On the advice of Martin Hurton, removed the new command type and just terminated the pipe in a reconnect situation, and notified the socket of the same. This handles the blocking properly, but at the cost of potentially losing in flight messages. However, this is a reasonable trade off given how much simpler it makes the patch.

src/command.hpp

@@ -47,7 +47,6 @@ namespace zmq
             own,
             attach,
             bind,
-            detach,
             activate_read,
             activate_write,
             hiccup,
@@ -88,11 +87,6 @@ namespace zmq
             struct {
                 zmq::pipe_t *pipe;
             } bind;
-            
-            //  Sent from session to socket to disconnect a pipe 
-            struct {
-                zmq::pipe_t *pipe;
-            } detach;
 
             //  Sent by pipe writer to inform dormant pipe reader that there
             //  are messages in the pipe.

src/object.cpp

@@ -92,11 +92,6 @@ void zmq::object_t::process_command (command_t &cmd_)
         process_seqnum ();
         break;
 
-    case command_t::detach:
-        process_detach (cmd_.args.detach.pipe);
-        process_seqnum ();
-        break;
-
     case command_t::hiccup:
         process_hiccup (cmd_.args.hiccup.pipe);
         break;
@@ -216,15 +211,6 @@ void zmq::object_t::send_bind (own_t *destination_, pipe_t *pipe_,
     send_command (cmd);
 }
 
-void zmq::object_t::send_detach (own_t *destination_, pipe_t *pipe_)
-{
-    command_t cmd;
-    cmd.destination = destination_;
-    cmd.type = command_t::detach;
-    cmd.args.detach.pipe = pipe_;
-    send_command (cmd);
-}
-
 void zmq::object_t::send_activate_read (pipe_t *destination_)
 {
     command_t cmd;
@@ -345,11 +331,6 @@ void zmq::object_t::process_bind (pipe_t *pipe_)
     zmq_assert (false);
 }
 
-void zmq::object_t::process_detach (pipe_t *pipe_)
-{
-    zmq_assert (false);
-}
-
 void zmq::object_t::process_activate_read ()
 {
     zmq_assert (false);

src/object.hpp

@@ -78,7 +78,6 @@ namespace zmq
              zmq::i_engine *engine_, bool inc_seqnum_ = true);
         void send_bind (zmq::own_t *destination_, zmq::pipe_t *pipe_,
              bool inc_seqnum_ = true);
-        void send_detach (own_t *destination_, pipe_t *pipe_);
         void send_activate_read (zmq::pipe_t *destination_);
         void send_activate_write (zmq::pipe_t *destination_,
              uint64_t msgs_read_);
@@ -100,7 +99,6 @@ namespace zmq
         virtual void process_own (zmq::own_t *object_);
         virtual void process_attach (zmq::i_engine *engine_);
         virtual void process_bind (zmq::pipe_t *pipe_);
-        virtual void process_detach (zmq::pipe_t *pipe_);
         virtual void process_activate_read ();
         virtual void process_activate_write (uint64_t msgs_read_);
         virtual void process_hiccup (void *pipe_);

src/session_base.cpp

@@ -411,11 +411,14 @@ void zmq::session_base_t::detached ()
     //  the socket object to resend all the subscriptions.
     if (pipe && (options.type == ZMQ_SUB || options.type == ZMQ_XSUB))
         pipe->hiccup ();
-        
+
-    //  For delayed connect situations, hiccup the socket to have it
+    //  For delayed connect situations, terminate the pipe
-    //  pause usage of this pipe
+    //  and reestablish later on
-    if (outpipe && options.delay_attach_on_connect == 1) 
+    if (pipe && options.delay_attach_on_connect == 1) {
-        send_detach(socket, outpipe);
+        pipe->terminate (false);
+        socket->terminated (pipe);
+        pipe = NULL;
+    }
 }
 
 void zmq::session_base_t::start_connecting (bool wait_)

src/socket_base.cpp

@@ -876,17 +876,6 @@ void zmq::socket_base_t::process_destroy ()
     destroyed = true;
 }
 
-void zmq::socket_base_t::process_detach (pipe_t *pipe_)
-{
-    //  If we are blocking connecting threads, drop this one
-    if (options.delay_attach_on_connect == 1) {
-        zmq_assert (pipe_);
-        pipes.erase (pipe_);
-        //  Let derived sockets know we're ditching this pipe
-        xterminated (pipe_);
-    }
-}
-
 int zmq::socket_base_t::xsetsockopt (int option_, const void *optval_,
     size_t optvallen_)
 {

src/socket_base.hpp

@@ -185,9 +185,6 @@ namespace zmq
         void process_bind (zmq::pipe_t *pipe_);
         void process_term (int linger_);
 
-        // Allow blocking reconnecting pipes
-        void process_detach (pipe_t *pipe_);
-
         //  Socket's mailbox object.
         mailbox_t mailbox;