Problem: can't process ZMTP 3.1 cancel/subscribe commands

Solution: add some msg helpers to parse commands, and check for
subscribe or cancel commands and process them accordingly in the xpub
and xsub classes.

.gitignore

@@ -147,6 +147,7 @@ test_app_meta
 test_security_zap
 test_socket_null
 test_xpub_verbose
+test_mock_pub_sub
 unittest_ip_resolver
 unittest_mtrie
 unittest_poller

Makefile.am

@@ -445,6 +445,7 @@ test_apps = \
 	tests/test_bind_after_connect_tcp \
 	tests/test_sodium \
 	tests/test_reconnect_ivl \
+	tests/test_mock_pub_sub \
 	tests/test_socket_null
 
 UNITY_CPPFLAGS = -I$(top_srcdir)/external/unity -DUNITY_USE_COMMAND_LINE_ARGS -DUNITY_EXCLUDE_FLOAT
@@ -699,6 +700,10 @@ tests_test_reconnect_ivl_SOURCES = tests/test_reconnect_ivl.cpp
 tests_test_reconnect_ivl_LDADD = src/libzmq.la ${UNITY_LIBS}
 tests_test_reconnect_ivl_CPPFLAGS = ${UNITY_CPPFLAGS}
 
+tests_test_mock_pub_sub_SOURCES = tests/test_mock_pub_sub.cpp
+tests_test_mock_pub_sub_LDADD = src/libzmq.la ${UNITY_LIBS}
+tests_test_mock_pub_sub_CPPFLAGS = ${UNITY_CPPFLAGS}
+
 if HAVE_CURVE
 
 test_apps += \

src/msg.cpp

@@ -444,6 +444,43 @@ bool zmq::msg_t::is_leave () const
     return _u.base.type == type_leave;
 }
 
+bool zmq::msg_t::is_ping () const
+{
+    return (_u.base.flags & CMD_TYPE_MASK) == ping;
+}
+
+bool zmq::msg_t::is_pong () const
+{
+    return (_u.base.flags & CMD_TYPE_MASK) == pong;
+}
+
+size_t zmq::msg_t::command_body_size () const
+{
+    if (this->is_ping () || this->is_pong ())
+        return this->size () - ping_cmd_name_size;
+    if (this->is_subscribe ())
+        return this->size () - sub_cmd_name_size;
+    if (this->is_cancel ())
+        return this->size () - cancel_cmd_name_size;
+
+    return 0;
+}
+
+void *zmq::msg_t::command_body ()
+{
+    unsigned char *data = NULL;
+    if (this->is_ping () || this->is_pong ())
+        data =
+          static_cast<unsigned char *> (this->data ()) + ping_cmd_name_size;
+    if (this->is_subscribe ())
+        data = static_cast<unsigned char *> (this->data ()) + sub_cmd_name_size;
+    if (this->is_cancel ())
+        data =
+          static_cast<unsigned char *> (this->data ()) + cancel_cmd_name_size;
+
+    return data;
+}
+
 void zmq::msg_t::add_refs (int refs_)
 {
     zmq_assert (refs_ >= 0);

src/msg.hpp

@@ -39,6 +39,9 @@
 #include "atomic_counter.hpp"
 #include "metadata.hpp"
 
+//  bits 2-5
+#define CMD_TYPE_MASK 0x1c
+
 //  Signature for free function to deallocate the message content.
 //  Note that it has to be declared as "C" so that it is the same as
 //  zmq_free_fn defined in zmq.h.
@@ -75,6 +78,12 @@ class msg_t
     {
         more = 1,    //  Followed by more parts
         command = 2, //  Command frame (see ZMTP spec)
+        //  Command types, use only bits 2-5 and compare with ==, not bitwise,
+        //  a command can never be of more that one type at the same time
+        ping = 4,
+        pong = 8,
+        subscribe = 12,
+        cancel = 16,
         credential = 32,
         routing_id = 64,
         shared = 128
@@ -115,6 +124,22 @@ class msg_t
     bool is_delimiter () const;
     bool is_join () const;
     bool is_leave () const;
+    bool is_ping () const;
+    bool is_pong () const;
+
+    //  These are called on each message received by the session_base class,
+    //  so get them inlined to avoid the overhead of 2 function calls per msg
+    inline bool is_subscribe () const
+    {
+        return (_u.base.flags & CMD_TYPE_MASK) == subscribe;
+    }
+    inline bool is_cancel () const
+    {
+        return (_u.base.flags & CMD_TYPE_MASK) == cancel;
+    }
+
+    size_t command_body_size () const;
+    void *command_body ();
     bool is_vsm () const;
     bool is_cmsg () const;
     bool is_lmsg () const;
@@ -145,6 +170,12 @@ class msg_t
         max_vsm_size =
           msg_t_size - (sizeof (metadata_t *) + 3 + 16 + sizeof (uint32_t))
     };
+    enum
+    {
+        ping_cmd_name_size = 5,   // 4PING
+        cancel_cmd_name_size = 7, // 6CANCEL
+        sub_cmd_name_size = 10    // 9SUBSCRIBE
+    };
 
   private:
     zmq::atomic_counter_t *refcnt ();

src/session_base.cpp

@@ -163,7 +163,9 @@ int zmq::session_base_t::pull_msg (msg_t *msg_)
 
 int zmq::session_base_t::push_msg (msg_t *msg_)
 {
-    if (msg_->flags () & msg_t::command)
+    //  pass subscribe/cancel to the sockets
+    if ((msg_->flags () & msg_t::command) && !msg_->is_subscribe ()
+        && !msg_->is_cancel ())
         return 0;
     if (_pipe && _pipe->write (msg_)) {
         int rc = msg_->init ();

src/stream_engine.cpp

@@ -1038,18 +1038,20 @@ void zmq::stream_engine_t::timer_event (int id_)
 int zmq::stream_engine_t::produce_ping_message (msg_t *msg_)
 {
     int rc = 0;
+    // 16-bit TTL + \4PING == 7
+    const size_t ping_ttl_len = msg_t::ping_cmd_name_size + 2;
     zmq_assert (_mechanism != NULL);
 
-    // 16-bit TTL + \4PING == 7
+    rc = msg_->init_size (ping_ttl_len);
-    rc = msg_->init_size (7);
     errno_assert (rc == 0);
     msg_->set_flags (msg_t::command);
     // Copy in the command message
-    memcpy (msg_->data (), "\4PING", 5);
+    memcpy (msg_->data (), "\4PING", msg_t::ping_cmd_name_size);
 
     uint16_t ttl_val = htons (_options.heartbeat_ttl);
-    memcpy ((static_cast<uint8_t *> (msg_->data ())) + 5, &ttl_val,
+    memcpy ((static_cast<uint8_t *> (msg_->data ()))
-            sizeof (ttl_val));
+              + msg_t::ping_cmd_name_size,
+            &ttl_val, sizeof (ttl_val));
 
     rc = _mechanism->encode (msg_);
     _next_msg = &stream_engine_t::pull_and_encode;
@@ -1075,11 +1077,17 @@ int zmq::stream_engine_t::produce_pong_message (msg_t *msg_)
 
 int zmq::stream_engine_t::process_heartbeat_message (msg_t *msg_)
 {
-    if (memcmp (msg_->data (), "\4PING", 5) == 0) {
+    if (msg_->is_ping ()) {
+        // 16-bit TTL + \4PING == 7
+        const size_t ping_ttl_len = msg_t::ping_cmd_name_size + 2;
+        const size_t ping_max_ctx_len = 16;
         uint16_t remote_heartbeat_ttl;
+
         // Get the remote heartbeat TTL to setup the timer
         memcpy (&remote_heartbeat_ttl,
-                static_cast<uint8_t *> (msg_->data ()) + 5, 2);
+                static_cast<uint8_t *> (msg_->data ())
+                  + msg_t::ping_cmd_name_size,
+                ping_ttl_len - msg_t::ping_cmd_name_size);
         remote_heartbeat_ttl = ntohs (remote_heartbeat_ttl);
         // The remote heartbeat is in 10ths of a second
         // so we multiply it by 100 to get the timer interval in ms.
@@ -1095,14 +1103,18 @@ int zmq::stream_engine_t::process_heartbeat_message (msg_t *msg_)
         //  here and store it. Truncate it if it's too long.
         //  Given the engine goes straight to out_event, sequential PINGs will
         //  not be a problem.
-        size_t context_len = msg_->size () - 7 > 16 ? 16 : msg_->size () - 7;
+        size_t context_len = msg_->size () - ping_ttl_len > ping_max_ctx_len
-        int rc = _pong_msg.init_size (5 + context_len);
+                               ? ping_max_ctx_len
+                               : msg_->size () - ping_ttl_len;
+        int rc = _pong_msg.init_size (msg_t::ping_cmd_name_size + context_len);
         errno_assert (rc == 0);
         _pong_msg.set_flags (msg_t::command);
-        memcpy (_pong_msg.data (), "\4PONG", 5);
+        memcpy (_pong_msg.data (), "\4PONG", msg_t::ping_cmd_name_size);
         if (context_len > 0)
-            memcpy ((static_cast<uint8_t *> (_pong_msg.data ())) + 5,
+            memcpy ((static_cast<uint8_t *> (_pong_msg.data ()))
-                    (static_cast<uint8_t *> (msg_->data ())) + 7, context_len);
+                      + msg_t::ping_cmd_name_size,
+                    (static_cast<uint8_t *> (msg_->data ())) + ping_ttl_len,
+                    context_len);
 
         _next_msg = &stream_engine_t::produce_pong_message;
         out_event ();
@@ -1115,15 +1127,28 @@ int zmq::stream_engine_t::process_command_message (msg_t *msg_)
 {
     const uint8_t cmd_name_size =
       *(static_cast<const uint8_t *> (msg_->data ()));
+    const size_t ping_name_size = msg_t::ping_cmd_name_size - 1;
+    const size_t sub_name_size = msg_t::sub_cmd_name_size - 1;
+    const size_t cancel_name_size = msg_t::cancel_cmd_name_size - 1;
     //  Malformed command
-    if (msg_->size () < cmd_name_size + sizeof (cmd_name_size))
+    if (unlikely (msg_->size () < cmd_name_size + sizeof (cmd_name_size)))
         return -1;
 
-    const uint8_t *cmd_name =
+    uint8_t *cmd_name = (static_cast<uint8_t *> (msg_->data ())) + 1;
-      (static_cast<const uint8_t *> (msg_->data ())) + 1;
+    if (cmd_name_size == ping_name_size
-    if (cmd_name_size == 4
+        && memcmp (cmd_name, "PING", cmd_name_size) == 0)
-        && (memcmp (cmd_name, "PING", cmd_name_size) == 0
+        msg_->set_flags (zmq::msg_t::ping);
-            || memcmp (cmd_name, "PONG", cmd_name_size) == 0))
+    if (cmd_name_size == ping_name_size
+        && memcmp (cmd_name, "PONG", cmd_name_size) == 0)
+        msg_->set_flags (zmq::msg_t::pong);
+    if (cmd_name_size == sub_name_size
+        && memcmp (cmd_name, "SUBSCRIBE", cmd_name_size) == 0)
+        msg_->set_flags (zmq::msg_t::subscribe);
+    if (cmd_name_size == cancel_name_size
+        && memcmp (cmd_name, "CANCEL", cmd_name_size) == 0)
+        msg_->set_flags (zmq::msg_t::cancel);
+
+    if (msg_->is_ping () || msg_->is_pong ())
         return process_heartbeat_message (msg_);
 
     return 0;

src/xpub.cpp

@@ -88,57 +88,89 @@ void zmq::xpub_t::xread_activated (pipe_t *pipe_)
     //  There are some subscriptions waiting. Let's process them.
     msg_t sub;
     while (pipe_->read (&sub)) {
-        //  Apply the subscription to the trie
-        const unsigned char *const data =
-          static_cast<const unsigned char *> (sub.data ());
-        const size_t size = sub.size ();
         metadata_t *metadata = sub.metadata ();
-        if (size > 0 && (*data == 0 || *data == 1)) {
+        unsigned char *msg_data = static_cast<unsigned char *> (sub.data ()),
-            if (_manual) {
+                      *data = NULL;
-                // Store manual subscription to use on termination
+        size_t size = 0;
-                if (*data == 0)
+        bool subscribe = false;
-                    _manual_subscriptions.rm (data + 1, size - 1, pipe_);
+
-                else
+        //  Apply the subscription to the trie
-                    _manual_subscriptions.add (data + 1, size - 1, pipe_);
+        if (sub.is_subscribe () || sub.is_cancel ()) {
+            data = static_cast<unsigned char *> (sub.command_body ());
+            size = sub.command_body_size ();
+            subscribe = sub.is_subscribe ();
+        } else if (sub.size () > 0) {
+            unsigned char first = *msg_data;
+            if (first == 0 || first == 1) {
+                data = msg_data + 1;
+                size = sub.size () - 1;
+                subscribe = first == 1;
+            }
+        } else {
+            //  Process user message coming upstream from xsub socket
+            _pending_data.push_back (blob_t (msg_data, sub.size ()));
+            if (metadata)
+                metadata->add_ref ();
+            _pending_metadata.push_back (metadata);
+            _pending_flags.push_back (sub.flags ());
+            sub.close ();
+            continue;
+        }
+
+        if (_manual) {
+            // Store manual subscription to use on termination
+            if (!subscribe)
+                _manual_subscriptions.rm (data, size, pipe_);
+            else
+                _manual_subscriptions.add (data, size, pipe_);
+
+            _pending_pipes.push_back (pipe_);
+
+            //  ZMTP 3.1 hack: we need to support sub/cancel commands, but
+            //  we can't give them back to userspace as it would be an API
+            //  breakage since the payload of the message is completely
+            //  different. Manually craft an old-style message instead.
+            data = data - 1;
+            size = size + 1;
+            if (subscribe)
+                *data = 1;
+            else
+                *data = 0;
+
+            _pending_data.push_back (blob_t (data, size));
+            if (metadata)
+                metadata->add_ref ();
+            _pending_metadata.push_back (metadata);
+            _pending_flags.push_back (0);
+        } else {
+            bool notify;
+            if (!subscribe) {
+                mtrie_t::rm_result rm_result =
+                  _subscriptions.rm (data, size, pipe_);
+                //  TODO reconsider what to do if rm_result == mtrie_t::not_found
+                notify = rm_result != mtrie_t::values_remain || _verbose_unsubs;
+            } else {
+                bool first_added = _subscriptions.add (data, size, pipe_);
+                notify = first_added || _verbose_subs;
+            }
+
+            //  If the request was a new subscription, or the subscription
+            //  was removed, or verbose mode is enabled, store it so that
+            //  it can be passed to the user on next recv call.
+            if (options.type == ZMQ_XPUB && notify) {
+                data = data - 1;
+                size = size + 1;
+                if (subscribe)
+                    *data = 1;
+                else
+                    *data = 0;
 
-                _pending_pipes.push_back (pipe_);
                 _pending_data.push_back (blob_t (data, size));
                 if (metadata)
                     metadata->add_ref ();
                 _pending_metadata.push_back (metadata);
                 _pending_flags.push_back (0);
-            } else {
-                bool notify;
-                if (*data == 0) {
-                    mtrie_t::rm_result rm_result =
-                      _subscriptions.rm (data + 1, size - 1, pipe_);
-                    //  TODO reconsider what to do if rm_result == mtrie_t::not_found
-                    notify =
-                      rm_result != mtrie_t::values_remain || _verbose_unsubs;
-                } else {
-                    bool first_added =
-                      _subscriptions.add (data + 1, size - 1, pipe_);
-                    notify = first_added || _verbose_subs;
-                }
-
-                //  If the request was a new subscription, or the subscription
-                //  was removed, or verbose mode is enabled, store it so that
-                //  it can be passed to the user on next recv call.
-                if (options.type == ZMQ_XPUB && notify) {
-                    _pending_data.push_back (blob_t (data, size));
-                    if (metadata)
-                        metadata->add_ref ();
-                    _pending_metadata.push_back (metadata);
-                    _pending_flags.push_back (0);
-                }
             }
-        } else {
-            //  Process user message coming upstream from xsub socket
-            _pending_data.push_back (blob_t (data, size));
-            if (metadata)
-                metadata->add_ref ();
-            _pending_metadata.push_back (metadata);
-            _pending_flags.push_back (sub.flags ());
         }
         sub.close ();
     }

src/xsub.cpp

@@ -96,18 +96,32 @@ int zmq::xsub_t::xsend (msg_t *msg_)
     size_t size = msg_->size ();
     unsigned char *data = static_cast<unsigned char *> (msg_->data ());
 
-    if (size > 0 && *data == 1) {
+    if (msg_->is_subscribe () || (size > 0 && *data == 1)) {
         //  Process subscribe message
         //  This used to filter out duplicate subscriptions,
         //  however this is alread done on the XPUB side and
         //  doing it here as well breaks ZMQ_XPUB_VERBOSE
         //  when there are forwarding devices involved.
-        _subscriptions.add (data + 1, size - 1);
+        if (msg_->is_subscribe ()) {
+            data = static_cast<unsigned char *> (msg_->command_body ());
+            size = msg_->command_body_size ();
+        } else {
+            data = data + 1;
+            size = size - 1;
+        }
+        _subscriptions.add (data, size);
         return _dist.send_to_all (msg_);
     }
-    if (size > 0 && *data == 0) {
+    if (msg_->is_cancel () || (size > 0 && *data == 0)) {
         //  Process unsubscribe message
-        if (_subscriptions.rm (data + 1, size - 1))
+        if (msg_->is_cancel ()) {
+            data = static_cast<unsigned char *> (msg_->command_body ());
+            size = msg_->command_body_size ();
+        } else {
+            data = data + 1;
+            size = size - 1;
+        }
+        if (_subscriptions.rm (data, size))
             return _dist.send_to_all (msg_);
     } else
         //  User message sent upstream to XPUB socket

tests/CMakeLists.txt

@@ -72,6 +72,7 @@ set(tests
         test_monitor
 		test_socket_null
 	test_reconnect_ivl
+        test_mock_pub_sub
 )
 if(ZMQ_HAVE_CURVE)
   list(APPEND tests 

tests/test_mock_pub_sub.cpp

@@ -0,0 +1,226 @@
+/*
+    Copyright (c) 2018 Contributors as noted in the AUTHORS file
+
+    This file is part of 0MQ.
+
+    0MQ is free software; you can redistribute it and/or modify it under
+    the terms of the GNU Lesser General Public License as published by
+    the Free Software Foundation; either version 3 of the License, or
+    (at your option) any later version.
+
+    0MQ is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "testutil.hpp"
+#include "testutil_unity.hpp"
+#if defined(ZMQ_HAVE_WINDOWS)
+#include <winsock2.h>
+#include <ws2tcpip.h>
+#include <stdexcept>
+#define close closesocket
+typedef SOCKET raw_socket;
+#else
+#include <arpa/inet.h>
+typedef int raw_socket;
+#endif
+
+
+void setUp ()
+{
+    setup_test_context ();
+}
+
+void tearDown ()
+{
+    teardown_test_context ();
+}
+
+//  Read one event off the monitor socket; return value and address
+//  by reference, if not null, and event number by value. Returns -1
+//  in case of error.
+
+static int get_monitor_event (void *monitor_)
+{
+    for (int i = 0; i < 2; i++) {
+        //  First frame in message contains event number and value
+        zmq_msg_t msg;
+        TEST_ASSERT_SUCCESS_ERRNO (zmq_msg_init (&msg));
+        if (zmq_msg_recv (&msg, monitor_, ZMQ_DONTWAIT) == -1) {
+            msleep (SETTLE_TIME);
+            continue; //  Interrupted, presumably
+        }
+        TEST_ASSERT_TRUE (zmq_msg_more (&msg));
+
+        uint8_t *data = (uint8_t *) zmq_msg_data (&msg);
+        uint16_t event = *(uint16_t *) (data);
+
+        //  Second frame in message contains event address
+        TEST_ASSERT_SUCCESS_ERRNO (zmq_msg_init (&msg));
+        if (zmq_msg_recv (&msg, monitor_, 0) == -1) {
+            return -1; //  Interrupted, presumably
+        }
+        TEST_ASSERT_FALSE (zmq_msg_more (&msg));
+
+        return event;
+    }
+    return -1;
+}
+
+static void recv_with_retry (raw_socket fd_, char *buffer_, int bytes_)
+{
+    int received = 0;
+    while (true) {
+        int rc = TEST_ASSERT_SUCCESS_RAW_ERRNO (
+          recv (fd_, buffer_ + received, bytes_ - received, 0));
+        TEST_ASSERT_GREATER_THAN_INT (0, rc);
+        received += rc;
+        TEST_ASSERT_LESS_OR_EQUAL_INT (bytes_, received);
+        if (received == bytes_)
+            break;
+    }
+}
+
+static void mock_handshake (raw_socket fd_)
+{
+    const uint8_t zmtp_greeting[33] = {0xff, 0, 0, 0,   0,   0,   0,   0, 0,
+                                       0x7f, 3, 0, 'N', 'U', 'L', 'L', 0};
+    char buffer[128];
+    memset (buffer, 0, sizeof (buffer));
+    memcpy (buffer, zmtp_greeting, sizeof (zmtp_greeting));
+
+    int rc = TEST_ASSERT_SUCCESS_RAW_ERRNO (send (fd_, buffer, 64, 0));
+    TEST_ASSERT_EQUAL_INT (64, rc);
+
+    recv_with_retry (fd_, buffer, 64);
+
+    const uint8_t zmtp_ready[27] = {
+      4,   25,  5,   'R', 'E', 'A', 'D', 'Y', 11, 'S', 'o', 'c', 'k', 'e',
+      't', '-', 'T', 'y', 'p', 'e', 0,   0,   0,  3,   'S', 'U', 'B'};
+
+    memset (buffer, 0, sizeof (buffer));
+    memcpy (buffer, zmtp_ready, 27);
+    rc = TEST_ASSERT_SUCCESS_RAW_ERRNO (send (fd_, buffer, 27, 0));
+    TEST_ASSERT_EQUAL_INT (27, rc);
+
+    //  greeting - XPUB so one extra byte
+    recv_with_retry (fd_, buffer, 28);
+}
+
+static void prep_server_socket (void **server_out_,
+                                void **mon_out_,
+                                char *endpoint_,
+                                size_t ep_length_)
+{
+    //  We'll be using this socket in raw mode
+    void *server = test_context_socket (ZMQ_XPUB);
+
+    int value = 0;
+    TEST_ASSERT_SUCCESS_ERRNO (
+      zmq_setsockopt (server, ZMQ_LINGER, &value, sizeof (value)));
+
+    bind_loopback_ipv4 (server, endpoint_, ep_length_);
+
+    //  Create and connect a socket for collecting monitor events on xpub
+    void *server_mon = test_context_socket (ZMQ_PAIR);
+
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_socket_monitor (
+      server, "inproc://monitor-dealer",
+      ZMQ_EVENT_CONNECTED | ZMQ_EVENT_DISCONNECTED | ZMQ_EVENT_ACCEPTED));
+
+    //  Connect to the inproc endpoint so we'll get events
+    TEST_ASSERT_SUCCESS_ERRNO (
+      zmq_connect (server_mon, "inproc://monitor-dealer"));
+
+    *server_out_ = server;
+    *mon_out_ = server_mon;
+}
+
+static void test_mock_sub (bool sub_command)
+{
+    int rc;
+    char my_endpoint[MAX_SOCKET_STRING];
+
+    void *server, *server_mon;
+    prep_server_socket (&server, &server_mon, my_endpoint, MAX_SOCKET_STRING);
+
+    struct sockaddr_in ip4addr;
+    raw_socket s;
+
+    ip4addr.sin_family = AF_INET;
+    ip4addr.sin_port = htons (atoi (strrchr (my_endpoint, ':') + 1));
+#if defined(ZMQ_HAVE_WINDOWS) && (_WIN32_WINNT < 0x0600)
+    ip4addr.sin_addr.s_addr = inet_addr ("127.0.0.1");
+#else
+    inet_pton (AF_INET, "127.0.0.1", &ip4addr.sin_addr);
+#endif
+
+    s = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
+    rc = TEST_ASSERT_SUCCESS_RAW_ERRNO (
+      connect (s, (struct sockaddr *) &ip4addr, sizeof ip4addr));
+    TEST_ASSERT_GREATER_THAN_INT (-1, rc);
+
+    // Mock a ZMTP 3 client so we can forcibly try sub commands
+    mock_handshake (s);
+
+    // By now everything should report as connected
+    rc = get_monitor_event (server_mon);
+    TEST_ASSERT_EQUAL_INT (ZMQ_EVENT_ACCEPTED, rc);
+
+    if (sub_command) {
+        const uint8_t sub[13] = {4,   11,  9,   'S', 'U', 'B', 'S',
+                                 'C', 'R', 'I', 'B', 'E', 'A'};
+        rc =
+          TEST_ASSERT_SUCCESS_RAW_ERRNO (send (s, (const char *) sub, 13, 0));
+        TEST_ASSERT_EQUAL_INT (13, rc);
+    } else {
+        const uint8_t sub[4] = {0, 2, 1, 'A'};
+        rc = TEST_ASSERT_SUCCESS_RAW_ERRNO (send (s, (const char *) sub, 4, 0));
+        TEST_ASSERT_EQUAL_INT (4, rc);
+    }
+
+    char buffer[16];
+    memset (buffer, 0, sizeof (buffer));
+    rc = zmq_recv (server, buffer, 2, 0);
+    TEST_ASSERT_EQUAL_INT (2, rc);
+    TEST_ASSERT_EQUAL_INT (0, memcmp (buffer, "\1A", 2));
+
+    rc = zmq_send (server, "ALOL", 4, 0);
+    TEST_ASSERT_EQUAL_INT (4, rc);
+
+    memset (buffer, 0, sizeof (buffer));
+    recv_with_retry (s, buffer, 6);
+    TEST_ASSERT_EQUAL_INT (0, memcmp (buffer, "\0\4ALOL", 6));
+
+    close (s);
+
+    test_context_socket_close (server);
+    test_context_socket_close (server_mon);
+}
+
+void test_mock_sub_command ()
+{
+    test_mock_sub (true);
+}
+
+void test_mock_sub_legacy ()
+{
+    test_mock_sub (false);
+}
+
+int main (void)
+{
+    setup_test_environment ();
+
+    UNITY_BEGIN ();
+
+    RUN_TEST (test_mock_sub_command);
+    RUN_TEST (test_mock_sub_legacy);
+
+    return UNITY_END ();
+}