Implement msgqueue_putback.

This function is called when we wish to return a message to the
queue after examining it.  It can also be used by the resender in
the REQ protocol.  Critically it does not disrupt the ordering
of other messages. This is a non-blocking operation.

1 files changed, 35 insertions, 0 deletions

diff --git a/src/core/msgqueue.c b/src/core/msgqueue.c
index 41127c50..3af4e081 100644
--- a/src/core/msgqueue.c
+++ b/src/core/msgqueue.c
@@ -198,6 +198,41 @@ nni_msgqueue_put_impl(nni_msgqueue *mq, nni_msg *msg,
 }
 
 
+// nni_msgqueue_putback will attempt to put a single message back
+// to the head of the queue.  It never blocks.  Message queues always
+// have room for at least one putback.
+int
+nni_msgqueue_putback(nni_msgqueue *mq, nni_msg *msg)
+{
+	nni_mutex_enter(&mq->mq_lock);
+
+	// if closed, we don't put more... this check is first!
+	if (mq->mq_closed) {
+		nni_mutex_exit(&mq->mq_lock);
+		return (NNG_ECLOSED);
+	}
+
+	// room in the queue?
+	if (mq->mq_len >= mq->mq_cap) {
+		nni_mutex_exit(&mq->mq_lock);
+		return (NNG_EAGAIN);
+	}
+
+	// Subtract one from the get index, possibly wrapping.
+	mq->mq_get--;
+	if (mq->mq_get == 0) {
+		mq->mq_get = mq->mq_cap;
+	}
+	mq->mq_msgs[mq->mq_get] = msg;
+	mq->mq_len++;
+	if (mq->mq_rwait) {
+		nni_cond_broadcast(&mq->mq_readable);
+	}
+	nni_mutex_exit(&mq->mq_lock);
+	return (0);
+}
+
+
 static int
 nni_msgqueue_get_impl(nni_msgqueue *mq, nni_msg **msgp,
     nni_time expire, nni_signal *signal)