEFINE(thread_stack, 2048);
+
+static void thread_fun(void *arg1, void *arg2, void *arg3)
+{
+	eventfd_t value;
+	struct eventfd_fixture *fixture = arg1;
+
+	zassert_ok(eventfd_read(fixture->fd, &value));
+	zassert_equal(value, 42);
+}
+
+ZTEST_F(eventfd, test_read_then_write_block)
+{
+	struct k_thread thread;
+
+	k_thread_create(&thread, thread_stack, K_THREAD_STACK_SIZEOF(thread_stack), thread_fun,
+			fixture, NULL, NULL, 0, 0, K_NO_WAIT);
+
+	k_msleep(100);
+
+	/* this write never occurs */
+	zassert_ok(eventfd_write(fixture->fd, 42));
+
+	/* unreachable code */
+	k_thread_join(&thread, K_FOREVER);
+}