kernel: queue, fifo: Add cancel_wait operation.

Currently, a queue/fifo getter chooses how long to wait for an
element. But there are scenarios when putter would know better,
there should be a way to expire getter's timeout to make it run
again. k_queue_cancel_wait() and k_fifo_cancel_wait() functions
do just that. They cause corresponding *_get() functions to return
with NULL value, as if timeout expired on getter's side (even
K_FOREVER).

This can be used to signal out of band conditions from putter to
getter, e.g. end of processing, error, configuration change, etc.
A specific event would be communicated to getter by other means
(e.g. using existing shared context structures).

Without this call, achieving the same effect would require e.g.
calling k_fifo_put() with a pointer to a special sentinal memory
structure - such structure would need to be allocated somewhere
and somehow, and getter would need to recognize it from a normal
data item. Having cancel_wait() functions offers an elegant
alternative. From this perspective, these calls can be seen as
an equivalent to e.g. k_fifo_put(fifo, NULL), except that such
call won't work in practice.

Change-Id: I47b7f690dc325a80943082bcf5345c41649e7024
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>

include/kernel.h

@@ -1274,6 +1274,20 @@ struct k_queue {
  */
 extern void k_queue_init(struct k_queue *queue);
 
+/**
+ * @brief Cancel waiting on a queue.
+ *
+ * This routine causes first thread pending on @a queue, if any, to
+ * return from k_queue_get() call with NULL value (as if timeout expired).
+ *
+ * @note Can be called by ISRs.
+ *
+ * @param queue Address of the queue.
+ *
+ * @return N/A
+ */
+extern void k_queue_cancel_wait(struct k_queue *queue);
+
 /**
  * @brief Append an element to the end of a queue.
  *
@@ -1445,6 +1459,22 @@ struct k_fifo {
 #define k_fifo_init(fifo) \
 	k_queue_init((struct k_queue *) fifo)
 
+/**
+ * @brief Cancel waiting on a fifo.
+ *
+ * This routine causes first thread pending on @a fifo, if any, to
+ * return from k_fifo_get() call with NULL value (as if timeout
+ * expired).
+ *
+ * @note Can be called by ISRs.
+ *
+ * @param fifo Address of the fifo.
+ *
+ * @return N/A
+ */
+#define k_fifo_cancel_wait(fifo) \
+	k_queue_cancel_wait((struct k_queue *) fifo)
+
 /**
  * @brief Add an element to a fifo.
  *