Problem: Socket monitoring only allows ZMQ_PAIR

Solution: Allow ZMQ_PUB and ZMQ_PUSH sockets types for the monitoring.
This way someone could create a ZMQ_PULL socket connected to multiple
monitoring sockets at the same time.

doc/zmq_socket_monitor_versioned.txt

@@ -11,6 +11,8 @@ zmq_socket_monitor_versioned - monitor socket events
 SYNOPSIS
 --------
 *int zmq_socket_monitor_versioned (void '*socket', char '*endpoint', uint64_t 'events', int 'event_version');*
+*int zmq_socket_monitor_versioned_typed (
+    void '*socket', char '*endpoint', uint64_t 'events', int 'event_version', int 'type');*
 
 *int zmq_socket_monitor_pipes_stats (void '*socket');*
 
@@ -56,6 +58,17 @@ 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_versioned_typed()_ is a generalisation of
+_zmq_socket_monitor_versioned_ that supports more monitoring socket types.
+The 'type' argument is used to specify the type of the monitoring socket.
+Supported types are 'ZMQ_PAIR' (which is the equivalent of
+_zmq_socket_monitor_versioned_), 'ZMQ_PUB' and 'ZMQ_PUSH'. Note that consumers
+of the events will have to be compatible with the socket type, for instance a
+monitoring socket of type 'ZMQ_PUB' will require consumers of type 'ZMQ_SUB'.
+In the case that the monitoring socket type is of 'ZMQ_PUB', the multipart
+message topic is the event number, thus consumers should subscribe to the
+events they want to receive.
+
 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.
@@ -215,6 +228,20 @@ sockets are required to use the inproc:// transport.
 The monitor 'endpoint' supplied does not exist.
 
 
+ERRORS - _zmq_socket_monitor_typed()_
+-------------------------------
+*ETERM*::
+The 0MQ 'context' associated with the specified 'socket' was terminated.
+
+*EPROTONOSUPPORT*::
+The transport protocol of the monitor 'endpoint' is not supported. Monitor
+sockets are required to use the inproc:// transport.
+
+*EINVAL*::
+The monitor 'endpoint' supplied does not exist or the specified socket 'type'
+is not supported.
+
+
 ERRORS - _zmq_socket_monitor_pipes_stats()_
 -------------------------------------------
 *ENOTSOCK*::

include/zmq.h

@@ -735,6 +735,8 @@ ZMQ_EXPORT int zmq_socket_monitor_versioned (void *s_,
                                              const char *addr_,
                                              uint64_t events_,
                                              int event_version_);
+ZMQ_EXPORT int zmq_socket_monitor_versioned_typed (
+  void *s_, const char *addr_, uint64_t events_, int event_version_, int type_);
 ZMQ_EXPORT int zmq_socket_monitor_pipes_stats (void *s);
 
 #endif // ZMQ_BUILD_DRAFT_API

src/socket_base.cpp

@@ -1639,7 +1639,8 @@ void zmq::socket_base_t::extract_flags (msg_t *msg_)
 
 int zmq::socket_base_t::monitor (const char *endpoint_,
                                  uint64_t events_,
-                                 int event_version_)
+                                 int event_version_,
+                                 int type_)
 {
     scoped_lock_t lock (_monitor_sync);
 
@@ -1670,14 +1671,31 @@ int zmq::socket_base_t::monitor (const char *endpoint_,
         errno = EPROTONOSUPPORT;
         return -1;
     }
+
     // already monitoring. Stop previous monitor before starting new one.
     if (_monitor_socket != NULL) {
         stop_monitor (true);
     }
+
+    // Check if the specified socket type is supported. It must be a
+    // one-way socket types that support the SNDMORE flag.
+    switch (type_) {
+        case ZMQ_PAIR:
+            break;
+        case ZMQ_PUB:
+            break;
+        case ZMQ_PUSH:
+            break;
+        default:
+            errno = EINVAL;
+            return -1;
+    }
+
     //  Register events to monitor
     _monitor_events = events_;
     options.monitor_event_version = event_version_;
-    _monitor_socket = zmq_socket (get_ctx (), ZMQ_PAIR);
+    //  Create a monitor socket of the specified type.
+    _monitor_socket = zmq_socket (get_ctx (), type_);
     if (_monitor_socket == NULL)
         return -1;
 

src/socket_base.hpp

@@ -119,7 +119,10 @@ class socket_base_t : public own_t,
     void lock ();
     void unlock ();
 
-    int monitor (const char *endpoint_, uint64_t events_, int event_version_);
+    int monitor (const char *endpoint_,
+                 uint64_t events_,
+                 int event_version_,
+                 int type_);
 
     void event_connected (const endpoint_uri_pair_t &endpoint_uri_pair_,
                           zmq::fd_t fd_);

src/zmq.cpp

@@ -275,7 +275,7 @@ int zmq_socket_monitor_versioned (void *s_,
     zmq::socket_base_t *s = as_socket_base_t (s_);
     if (!s)
         return -1;
-    return s->monitor (addr_, events_, event_version_);
+    return s->monitor (addr_, events_, event_version_, ZMQ_PAIR);
 }
 
 int zmq_socket_monitor (void *s_, const char *addr_, int events_)
@@ -283,6 +283,16 @@ int zmq_socket_monitor (void *s_, const char *addr_, int events_)
     return zmq_socket_monitor_versioned (s_, addr_, events_, 1);
 }
 
+int zmq_socket_monitor_versioned_typed (
+  void *s_, const char *addr_, uint64_t events_, int event_version_, int type_)
+{
+    zmq::socket_base_t *s = as_socket_base_t (s_);
+    if (!s)
+        return -1;
+
+    return s->monitor (addr_, events_, event_version_, type_);
+}
+
 int zmq_join (void *s_, const char *group_)
 {
     zmq::socket_base_t *s = as_socket_base_t (s_);

src/zmq_draft.h

@@ -133,6 +133,8 @@ int zmq_socket_monitor_versioned (void *s_,
                                   const char *addr_,
                                   uint64_t events_,
                                   int event_version_);
+int zmq_socket_monitor_versioned_typed (
+  void *s_, const char *addr_, uint64_t events_, int event_version_, int type_);
 int zmq_socket_monitor_pipes_stats (void *s_);
 
 #endif // ZMQ_BUILD_DRAFT_API

tests/test_monitor.cpp

@@ -126,8 +126,21 @@ void 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)
-void test_monitor_versioned_basic (bind_function_t bind_function_,
+void test_monitor_versioned_typed_invalid_socket_type ()
-                                   const char *expected_prefix_)
+{
+    void *client = test_context_socket (ZMQ_DEALER);
+
+    //  Socket monitoring only works with ZMQ_PAIR, ZMQ_PUB and ZMQ_PUSH.
+    TEST_ASSERT_FAILURE_ERRNO (
+      EINVAL, zmq_socket_monitor_versioned_typed (
+                client, "inproc://invalid-socket-type", 0, 2, ZMQ_CLIENT));
+
+    test_context_socket_close_zero_linger (client);
+}
+
+void test_monitor_versioned_typed_basic (bind_function_t bind_function_,
+                                         const char *expected_prefix_,
+                                         int type_)
 {
     char server_endpoint[MAX_SOCKET_STRING];
 
@@ -136,14 +149,36 @@ void test_monitor_versioned_basic (bind_function_t bind_function_,
     void *server = test_context_socket (ZMQ_DEALER);
 
     //  Monitor all events on client and server sockets
-    TEST_ASSERT_SUCCESS_ERRNO (zmq_socket_monitor_versioned (
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_socket_monitor_versioned_typed (
-      client, "inproc://monitor-client", ZMQ_EVENT_ALL_V2, 2));
+      client, "inproc://monitor-client", ZMQ_EVENT_ALL_V2, 2, type_));
-    TEST_ASSERT_SUCCESS_ERRNO (zmq_socket_monitor_versioned (
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_socket_monitor_versioned_typed (
-      server, "inproc://monitor-server", ZMQ_EVENT_ALL_V2, 2));
+      server, "inproc://monitor-server", ZMQ_EVENT_ALL_V2, 2, type_));
+
+    //  Choose the appropriate consumer socket type.
+    int mon_type;
+    switch (type_) {
+        case ZMQ_PAIR:
+            mon_type = ZMQ_PAIR;
+            break;
+        case ZMQ_PUSH:
+            mon_type = ZMQ_PULL;
+            break;
+        case ZMQ_PUB:
+            mon_type = ZMQ_SUB;
+            break;
+    }
 
     //  Create two sockets for collecting monitor events
-    void *client_mon = test_context_socket (ZMQ_PAIR);
+    void *client_mon = test_context_socket (mon_type);
-    void *server_mon = test_context_socket (ZMQ_PAIR);
+    void *server_mon = test_context_socket (mon_type);
+
+    //  Additionally subscribe to all events if a PUB socket is used.
+    if (type_ == ZMQ_PUB) {
+        TEST_ASSERT_SUCCESS_ERRNO (
+          zmq_setsockopt (client_mon, ZMQ_SUBSCRIBE, "", 0));
+        TEST_ASSERT_SUCCESS_ERRNO (
+          zmq_setsockopt (server_mon, ZMQ_SUBSCRIBE, "", 0));
+    }
 
     //  Connect these to the inproc endpoints so they'll get events
     TEST_ASSERT_SUCCESS_ERRNO (
@@ -220,30 +255,44 @@ void test_monitor_versioned_basic (bind_function_t bind_function_,
     //  TODO why does this use zero_linger?
     test_context_socket_close_zero_linger (client_mon);
     test_context_socket_close_zero_linger (server_mon);
+
+    //  Wait for the monitor socket's endpoint to be available
+    //  for reuse.
+    msleep (SETTLE_TIME);
 }
 
 void test_monitor_versioned_basic_tcp_ipv4 ()
 {
     static const char prefix[] = "tcp://127.0.0.1:";
-    test_monitor_versioned_basic (bind_loopback_ipv4, prefix);
+    // Calling 'monitor_versioned_typed' with ZMQ_PAIR is the equivalent of
+    // calling 'monitor_versioned'.
+    test_monitor_versioned_typed_basic (bind_loopback_ipv4, prefix, ZMQ_PAIR);
+    test_monitor_versioned_typed_basic (bind_loopback_ipv4, prefix, ZMQ_PUB);
+    test_monitor_versioned_typed_basic (bind_loopback_ipv4, prefix, ZMQ_PUSH);
 }
 
 void test_monitor_versioned_basic_tcp_ipv6 ()
 {
     static const char prefix[] = "tcp://[::1]:";
-    test_monitor_versioned_basic (bind_loopback_ipv6, prefix);
+    test_monitor_versioned_typed_basic (bind_loopback_ipv6, prefix, ZMQ_PAIR);
+    test_monitor_versioned_typed_basic (bind_loopback_ipv6, prefix, ZMQ_PUB);
+    test_monitor_versioned_typed_basic (bind_loopback_ipv6, prefix, ZMQ_PUSH);
 }
 
 void test_monitor_versioned_basic_ipc ()
 {
     static const char prefix[] = "ipc://";
-    test_monitor_versioned_basic (bind_loopback_ipc, prefix);
+    test_monitor_versioned_typed_basic (bind_loopback_ipc, prefix, ZMQ_PAIR);
+    test_monitor_versioned_typed_basic (bind_loopback_ipc, prefix, ZMQ_PUB);
+    test_monitor_versioned_typed_basic (bind_loopback_ipc, prefix, ZMQ_PUSH);
 }
 
 void test_monitor_versioned_basic_tipc ()
 {
     static const char prefix[] = "tipc://";
-    test_monitor_versioned_basic (bind_loopback_tipc, prefix);
+    test_monitor_versioned_typed_basic (bind_loopback_tipc, prefix, ZMQ_PAIR);
+    test_monitor_versioned_typed_basic (bind_loopback_tipc, prefix, ZMQ_PUB);
+    test_monitor_versioned_typed_basic (bind_loopback_tipc, prefix, ZMQ_PUSH);
 }
 
 #ifdef ZMQ_EVENT_PIPES_STATS
@@ -385,6 +434,7 @@ int main ()
 #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_typed_invalid_socket_type);
     RUN_TEST (test_monitor_versioned_basic_tcp_ipv4);
     RUN_TEST (test_monitor_versioned_basic_tcp_ipv6);
     RUN_TEST (test_monitor_versioned_basic_ipc);