Problem: cannot use pre-allocated FDs. Fixes #777

Solution: add new [set|get]sockopt ZMQ_PRE_ALLOCATED_FD to allow
users to let ZMQ use a pre-allocated file descriptor instead of
allocating a new one. Update [set|get]sockopt documentation and
test accordingly.

The main use case for this feature is a socket-activated systemd
service. For more information about this feature see:
http://0pointer.de/blog/projects/socket-activation.html

doc/zmq_getsockopt.txt

@@ -464,6 +464,19 @@ Default value:: null string
 Applicable socket types:: all, when using TCP or IPC transports
 
 
+ZMQ_PRE_ALLOCATED_FD: Retrieve the pre-allocated socket file descriptor
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The 'ZMQ_PRE_ALLOCATED_FD' option shall retrieve the pre-allocated file
+descriptor that has been assigned to a ZMQ socket, if any. -1 shall be
+returned if a pre-allocated file descriptor was not set for the socket.
+
+[horizontal]
+Option value type:: int
+Option value unit:: file descriptor
+Default value:: -1
+Applicable socket types:: all bound sockets, when using IPC or TCP transport
+
+
 ZMQ_RATE: Retrieve multicast data rate
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The 'ZMQ_RATE' option shall retrieve the maximum send or receive data rate for

doc/zmq_setsockopt.txt

@@ -492,6 +492,28 @@ Default value:: not set
 Applicable socket types:: all, when using TCP transport
 
 
+ZMQ_PRE_ALLOCATED_FD: Set the pre-allocated socket file descriptor
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When set to a positive integer value before zmq_bind is called on the socket,
+the socket shall use the corresponding file descriptor for connections over
+TCP or IPC instead of allocating a new file descriptor.
+Useful for writing systemd socket activated services. If set to -1 (default),
+a new file descriptor will be allocated instead (default behaviour).
+
+NOTE: if set after calling zmq_bind, this option shall have no effect.
+NOTE: the file descriptor passed through MUST have been ran through the "bind"
+      and "listen" system calls beforehand. Also, socket option that would
+      normally be passed through zmq_setsockopt like TCP buffers length,
+      IP_TOS or SO_REUSEADDR MUST be set beforehand by the caller, as they
+      must be set before the socket is bound.
+
+[horizontal]
+Option value type:: int
+Option value unit:: file descriptor
+Default value:: -1
+Applicable socket types:: all bound sockets, when using IPC or TCP transport
+
+
 ZMQ_PROBE_ROUTER: bootstrap connections to ROUTER sockets
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 When set to 1, the socket will automatically send an empty message when a

include/zmq.h

@@ -344,6 +344,7 @@ ZMQ_EXPORT const char *zmq_msg_group (zmq_msg_t *msg);
 #define ZMQ_VMCI_BUFFER_MIN_SIZE 86
 #define ZMQ_VMCI_BUFFER_MAX_SIZE 87
 #define ZMQ_VMCI_CONNECT_TIMEOUT 88
+#define ZMQ_PRE_ALLOCATED_FD 89
 
 /*  Message options                                                           */
 #define ZMQ_MORE 1

src/options.cpp

@@ -77,7 +77,8 @@ zmq::options_t::options_t () :
     connected (false),
     heartbeat_ttl (0),
     heartbeat_interval (0),
-    heartbeat_timeout (-1)
+    heartbeat_timeout (-1),
+    pre_allocated_fd (-1)
 {
 #if defined ZMQ_HAVE_VMCI
     vmci_buffer_size = 0;
@@ -621,6 +622,13 @@ int zmq::options_t::setsockopt (int option_, const void *optval_,
             break;
 #       endif
 
+        case ZMQ_PRE_ALLOCATED_FD:
+            if (is_int && value >= -1) {
+                pre_allocated_fd = value;
+                return 0;
+            }
+            break;
+
         default:
 #if defined (ZMQ_ACT_MILITANT)
             //  There are valid scenarios for probing with unknown socket option
@@ -1031,6 +1039,13 @@ int zmq::options_t::getsockopt (int option_, void *optval_, size_t *optvallen_)
             }
             break;
 
+        case ZMQ_PRE_ALLOCATED_FD:
+            if (is_int) {
+                *value = pre_allocated_fd;
+                return 0;
+            }
+            break;
+
         default:
 #if defined (ZMQ_ACT_MILITANT)
             malformed = false;

src/options.hpp

@@ -231,6 +231,11 @@ namespace zmq
         uint64_t vmci_buffer_max_size;
         int vmci_connect_timeout;
 #       endif
+
+        //  When creating a new ZMQ socket, if this option is set the value
+        //  will be used as the File Descriptor instead of allocating a new
+        //  one via the socket () system call.
+        int pre_allocated_fd;
     };
 }
 

tests/test_setsockopt.cpp

@@ -70,9 +70,37 @@ void test_setsockopt_tcp_send_buffer()
     zmq_ctx_term(ctx);
 }
 
+void test_setsockopt_pre_allocated_fd()
+{
+    int rc;
+    void *ctx = zmq_ctx_new();
+    void *socket = zmq_socket(ctx, ZMQ_PUSH);
+
+    int val = 0;
+    size_t placeholder = sizeof(val);
+
+    rc = zmq_getsockopt(socket, ZMQ_PRE_ALLOCATED_FD, &val, &placeholder);
+    assert(rc == 0);
+    assert(val == -1);
+
+    val = 3;
+
+    rc = zmq_setsockopt(socket, ZMQ_PRE_ALLOCATED_FD, &val, sizeof(val));
+    assert(rc == 0);
+    assert(val == 3);
+
+    rc = zmq_getsockopt(socket, ZMQ_PRE_ALLOCATED_FD, &val, &placeholder);
+    assert(rc == 0);
+    assert(val == 3);
+
+    zmq_close(socket);
+    zmq_ctx_term(ctx);
+}
+
 
 int main()
 {
     test_setsockopt_tcp_recv_buffer();
     test_setsockopt_tcp_send_buffer();
+    test_setsockopt_pre_allocated_fd();
 }