2 files changed, 41 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)
diff --git a/src/core/msgqueue.h b/src/core/msgqueue.h
index 5efbc245..900fed2b 100644
--- a/src/core/msgqueue.h
+++ b/src/core/msgqueue.h
@@ -45,6 +45,12 @@ extern void nni_msgqueue_destroy(nni_msgqueue *);
 // the caller is not permitted to access it further.
 extern int nni_msgqueue_put(nni_msgqueue *, nni_msg *);
 
+// nni_msgqueue_putback returns a message to the head of the queue.
+// This is a non-blocking operation, and it returns EAGAIN if there
+// is no room.  There is always at least room for one putback after
+// a message is retried with nni_msgqueue_get.
+extern int nni_msgqueue_putback(nni_msgqueue *, nni_msg *);
+
 // nni_msgqueue_get gets the message from the queue.  It blocks until a
 // message is available, or the queue is closed, returning either 0 on
 // success or NNG_ECLOSED if the queue was closed.  If a message is