Problem: eventfd leaks socket on fork+exec

Solution: if available, use eventfd with EFD_CLOEXEC flag to make
the process close the socket on fork+exec

CMakeLists.txt

@@ -183,6 +183,9 @@ check_include_files (ifaddrs.h ZMQ_HAVE_IFADDRS)
 check_include_files (windows.h ZMQ_HAVE_WINDOWS)
 check_include_files (sys/uio.h ZMQ_HAVE_UIO)
 check_include_files (sys/eventfd.h ZMQ_HAVE_EVENTFD)
+if (ZMQ_HAVE_EVENTFD)
+  zmq_check_efd_cloexec ()
+endif ()
 
 check_library_exists (ws2_32 fopen "" HAVE_WS2_32) # TODO: Why doesn't something logical like WSAStartup work?
 check_library_exists (ws2 fopen "" HAVE_WS2)

acinclude.m4

@@ -614,6 +614,29 @@ int main (int argc, char *argv [])
     AS_IF([test "x$libzmq_cv_sock_cloexec" = "xyes"], [$1], [$2])
 }])
 
+dnl ################################################################################
+dnl # LIBZMQ_CHECK_EVENTFD_CLOEXEC([action-if-found], [action-if-not-found])          #
+dnl # Check if EFD_CLOEXEC is supported                                           #
+dnl ################################################################################
+AC_DEFUN([LIBZMQ_CHECK_EVENTFD_CLOEXEC], [{
+    AC_CACHE_CHECK([whether EFD_CLOEXEC is supported], [libzmq_cv_efd_cloexec],
+        [AC_TRY_RUN([/* EFD_CLOEXEC test */
+#include <sys/eventfd.h>
+
+int main (int argc, char *argv [])
+{
+    int s = eventfd (0, EFD_CLOEXEC);
+    return (s == -1);
+}
+        ],
+        [libzmq_cv_efd_cloexec="yes"],
+        [libzmq_cv_efd_cloexec="no"],
+        [libzmq_cv_efd_cloexec="not during cross-compile"]
+        )]
+    )
+    AS_IF([test "x$libzmq_cv_efd_cloexec" = "xyes"], [$1], [$2])
+}])
+
 dnl ################################################################################
 dnl # LIBZMQ_CHECK_ATOMIC_INSTRINSICS([action-if-found], [action-if-not-found])    #
 dnl # Check if compiler supoorts __atomic_Xxx intrinsics                           #

builds/cmake/Modules/ZMQSourceRunChecks.cmake

@@ -16,6 +16,21 @@ int main(int argc, char *argv [])
     ZMQ_HAVE_SOCK_CLOEXEC)
 endmacro()
 
+macro(zmq_check_efd_cloexec)
+  message(STATUS "Checking whether EFD_CLOEXEC is supported")
+  check_c_source_runs(
+    "
+#include <sys/eventfd.h>
+
+int main(int argc, char *argv [])
+{
+    int s = eventfd (0, EFD_CLOEXEC);
+    return(s == -1);
+}
+"
+    ZMQ_HAVE_EVENTFD_CLOEXEC)
+endmacro()
+
 # TCP keep-alives Checks.
 
 macro(zmq_check_so_keepalive)

builds/cmake/platform.hpp.in

@@ -17,6 +17,7 @@
 #cmakedefine ZMQ_HAVE_UIO
 
 #cmakedefine ZMQ_HAVE_EVENTFD
+#cmakedefine ZMQ_HAVE_EVENTFD_CLOEXEC
 #cmakedefine ZMQ_HAVE_IFADDRS
 
 #cmakedefine ZMQ_HAVE_SO_PEERCRED

configure.ac

@@ -358,8 +358,14 @@ AC_ARG_ENABLE([eventfd],
 
 if test "x$zmq_enable_eventfd" = "xyes"; then
     # Check if we have eventfd.h header file.
-    AC_CHECK_HEADERS(sys/eventfd.h,
-        [AC_DEFINE(ZMQ_HAVE_EVENTFD, 1, [Have eventfd extension])])
+    AC_CHECK_HEADERS(sys/eventfd.h, [
+        AC_DEFINE(ZMQ_HAVE_EVENTFD, 1, [Have eventfd extension])
+        LIBZMQ_CHECK_EVENTFD_CLOEXEC([
+            AC_DEFINE([ZMQ_HAVE_EVENTFD_CLOEXEC],
+               [1],
+               [Whether EFD_CLOEXEC is defined and functioning.])
+            ])
+    ])
 fi
 
 # Conditionally build performance measurement tools

src/signaler.cpp

@@ -381,7 +381,14 @@ void zmq::signaler_t::forked ()
 int zmq::signaler_t::make_fdpair (fd_t *r_, fd_t *w_)
 {
 #if defined ZMQ_HAVE_EVENTFD
-    fd_t fd = eventfd (0, 0);
+    int flags = 0;
+#if defined ZMQ_HAVE_EVENTFD_CLOEXEC
+    //  Setting this option result in sane behaviour when exec() functions
+    //  are used. Old sockets are closed and don't block TCP ports, avoid
+    //  leaks, etc.
+    flags |= EFD_CLOEXEC;
+#endif
+    fd_t fd = eventfd (0, flags);
     if (fd == -1) {
         errno_assert (errno == ENFILE || errno == EMFILE);
         *w_ = *r_ = -1;