Problem: Missing doc & unit tests for zmq_poller_fd

Solution: Add doc & unit tests

doc/zmq_poller.txt

@@ -21,11 +21,16 @@ SYNOPSIS
 *int zmq_poller_modify_fd (void *'poller', int 'fd', short 'events');*
 *int zmq_poller_remove_fd (void *'poller', int 'fd');*
 
+*int zmq_poller_wait (void *'poller',
+                          zmq_poller_event_t *'event',
+                          long 'timeout');*
 *int zmq_poller_wait_all (void *'poller',
                           zmq_poller_event_t *'events',
                           int 'n_events',
                           long 'timeout');*
 
+*int zmq_poller_fd (void *'poller');*
+
 DESCRIPTION
 -----------
 The _zmq_poller_*_ functions provide a mechanism for applications to multiplex
@@ -124,6 +129,10 @@ The client does therefore not need to initialize the contents of the events
 array before a call to _zmq_poller_wait_all_. It is unspecified whether the
 the remaining elements of 'events' are written to by _zmq_poller_wait_all_.
 
+_zmq_poller_fd_ returns the file descriptor associated with the zmq_poller.
+The zmq_poller is only guaranteed to have a file descriptor if
+at least one thread-safe socket is currently registered.
+
 EVENT TYPES
 -----------
 
@@ -237,6 +246,9 @@ available.
 *EAGAIN*::
 No registered event was signalled before the timeout was reached.
 
+On _zmq_poller_fd:
+*EINVAL*::
+The poller has no associated file descriptor.
 
 EXAMPLE
 -------

tests/test_poller.cpp

@@ -257,6 +257,36 @@ void test_with_valid_poller (extra_poller_func_t extra_func_)
     test_context_socket_close (socket);
 }
 
+void test_call_poller_fd_no_signaler ()
+{
+    void *socket = test_context_socket (ZMQ_PAIR);
+
+    void *poller = zmq_poller_new ();
+    TEST_ASSERT_NOT_NULL (poller);
+
+    TEST_ASSERT_SUCCESS_ERRNO (
+      zmq_poller_add (poller, socket, NULL, ZMQ_POLLIN));
+
+    TEST_ASSERT_FAILURE_ERRNO (EINVAL, zmq_poller_fd (poller));
+
+    test_context_socket_close (socket);
+}
+
+void test_call_poller_fd ()
+{
+    void *socket = test_context_socket (ZMQ_CLIENT);
+
+    void *poller = zmq_poller_new ();
+    TEST_ASSERT_NOT_NULL (poller);
+
+    TEST_ASSERT_SUCCESS_ERRNO (
+      zmq_poller_add (poller, socket, NULL, ZMQ_POLLIN));
+
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_poller_fd (poller));
+
+    test_context_socket_close (socket);
+}
+
 void call_poller_wait_null_event_fails (void *poller_)
 {
     TEST_ASSERT_FAILURE_ERRNO (EFAULT, zmq_poller_wait (poller_, NULL, 0));
@@ -652,6 +682,9 @@ int main (void)
     RUN_TEST (test_call_poller_wait_all_empty_without_timeout_fails);
     RUN_TEST (test_call_poller_wait_all_empty_with_timeout_fails);
 
+    RUN_TEST (test_call_poller_fd_no_signaler);
+    RUN_TEST (test_call_poller_fd);
+
     RUN_TEST (test_poll_basic);
     RUN_TEST (test_poll_fd);
     RUN_TEST (test_poll_client_server);