write_queue: Enable updating max_length field

Dequeue and free any excess messages, in case the new queue length
is shorter than the old.

Related: OS#5774
Change-Id: Ibfe51a2faf29f8ae160a9c330c9af0d09b5a9002

include/osmocom/core/write_queue.h

@@ -50,6 +50,7 @@ void osmo_wqueue_init(struct osmo_wqueue *queue, int max_length);
 void osmo_wqueue_clear(struct osmo_wqueue *queue);
 int osmo_wqueue_enqueue(struct osmo_wqueue *queue, struct msgb *data);
 int osmo_wqueue_enqueue_quiet(struct osmo_wqueue *queue, struct msgb *data);
+size_t osmo_wqueue_set_maxlen(struct osmo_wqueue *queue, unsigned int len);
 int osmo_wqueue_bfd_cb(struct osmo_fd *fd, unsigned int what);
 
 /*! @} */

src/core/libosmocore.map

@@ -565,6 +565,7 @@ osmo_wqueue_bfd_cb;
 osmo_wqueue_clear;
 osmo_wqueue_enqueue;
 osmo_wqueue_enqueue_quiet;
+osmo_wqueue_set_maxlen;
 osmo_wqueue_init;
 rate_ctr_add;
 rate_ctr_difference;

src/core/write_queue.c

@@ -147,4 +147,24 @@ void osmo_wqueue_clear(struct osmo_wqueue *queue)
 	queue->bfd.when &= ~OSMO_FD_WRITE;
 }
 
+/* Update write queue length & drop excess messages.
+ * \param[in] queue linked list header of message queue
+ * \param[in] len new max. wqueue length
+ * \returns Number of messages dropped.
+ *
+ * Messages beyond the new maximum message queue size will be dropped.
+ */
+size_t osmo_wqueue_set_maxlen(struct osmo_wqueue *queue, unsigned int len)
+{
+	size_t dropped_msgs = 0;
+	struct msgb *msg;
+	queue->max_length = len;
+	while (queue->current_length > queue->max_length) {
+		msg = msgb_dequeue_count(&queue->msg_queue, &queue->current_length);
+		msgb_free(msg);
+		dropped_msgs++;
+	}
+	return dropped_msgs;
+}
+
 /*! @} */

tests/write_queue/wqueue_test.c

@@ -1,3 +1,14 @@
+/*
+ * (C) 2023 by sysmocom - s.f.m.c. GmbH.
+ * Authors: Holger Hans Peter Freyther
+ *	    Alexander Rehbein
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
 #include <osmocom/core/logging.h>
 #include <osmocom/core/utils.h>
 #include <osmocom/core/write_queue.h>
@@ -15,6 +26,7 @@ static void test_wqueue_limit(void)
 	struct msgb *msg;
 	struct osmo_wqueue wqueue;
 	int rc;
+	size_t dropped_msgs;
 
 	osmo_wqueue_init(&wqueue, 0);
 	OSMO_ASSERT(wqueue.max_length == 0);
@@ -63,6 +75,46 @@ static void test_wqueue_limit(void)
 	OSMO_ASSERT(wqueue.current_length == 2);
 	msgb_free(msg);
 	osmo_wqueue_clear(&wqueue);
+
+	/* Update limit */
+	OSMO_ASSERT(osmo_wqueue_set_maxlen(&wqueue, 5) == 0);
+	OSMO_ASSERT(osmo_wqueue_set_maxlen(&wqueue, 1) == 0);
+	OSMO_ASSERT(osmo_wqueue_set_maxlen(&wqueue, 4) == 0);
+
+	/* Add three, update limit to 1 */
+	OSMO_ASSERT(wqueue.max_length == 4);
+	msg = msgb_alloc(4096, "msg6");
+	rc = osmo_wqueue_enqueue(&wqueue, msg);
+	OSMO_ASSERT(rc == 0);
+	OSMO_ASSERT(wqueue.current_length == 1);
+	msg = msgb_alloc(4096, "msg7");
+	rc = osmo_wqueue_enqueue(&wqueue, msg);
+	OSMO_ASSERT(rc == 0);
+	OSMO_ASSERT(wqueue.current_length == 2);
+	msg = msgb_alloc(4096, "msg8");
+	rc = osmo_wqueue_enqueue(&wqueue, msg);
+	OSMO_ASSERT(wqueue.current_length == 3);
+	dropped_msgs = osmo_wqueue_set_maxlen(&wqueue, 1);
+	OSMO_ASSERT(dropped_msgs == 2);
+	osmo_wqueue_clear(&wqueue);
+
+	/* Add three, reduce limit to 3 from 6 */
+	OSMO_ASSERT(osmo_wqueue_set_maxlen(&wqueue, 6) == 0);
+	OSMO_ASSERT(wqueue.max_length == 6);
+	msg = msgb_alloc(4096, "msg9");
+	rc = osmo_wqueue_enqueue(&wqueue, msg);
+	OSMO_ASSERT(rc == 0);
+	OSMO_ASSERT(wqueue.current_length == 1);
+	msg = msgb_alloc(4096, "msg10");
+	rc = osmo_wqueue_enqueue(&wqueue, msg);
+	OSMO_ASSERT(rc == 0);
+	OSMO_ASSERT(wqueue.current_length == 2);
+	msg = msgb_alloc(4096, "msg11");
+	rc = osmo_wqueue_enqueue(&wqueue, msg);
+	OSMO_ASSERT(wqueue.current_length == 3);
+	dropped_msgs = osmo_wqueue_set_maxlen(&wqueue, 3);
+	OSMO_ASSERT(dropped_msgs == 0);
+	osmo_wqueue_clear(&wqueue);
 }
 
 int main(int argc, char **argv)