Problem: inconsistent error handling and unnecessary code duplication

Solution: make error handling consistent and use retired_fd to remove code duplication

src/ip.cpp

@@ -76,14 +76,13 @@ zmq::fd_t zmq::open_socket (int domain_, int type_, int protocol_)
     type_ |= SOCK_CLOEXEC;
 #endif
 
-    fd_t s = socket (domain_, type_, protocol_);
+    const fd_t s = socket (domain_, type_, protocol_);
+    if (s == retired_fd) {
 #ifdef ZMQ_HAVE_WINDOWS
-    if (s == INVALID_SOCKET)
-        return INVALID_SOCKET;
-#else
-    if (s == -1)
-        return -1;
+        errno = wsa_error_to_errno (WSAGetLastError ());
 #endif
+        return retired_fd;
+    }
 
         //  If there's no SOCK_CLOEXEC, let's try the second best option. Note that
         //  race condition can cause socket not to be closed (if fork happens

src/socks_connecter.cpp

@@ -309,13 +309,8 @@ int zmq::socks_connecter_t::connect_to_proxy ()
 
     //  Create the socket.
     s = open_socket (tcp_addr->family (), SOCK_STREAM, IPPROTO_TCP);
-#ifdef ZMQ_HAVE_WINDOWS
-    if (s == INVALID_SOCKET)
-        return -1;
-#else
-    if (s == -1)
+    if (s == retired_fd)
         return -1;
-#endif
 
     //  On some systems, IPv4 mapping in IPv6 sockets is disabled by default.
     //  Switch it on in such cases.

src/tcp_connecter.cpp

@@ -279,15 +279,9 @@ int zmq::tcp_connecter_t::open ()
         s = open_socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
     }
 
-#ifdef ZMQ_HAVE_WINDOWS
-    if (s == INVALID_SOCKET) {
-        errno = wsa_error_to_errno (WSAGetLastError ());
+    if (s == retired_fd) {
         return -1;
     }
-#else
-    if (s == -1)
-        return -1;
-#endif
 
     //  On some systems, IPv4 mapping in IPv6 sockets is disabled by default.
     //  Switch it on in such cases.

src/tcp_listener.cpp

@@ -195,20 +195,15 @@ int zmq::tcp_listener_t::set_address (const char *addr_)
         s = open_socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
     }
 
-#ifdef ZMQ_HAVE_WINDOWS
-    if (s == INVALID_SOCKET) {
-        errno = wsa_error_to_errno (WSAGetLastError ());
+    if (s == retired_fd) {
         return -1;
     }
-#if !defined _WIN32_WCE && !defined ZMQ_HAVE_WINDOWS_UWP
+#if defined ZMQ_HAVE_WINDOWS && !defined _WIN32_WCE                            \
+  && !defined ZMQ_HAVE_WINDOWS_UWP
     //  On Windows, preventing sockets to be inherited by child processes.
     BOOL brc = SetHandleInformation ((HANDLE) s, HANDLE_FLAG_INHERIT, 0);
     win_assert (brc);
 #endif
-#else
-    if (s == -1)
-        return -1;
-#endif
 
     //  On some systems, IPv4 mapping in IPv6 sockets is disabled by default.
     //  Switch it on in such cases.
@@ -306,26 +301,25 @@ zmq::fd_t zmq::tcp_listener_t::accept ()
       ::accept (s, reinterpret_cast<struct sockaddr *> (&ss), &ss_len);
 #endif
 
+    if (sock == retired_fd) {
 #ifdef ZMQ_HAVE_WINDOWS
-    if (sock == INVALID_SOCKET) {
         const int last_error = WSAGetLastError ();
         wsa_assert (last_error == WSAEWOULDBLOCK || last_error == WSAECONNRESET
                     || last_error == WSAEMFILE || last_error == WSAENOBUFS);
-        return retired_fd;
-    }
-#if !defined _WIN32_WCE && !defined ZMQ_HAVE_WINDOWS_UWP
-    //  On Windows, preventing sockets to be inherited by child processes.
-    BOOL brc = SetHandleInformation ((HANDLE) sock, HANDLE_FLAG_INHERIT, 0);
-    win_assert (brc);
-#endif
 #else
-    if (sock == -1) {
         errno_assert (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR
                       || errno == ECONNABORTED || errno == EPROTO
                       || errno == ENOBUFS || errno == ENOMEM || errno == EMFILE
                       || errno == ENFILE);
+#endif
         return retired_fd;
     }
+
+#if defined ZMQ_HAVE_WINDOWS && !defined _WIN32_WCE                            \
+  && !defined ZMQ_HAVE_WINDOWS_UWP
+    //  On Windows, preventing sockets to be inherited by child processes.
+    BOOL brc = SetHandleInformation ((HANDLE) sock, HANDLE_FLAG_INHERIT, 0);
+    win_assert (brc);
 #endif
 
 #if (!defined ZMQ_HAVE_SOCK_CLOEXEC || !defined HAVE_ACCEPT4)                  \