Problem: cannot monitor state of queues at runtime

Solution: add API and ZMQ_EVENT_PIPES_STATS event which generates 2
values, one for the egress and one for the ingress pipes respectively.
Refactor the events code to be able to send multiple values.

doc/zmq_socket_monitor_versioned.txt

@@ -12,6 +12,8 @@ SYNOPSIS
 --------
 *int zmq_socket_monitor_versioned (void '*socket', char '*endpoint', uint64_t 'events', int 'event_version');*
 
+*int zmq_socket_monitor_pipes_stats (void '*socket');*
+
 
 DESCRIPTION
 -----------
@@ -41,18 +43,23 @@ Each event is sent in multiple frames. The first frame contains an event
 number (64 bits). The number and content of further frames depend on this
 event number.
 
-Unless it is specified differently, the second frame contains an event
-value (64 bits) that provides additional data according to the event number.
-Some events might define additional value frames following the second one.
-The third and fourth frames contain strings that specifies the affected
-connection or endpoint. The third frame contains a string denoting the local
-endpoint, while the fourth frame contains a string denoting the remote endpoint.
+Unless it is specified differently, the second frame contains the number of
+value frames that will follow it as a 64 bits integer. The third frame to N-th
+frames contain an event value (64 bits) that provides additional data according
+to the event number. Each event type might have a different number of values.
+The second-to-last and last frames contain strings that specifies the affected
+connection or endpoint. The former frame contains a string denoting the local
+endpoint, while the latter frame contains a string denoting the remote endpoint.
 Either of these may be empty, depending on the event type and whether the
 connection uses a bound or connected local endpoint.
 
 Note that the format of the second and further frames, and also the number of
 frames, may be different for events added in the future.
 
+The _zmq_socket_monitor_pipes_stats()_ method triggers an event of type
+ZMQ_EVENT_PIPES_STATS for each connected peer of the monitored socket.
+NOTE: _zmq_socket_monitor_pipes_stats()_ is in DRAFT state.
+
 ----
 Monitoring events are only generated by some transports: At the moment these
 are SOCKS, TCP, IPC, and TIPC. Note that it is not an error to call
@@ -168,17 +175,35 @@ The ZMTP security mechanism handshake failed due to an authentication failure.
 The event value is the status code returned by the ZAP handler (i.e. 300, 
 400 or 500).
 
+----
+
+Supported events (v2)
+----------------
+
+ZMQ_EVENT_PIPE_STATS
+~~~~~~~~~~~~~~~~~~~~
+This event provides two values, the number of messages in each of the two
+queues associated with the returned endpoint (respectively egress and ingress).
+This event only triggers after calling the function
+_zmq_socket_monitor_pipes_stats()_.
+NOTE: this measurement is asynchronous, so by the time the message is received
+the internal state might have already changed.
+NOTE: when the monitored socket and the monitor are not used in a poll, the
+event might not be delivered until an API has been called on the monitored
+socket, like zmq_getsockopt for example (the option is irrelevant).
+NOTE: in DRAFT state, not yet available in stable releases.
+
 
 
 RETURN VALUE
 ------------
-The _zmq_socket_monitor()_ function returns a value of 0 or greater if
-successful. Otherwise it returns `-1` and sets 'errno' to one of the values
-defined below.
+The _zmq_socket_monitor()_ and _zmq_socket_monitor_pipes_stats()_ functions
+return a value of 0 or greater if successful. Otherwise they return `-1` and
+set 'errno' to one of the values defined below.
 
 
-ERRORS
-------
+ERRORS - _zmq_socket_monitor()_
+-------------------------------
 *ETERM*::
 The 0MQ 'context' associated with the specified 'socket' was terminated.
 
@@ -189,11 +214,23 @@ sockets are required to use the inproc:// transport.
 *EINVAL*::
 The monitor 'endpoint' supplied does not exist.
 
+
+ERRORS - _zmq_socket_monitor_pipes_stats()_
+-------------------------------------------
+*ENOTSOCK*::
+The 'socket' parameter was not a valid 0MQ socket.
+
+*EINVAL*::
+The socket did not have monitoring enabled.
+
+*EAGAIN*::
+The monitored socket did not have any connections to monitor yet.
+
 EXAMPLE
 -------
 .Monitoring client and server sockets
 ----
-//  Read one event off the monitor socket; return value and address
+//  Read one event off the monitor socket; return values and addresses
 //  by reference, if not null, and event number by value. Returns -1
 //  in case of error.
 
@@ -211,18 +248,29 @@ get_monitor_event (void *monitor, uint64_t *value, char **local_address, char **
     memcpy (&event, zmq_msg_data (&msg), sizeof (event));
     zmq_msg_close (&msg);
 
-    //  Second frame to Nth frame (depends on the event) in message
-    //  contains event value
+    //  Second frame in message contains the number of values
     zmq_msg_init (&msg);
     if (zmq_msg_recv (&msg, monitor, 0) == -1)
         return -1;              //  Interrupted, presumably
     assert (zmq_msg_more (&msg));
 
-    if (value_)
-        memcpy (value_, zmq_msg_data (&msg), sizeof *value_);
+    uint64_t value_count;
+    memcpy (&value_count, zmq_msg_data (&msg), sizeof (value_count));
     zmq_msg_close (&msg);
 
-    //  Third frame in message contains local address
+    for (uint64_t i = 0; i < value_count; ++i) {
+        //  Subsequent frames in message contain event values
+        zmq_msg_init (&msg);
+        if (zmq_msg_recv (&msg, monitor, 0) == -1)
+            return -1;              //  Interrupted, presumably
+        assert (zmq_msg_more (&msg));
+
+        if (value_ && value_ + i)
+            memcpy (value_ + i, zmq_msg_data (&msg), sizeof (*value_));
+        zmq_msg_close (&msg);
+    }
+
+    //  Second-to-last frame in message contains local address
     zmq_msg_init (&msg);
     if (zmq_msg_recv (&msg, monitor, 0) == -1)
         return -1;              //  Interrupted, presumably
@@ -237,7 +285,7 @@ get_monitor_event (void *monitor, uint64_t *value, char **local_address, char **
     }
     zmq_msg_close (&msg);
 
-    //  Fourth frame in message contains remote address
+    //  Last frame in message contains remote address
     zmq_msg_init (&msg);
     if (zmq_msg_recv (&msg, monitor, 0) == -1)
         return -1;              //  Interrupted, presumably

include/zmq.h

@@ -726,16 +726,20 @@ ZMQ_EXPORT int zmq_socket_get_peer_state (void *socket,
                                           const void *routing_id,
                                           size_t routing_id_size);
 
+/*  DRAFT Socket monitoring events                                            */
+#define ZMQ_EVENT_PIPES_STATS 0x10000
+
 #define ZMQ_CURRENT_EVENT_VERSION 1
 #define ZMQ_CURRENT_EVENT_VERSION_DRAFT 2
 
 #define ZMQ_EVENT_ALL_V1 ZMQ_EVENT_ALL
-#define ZMQ_EVENT_ALL_V2 ZMQ_EVENT_ALL_V1
+#define ZMQ_EVENT_ALL_V2 ZMQ_EVENT_ALL_V1 | ZMQ_EVENT_PIPES_STATS
 
 ZMQ_EXPORT int zmq_socket_monitor_versioned (void *s_,
                                              const char *addr_,
                                              uint64_t events_,
                                              int event_version_);
+ZMQ_EXPORT int zmq_socket_monitor_pipes_stats (void *s);
 
 #endif // ZMQ_BUILD_DRAFT_API
 

src/command.hpp

@@ -32,6 +32,7 @@
 
 #include <string>
 #include "stdint.hpp"
+#include "endpoint.hpp"
 
 namespace zmq
 {
@@ -73,6 +74,8 @@ __declspec(align (64))
         reap,
         reaped,
         inproc_connected,
+        pipe_peer_stats,
+        pipe_stats_publish,
         done
     } type;
 
@@ -186,6 +189,23 @@ __declspec(align (64))
         {
         } reaped;
 
+        //  Send application-side pipe count and ask to send monitor event
+        struct
+        {
+            uint64_t queue_count;
+            zmq::own_t *socket_base;
+            endpoint_uri_pair_t *endpoint_pair;
+        } pipe_peer_stats;
+
+        //  Collate application thread and I/O thread pipe counts and endpoints
+        //  and send as event
+        struct
+        {
+            uint64_t outbound_queue_count;
+            uint64_t inbound_queue_count;
+            endpoint_uri_pair_t *endpoint_pair;
+        } pipe_stats_publish;
+
         //  Sent by reaper thread to the term thread when all the sockets
         //  are successfully deallocated.
         struct

src/object.cpp

@@ -107,6 +107,19 @@ void zmq::object_t::process_command (command_t &cmd_)
             process_hiccup (cmd_.args.hiccup.pipe);
             break;
 
+        case command_t::pipe_peer_stats:
+            process_pipe_peer_stats (cmd_.args.pipe_peer_stats.queue_count,
+                                     cmd_.args.pipe_peer_stats.socket_base,
+                                     cmd_.args.pipe_peer_stats.endpoint_pair);
+            break;
+
+        case command_t::pipe_stats_publish:
+            process_pipe_stats_publish (
+              cmd_.args.pipe_stats_publish.outbound_queue_count,
+              cmd_.args.pipe_stats_publish.inbound_queue_count,
+              cmd_.args.pipe_stats_publish.endpoint_pair);
+            break;
+
         case command_t::pipe_term:
             process_pipe_term ();
             break;
@@ -285,6 +298,35 @@ void zmq::object_t::send_hiccup (pipe_t *destination_, void *pipe_)
     send_command (cmd);
 }
 
+void zmq::object_t::send_pipe_peer_stats (pipe_t *destination_,
+                                          uint64_t queue_count_,
+                                          own_t *socket_base_,
+                                          endpoint_uri_pair_t *endpoint_pair_)
+{
+    command_t cmd;
+    cmd.destination = destination_;
+    cmd.type = command_t::pipe_peer_stats;
+    cmd.args.pipe_peer_stats.queue_count = queue_count_;
+    cmd.args.pipe_peer_stats.socket_base = socket_base_;
+    cmd.args.pipe_peer_stats.endpoint_pair = endpoint_pair_;
+    send_command (cmd);
+}
+
+void zmq::object_t::send_pipe_stats_publish (
+  own_t *destination_,
+  uint64_t outbound_queue_count_,
+  uint64_t inbound_queue_count_,
+  endpoint_uri_pair_t *endpoint_pair_)
+{
+    command_t cmd;
+    cmd.destination = destination_;
+    cmd.type = command_t::pipe_stats_publish;
+    cmd.args.pipe_stats_publish.outbound_queue_count = outbound_queue_count_;
+    cmd.args.pipe_stats_publish.inbound_queue_count = inbound_queue_count_;
+    cmd.args.pipe_stats_publish.endpoint_pair = endpoint_pair_;
+    send_command (cmd);
+}
+
 void zmq::object_t::send_pipe_term (pipe_t *destination_)
 {
     command_t cmd;
@@ -422,6 +464,20 @@ void zmq::object_t::process_hiccup (void *)
     zmq_assert (false);
 }
 
+void zmq::object_t::process_pipe_peer_stats (uint64_t,
+                                             own_t *,
+                                             endpoint_uri_pair_t *)
+{
+    zmq_assert (false);
+}
+
+void zmq::object_t::process_pipe_stats_publish (uint64_t,
+                                                uint64_t,
+                                                endpoint_uri_pair_t *)
+{
+    zmq_assert (false);
+}
+
 void zmq::object_t::process_pipe_term ()
 {
     zmq_assert (false);

src/object.hpp

@@ -32,6 +32,7 @@
 
 #include <string>
 #include "stdint.hpp"
+#include "endpoint.hpp"
 
 namespace zmq
 {
@@ -96,6 +97,14 @@ class object_t
     void send_activate_read (zmq::pipe_t *destination_);
     void send_activate_write (zmq::pipe_t *destination_, uint64_t msgs_read_);
     void send_hiccup (zmq::pipe_t *destination_, void *pipe_);
+    void send_pipe_peer_stats (zmq::pipe_t *destination_,
+                               uint64_t queue_count_,
+                               zmq::own_t *socket_base,
+                               endpoint_uri_pair_t *endpoint_pair_);
+    void send_pipe_stats_publish (zmq::own_t *destination_,
+                                  uint64_t outbound_queue_count_,
+                                  uint64_t inbound_queue_count_,
+                                  endpoint_uri_pair_t *endpoint_pair_);
     void send_pipe_term (zmq::pipe_t *destination_);
     void send_pipe_term_ack (zmq::pipe_t *destination_);
     void send_pipe_hwm (zmq::pipe_t *destination_, int inhwm_, int outhwm_);
@@ -117,6 +126,13 @@ class object_t
     virtual void process_activate_read ();
     virtual void process_activate_write (uint64_t msgs_read_);
     virtual void process_hiccup (void *pipe_);
+    virtual void process_pipe_peer_stats (uint64_t queue_count_,
+                                          zmq::own_t *socket_base_,
+                                          endpoint_uri_pair_t *endpoint_pair_);
+    virtual void
+    process_pipe_stats_publish (uint64_t outbound_queue_count_,
+                                uint64_t inbound_queue_count_,
+                                endpoint_uri_pair_t *endpoint_pair_);
     virtual void process_pipe_term ();
     virtual void process_pipe_term_ack ();
     virtual void process_pipe_hwm (int inhwm_, int outhwm_);

src/pipe.cpp

@@ -563,3 +563,19 @@ const zmq::endpoint_uri_pair_t &zmq::pipe_t::get_endpoint_pair () const
 {
     return _endpoint_pair;
 }
+
+void zmq::pipe_t::send_stats_to_peer (own_t *socket_base_)
+{
+    endpoint_uri_pair_t *ep =
+      new (std::nothrow) endpoint_uri_pair_t (_endpoint_pair);
+    send_pipe_peer_stats (_peer, _msgs_written - _peers_msgs_read, socket_base_,
+                          ep);
+}
+
+void zmq::pipe_t::process_pipe_peer_stats (uint64_t queue_count_,
+                                           own_t *socket_base_,
+                                           endpoint_uri_pair_t *endpoint_pair_)
+{
+    send_pipe_stats_publish (socket_base_, queue_count_,
+                             _msgs_written - _peers_msgs_read, endpoint_pair_);
+}

src/pipe.hpp

@@ -145,6 +145,8 @@ class pipe_t : public object_t,
     void set_endpoint_pair (endpoint_uri_pair_t endpoint_pair_);
     const endpoint_uri_pair_t &get_endpoint_pair () const;
 
+    void send_stats_to_peer (own_t *socket_base_);
+
   private:
     //  Type of the underlying lock-free pipe.
     typedef ypipe_base_t<msg_t> upipe_t;
@@ -153,6 +155,9 @@ class pipe_t : public object_t,
     void process_activate_read ();
     void process_activate_write (uint64_t msgs_read_);
     void process_hiccup (void *pipe_);
+    void process_pipe_peer_stats (uint64_t queue_count_,
+                                  own_t *socket_base_,
+                                  endpoint_uri_pair_t *endpoint_pair_);
     void process_pipe_term ();
     void process_pipe_term_ack ();
     void process_pipe_hwm (int inhwm_, int outhwm_);

src/session_base.cpp

@@ -409,6 +409,11 @@ void zmq::session_base_t::process_attach (i_engine *engine_)
         zmq_assert (!_pipe);
         _pipe = pipes[0];
 
+        //  The endpoints strings are not set on bind, set them here so that
+        //  events can use them.
+        pipes[0]->set_endpoint_pair (engine_->get_endpoint ());
+        pipes[1]->set_endpoint_pair (engine_->get_endpoint ());
+
         //  Ask socket to plug into the remote end of the pipe.
         send_bind (_socket, pipes[1]);
     }

src/socket_base.cpp

@@ -1421,6 +1421,45 @@ void zmq::socket_base_t::process_term_endpoint (std::string *endpoint_)
     delete endpoint_;
 }
 
+void zmq::socket_base_t::process_pipe_stats_publish (
+  uint64_t outbound_queue_count_,
+  uint64_t inbound_queue_count_,
+  endpoint_uri_pair_t *endpoint_pair_)
+{
+    uint64_t values[2] = {outbound_queue_count_, inbound_queue_count_};
+    event (*endpoint_pair_, values, 2, ZMQ_EVENT_PIPES_STATS);
+    delete endpoint_pair_;
+}
+
+/*
+ * There are 2 pipes per connection, and the inbound one _must_ be queried from
+ * the I/O thread. So ask the outbound pipe, in the application thread, to send
+ * a message (pipe_peer_stats) to its peer. The message will carry the outbound
+ * pipe stats and endpoint, and the reference to the socket object.
+ * The inbound pipe on the I/O thread will then add its own stats and endpoint,
+ * and write back a message to the socket object (pipe_stats_publish) which
+ * will raise an event with the data.
+ */
+int zmq::socket_base_t::query_pipes_stats ()
+{
+    {
+        scoped_lock_t lock (_monitor_sync);
+        if (!(_monitor_events & ZMQ_EVENT_PIPES_STATS)) {
+            errno = EINVAL;
+            return -1;
+        }
+    }
+    if (_pipes.size () == 0) {
+        errno = EAGAIN;
+        return -1;
+    }
+    for (pipes_t::size_type i = 0; i != _pipes.size (); ++i) {
+        _pipes[i]->send_stats_to_peer (this);
+    }
+
+    return 0;
+}
+
 void zmq::socket_base_t::update_pipe_options (int option_)
 {
     if (option_ == ZMQ_SNDHWM || option_ == ZMQ_RCVHWM) {

src/socket_base.hpp

@@ -157,6 +157,11 @@ class socket_base_t : public own_t,
     virtual int get_peer_state (const void *routing_id_,
                                 size_t routing_id_size_) const;
 
+    //  Request for pipes statistics - will generate a ZMQ_EVENT_PIPES_STATS
+    //  after gathering the data asynchronously. Requires event monitoring to
+    //  be enabled.
+    int query_pipes_stats ();
+
   protected:
     socket_base_t (zmq::ctx_t *parent_,
                    uint32_t tid_,
@@ -278,6 +283,9 @@ class socket_base_t : public own_t,
     //  Handlers for incoming commands.
     void process_stop ();
     void process_bind (zmq::pipe_t *pipe_);
+    void process_pipe_stats_publish (uint64_t outbound_queue_count_,
+                                     uint64_t inbound_queue_count_,
+                                     endpoint_uri_pair_t *endpoint_pair_);
     void process_term (int linger_);
     void process_term_endpoint (std::string *endpoint_);
 

src/zmq.cpp

@@ -1452,3 +1452,11 @@ int zmq_has (const char *capability_)
     //  Whatever the application asked for, we don't have
     return false;
 }
+
+int zmq_socket_monitor_pipes_stats (void *s_)
+{
+    zmq::socket_base_t *s = as_socket_base_t (s_);
+    if (!s)
+        return -1;
+    return s->query_pipes_stats ();
+}

src/zmq_draft.h

@@ -123,10 +123,20 @@ int zmq_socket_get_peer_state (void *socket_,
                                const void *routing_id_,
                                size_t routing_id_size_);
 
+/*  DRAFT Socket monitoring events                                            */
+#define ZMQ_EVENT_PIPES_STATS 0x10000
+
+#define ZMQ_CURRENT_EVENT_VERSION 1
+#define ZMQ_CURRENT_EVENT_VERSION_DRAFT 2
+
+#define ZMQ_EVENT_ALL_V1 ZMQ_EVENT_ALL
+#define ZMQ_EVENT_ALL_V2 ZMQ_EVENT_ALL_V1 | ZMQ_EVENT_PIPES_STATS
+
 int zmq_socket_monitor_versioned (void *s_,
                                   const char *addr_,
                                   uint64_t events_,
                                   int event_version_);
+int zmq_socket_monitor_pipes_stats (void *s_);
 
 #endif // ZMQ_BUILD_DRAFT_API
 

tests/test_monitor.cpp

@@ -50,6 +50,13 @@ void test_monitor_invalid_protocol_fails ()
     TEST_ASSERT_FAILURE_ERRNO (
       EPROTONOSUPPORT, zmq_socket_monitor (client, "tcp://127.0.0.1:*", 0));
 
+#ifdef ZMQ_EVENT_PIPES_STATS
+    //  Stats command needs to be called on a valid socket with monitoring
+    //  enabled
+    TEST_ASSERT_FAILURE_ERRNO (ENOTSOCK, zmq_socket_monitor_pipes_stats (NULL));
+    TEST_ASSERT_FAILURE_ERRNO (EINVAL, zmq_socket_monitor_pipes_stats (client));
+#endif
+
     test_context_socket_close_zero_linger (client);
 }
 
@@ -121,8 +128,9 @@ void test_monitor_basic ()
     test_context_socket_close_zero_linger (server_mon);
 }
 
-#if (defined ZMQ_CURRENT_EVENT_VERSION && ZMQ_CURRENT_EVENT_VERSION >= 2) || \
-    (defined ZMQ_CURRENT_EVENT_VERSION && ZMQ_CURRENT_EVENT_VERSION_DRAFT >= 2)
+#if (defined ZMQ_CURRENT_EVENT_VERSION && ZMQ_CURRENT_EVENT_VERSION >= 2)      \
+  || (defined ZMQ_CURRENT_EVENT_VERSION                                        \
+      && ZMQ_CURRENT_EVENT_VERSION_DRAFT >= 2)
 void test_monitor_versioned_basic (bind_function_t bind_function_,
                                    const char *expected_prefix_)
 {
@@ -242,6 +250,133 @@ void test_monitor_versioned_basic_tipc ()
     static const char prefix[] = "tipc://";
     test_monitor_versioned_basic (bind_loopback_tipc, prefix);
 }
+
+#ifdef ZMQ_EVENT_PIPES_STATS
+void test_monitor_versioned_stats (bind_function_t bind_function_,
+                                   const char *expected_prefix_)
+{
+    char server_endpoint[MAX_SOCKET_STRING];
+    const int pulls_count = 4;
+    void *pulls[pulls_count];
+
+    //  We'll monitor these two sockets
+    void *push = test_context_socket (ZMQ_PUSH);
+
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_socket_monitor_versioned (
+      push, "inproc://monitor-push", ZMQ_EVENT_PIPES_STATS, 2));
+
+    //  Should fail if there are no pipes to monitor
+    TEST_ASSERT_FAILURE_ERRNO (EAGAIN, zmq_socket_monitor_pipes_stats (push));
+
+    void *push_mon = test_context_socket (ZMQ_PAIR);
+
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_connect (push_mon, "inproc://monitor-push"));
+
+    //  Set lower HWM - queues will be filled so we should see it in the stats
+    int send_hwm = 500;
+    TEST_ASSERT_SUCCESS_ERRNO (
+      zmq_setsockopt (push, ZMQ_SNDHWM, &send_hwm, sizeof (send_hwm)));
+    //  Set very low TCP buffers so that messages cannot be stored in-flight
+    const int tcp_buffer_size = 4096;
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_setsockopt (
+      push, ZMQ_SNDBUF, &tcp_buffer_size, sizeof (tcp_buffer_size)));
+    bind_function_ (push, server_endpoint, sizeof (server_endpoint));
+
+    int ipv6_;
+    size_t ipv6_size_ = sizeof (ipv6_);
+    TEST_ASSERT_SUCCESS_ERRNO (
+      zmq_getsockopt (push, ZMQ_IPV6, &ipv6_, &ipv6_size_));
+    for (int i = 0; i < pulls_count; ++i) {
+        pulls[i] = test_context_socket (ZMQ_PULL);
+        TEST_ASSERT_SUCCESS_ERRNO (
+          zmq_setsockopt (pulls[i], ZMQ_IPV6, &ipv6_, sizeof (int)));
+        int timeout_ms = 10;
+        TEST_ASSERT_SUCCESS_ERRNO (zmq_setsockopt (
+          pulls[i], ZMQ_RCVTIMEO, &timeout_ms, sizeof (timeout_ms)));
+        TEST_ASSERT_SUCCESS_ERRNO (
+          zmq_setsockopt (pulls[i], ZMQ_RCVHWM, &send_hwm, sizeof (send_hwm)));
+        TEST_ASSERT_SUCCESS_ERRNO (zmq_setsockopt (
+          pulls[i], ZMQ_RCVBUF, &tcp_buffer_size, sizeof (tcp_buffer_size)));
+        TEST_ASSERT_SUCCESS_ERRNO (zmq_connect (pulls[i], server_endpoint));
+    }
+
+    //  Send until we block
+    int send_count = 0;
+    //  Saturate the TCP buffers too
+    char data[tcp_buffer_size * 2];
+    memset (data, 0, sizeof (data));
+    //  Saturate all pipes - send + receive - on all connections
+    while (send_count < send_hwm * 2 * pulls_count) {
+        TEST_ASSERT_EQUAL_INT (sizeof (data),
+                               zmq_send (push, data, sizeof (data), 0));
+        ++send_count;
+    }
+
+    //  Drain one of the pulls - doesn't matter how many messages, at least one
+    send_count = send_count / 4;
+    do {
+        zmq_recv (pulls[0], data, sizeof (data), 0);
+        --send_count;
+    } while (send_count > 0);
+
+    //  To kick the application thread, do a dummy getsockopt - users here
+    //  should use the monitor and the other sockets in a poll.
+    unsigned long int dummy;
+    size_t dummy_size = sizeof (dummy);
+    msleep (SETTLE_TIME);
+    //  Note that the pipe stats on the sender will not get updated until the
+    //  receiver has processed at least lwm ((hwm + 1) / 2) messages AND until
+    //  the application thread has ran through the mailbox, as the update is
+    //  delivered via a message (send_activate_write)
+    zmq_getsockopt (push, ZMQ_EVENTS, &dummy, &dummy_size);
+
+    //  Ask for stats and check that they match
+    zmq_socket_monitor_pipes_stats (push);
+
+    msleep (SETTLE_TIME);
+    zmq_getsockopt (push, ZMQ_EVENTS, &dummy, &dummy_size);
+
+    for (int i = 0; i < pulls_count; ++i) {
+        char *push_local_address = NULL;
+        char *push_remote_address = NULL;
+        uint64_t queue_stat[2];
+        int64_t event = get_monitor_event_v2 (
+          push_mon, queue_stat, &push_local_address, &push_remote_address);
+        TEST_ASSERT_EQUAL_STRING (server_endpoint, push_local_address);
+        TEST_ASSERT_EQUAL_STRING_LEN (expected_prefix_, push_remote_address,
+                                      strlen (expected_prefix_));
+        TEST_ASSERT_EQUAL_INT (ZMQ_EVENT_PIPES_STATS, event);
+        TEST_ASSERT_EQUAL_INT (i == 0 ? 0 : send_hwm, queue_stat[0]);
+        TEST_ASSERT_EQUAL_INT (0, queue_stat[1]);
+        free (push_local_address);
+        free (push_remote_address);
+    }
+
+    //  Close client and server
+    test_context_socket_close_zero_linger (push_mon);
+    test_context_socket_close_zero_linger (push);
+    for (int i = 0; i < pulls_count; ++i)
+        test_context_socket_close_zero_linger (pulls[i]);
+}
+
+void test_monitor_versioned_stats_tcp_ipv4 ()
+{
+    static const char prefix[] = "tcp://127.0.0.1:";
+    test_monitor_versioned_stats (bind_loopback_ipv4, prefix);
+}
+
+void test_monitor_versioned_stats_tcp_ipv6 ()
+{
+    static const char prefix[] = "tcp://[::1]:";
+    test_monitor_versioned_stats (bind_loopback_ipv6, prefix);
+}
+
+void test_monitor_versioned_stats_ipc ()
+{
+    static const char prefix[] = "ipc://";
+    test_monitor_versioned_stats (bind_loopback_ipc, prefix);
+}
+#endif // ZMQ_EVENT_PIPES_STATS
 #endif
 
 int main ()
@@ -252,12 +387,18 @@ int main ()
     RUN_TEST (test_monitor_invalid_protocol_fails);
     RUN_TEST (test_monitor_basic);
 
-#if (defined ZMQ_CURRENT_EVENT_VERSION && ZMQ_CURRENT_EVENT_VERSION >= 2) || \
-    (defined ZMQ_CURRENT_EVENT_VERSION && ZMQ_CURRENT_EVENT_VERSION_DRAFT >= 2)
+#if (defined ZMQ_CURRENT_EVENT_VERSION && ZMQ_CURRENT_EVENT_VERSION >= 2)      \
+  || (defined ZMQ_CURRENT_EVENT_VERSION                                        \
+      && ZMQ_CURRENT_EVENT_VERSION_DRAFT >= 2)
     RUN_TEST (test_monitor_versioned_basic_tcp_ipv4);
     RUN_TEST (test_monitor_versioned_basic_tcp_ipv6);
     RUN_TEST (test_monitor_versioned_basic_ipc);
     RUN_TEST (test_monitor_versioned_basic_tipc);
+#ifdef ZMQ_EVENT_PIPES_STATS
+    RUN_TEST (test_monitor_versioned_stats_tcp_ipv4);
+    RUN_TEST (test_monitor_versioned_stats_tcp_ipv6);
+    RUN_TEST (test_monitor_versioned_stats_ipc);
+#endif
 #endif
 
     return UNITY_END ();

tests/testutil_monitoring.hpp

@@ -241,7 +241,7 @@ static int64_t get_monitor_event_internal_v2 (void *monitor_,
         zmq_msg_close (&msg);
     }
 
-    //  Third frame in message contains local address
+    //  Second-to-last frame in message contains local address
     zmq_msg_init (&msg);
     int res = zmq_msg_recv (&msg, monitor_, recv_flag_) == -1;
     assert (res != -1);
@@ -256,7 +256,7 @@ static int64_t get_monitor_event_internal_v2 (void *monitor_,
     }
     zmq_msg_close (&msg);
 
-    //  Fourth and last frame in message contains remote address
+    //  Last frame in message contains remote address
     zmq_msg_init (&msg);
     res = zmq_msg_recv (&msg, monitor_, recv_flag_) == -1;
     assert (res != -1);