Problem: No support to query poller size

Solution: Add zmq_poller_size that queries the number
of objects registered, allowing safer usages of poller
to avoid livelock situations.

doc/zmq_poller.txt

@@ -14,6 +14,8 @@ SYNOPSIS
 
 *int zmq_poller_destroy (void ****'poller_p');*
 
+*int zmq_poller_size (void *'poller');*
+
 *int zmq_poller_add (void *'poller', void *'socket', void *'user_data', short 'events');*
 
 *int zmq_poller_modify (void *'poller', void *'socket', short 'events');*
@@ -51,6 +53,11 @@ instance. _zmq_poller_destroy_ sets the passed pointer to NULL in case of a
 successful execution. _zmq_poller_destroy_ implicitly unregisters all
 registered sockets and file descriptors.
 
+_zmq_poller_size_ queries the number of sockets or file descriptors registered
+with a poller. The initial size of a poller is 0, a successful add operation
+increases the size by 1 and a successful remove operation decreases the size
+by 1. The size is unaffected by the events specified.
+
 _zmq_poller_add_, _zmq_poller_modify_ and _zmq_poller_remove_ manage the 0MQ
 sockets registered with a poller.
 
@@ -129,7 +136,8 @@ registered objects. Otherwise, a livelock situation may result: If more than
 'n_events' registered objects have an active event on each call to 
 _zmq_poller_wait_all_, it might happen that the same subset of registered 
 objects is always returned, and the caller never notices the events on the 
-others.
+others. The number of objects registered can be queried with
+_zmq_poller_size_.
 
 _zmq_poller_wait_all_ returns the number of valid elements. The valid elements
 are placed in positions '0' to 'n_events - 1' in the 'events' array. All
@@ -219,6 +227,12 @@ On _zmq_poller_destroy_:
 _poller_p_ did not point to a valid poller. Note that passing an invalid pointer (e.g.
 pointer to deallocated memory) may cause undefined behaviour (e.g. an access violation).
 
+On _zmq_poller_size_:
+*EFAULT*::
+_poller_ did not point to a valid poller. Note that passing an
+invalid pointer (e.g. pointer to deallocated memory) may cause undefined
+behaviour (e.g. an access violation).
+
 On _zmq_poller_add_, _zmq_poller_modify_ and _zmq_poller_remove_:
 *EFAULT*::
 _poller_ did not point to a valid poller. Note that passing an
@@ -264,7 +278,7 @@ available.
 *EAGAIN*::
 No registered event was signalled before the timeout was reached.
 
-On _zmq_poller_fd:
+On _zmq_poller_fd_:
 *EINVAL*::
 The poller has no associated file descriptor.
 *EFAULT*::

include/zmq.h

@@ -735,6 +735,7 @@ typedef struct zmq_poller_event_t
 
 ZMQ_EXPORT void *zmq_poller_new (void);
 ZMQ_EXPORT int zmq_poller_destroy (void **poller_p);
+ZMQ_EXPORT int zmq_poller_size (void *poller);
 ZMQ_EXPORT int
 zmq_poller_add (void *poller, void *socket, void *user_data, short events);
 ZMQ_EXPORT int zmq_poller_modify (void *poller, void *socket, short events);

src/zmq.cpp

@@ -1219,6 +1219,14 @@ static int check_poller_fd_registration_args (void *const poller_,
     return 0;
 }
 
+int zmq_poller_size (void *poller_)
+{
+    if (-1 == check_poller (poller_))
+        return -1;
+
+    return (static_cast<zmq::socket_poller_t *> (poller_))->size ();
+}
+
 int zmq_poller_add (void *poller_, void *s_, void *user_data_, short events_)
 {
     if (-1 == check_poller_registration_args (poller_, s_)

src/zmq_draft.h

@@ -124,6 +124,7 @@ typedef struct zmq_poller_event_t
 
 void *zmq_poller_new (void);
 int zmq_poller_destroy (void **poller_p_);
+int zmq_poller_size (void *poller_);
 int zmq_poller_add (void *poller_,
                     void *socket_,
                     void *user_data_,

tests/test_poller.cpp

@@ -60,6 +60,17 @@ void test_null_poller_pointers_destroy_indirect ()
     TEST_ASSERT_FAILURE_ERRNO (EFAULT, zmq_poller_destroy (&null_poller));
 }
 
+void test_null_poller_pointers_size_direct ()
+{
+    TEST_ASSERT_FAILURE_ERRNO (EFAULT, zmq_poller_size (NULL));
+}
+
+void test_null_poller_pointers_size_indirect ()
+{
+    void *null_poller = NULL;
+    TEST_ASSERT_FAILURE_ERRNO (EFAULT, zmq_poller_size (&null_poller));
+}
+
 void test_null_poller_pointers_add_direct ()
 {
     void *socket = test_context_socket (ZMQ_PAIR);
@@ -330,6 +341,42 @@ TEST_CASE_FUNC_PARAM (call_poller_wait_all_null_event_fails_event_count_nonzero,
 TEST_CASE_FUNC_PARAM (call_poller_wait_all_null_event_fails_event_count_zero,
                       test_with_valid_poller)
 
+void call_poller_size (void *poller_, void *socket_)
+{
+    int rc = zmq_poller_size (poller_);
+    TEST_ASSERT_SUCCESS_ERRNO (rc);
+    TEST_ASSERT_EQUAL (rc, 0);
+
+    TEST_ASSERT_SUCCESS_ERRNO (
+      zmq_poller_add (poller_, socket_, NULL, ZMQ_POLLIN));
+    rc = zmq_poller_size (poller_);
+    TEST_ASSERT_SUCCESS_ERRNO (rc);
+    TEST_ASSERT_EQUAL (rc, 1);
+
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_poller_modify (poller_, socket_, 0));
+    rc = zmq_poller_size (poller_);
+    TEST_ASSERT_SUCCESS_ERRNO (rc);
+    TEST_ASSERT_EQUAL (rc, 1);
+
+    fd_t plain_socket = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
+    TEST_ASSERT_SUCCESS_ERRNO (
+      zmq_poller_add_fd (poller_, plain_socket, NULL, ZMQ_POLLOUT));
+    rc = zmq_poller_size (poller_);
+    TEST_ASSERT_SUCCESS_ERRNO (rc);
+    TEST_ASSERT_EQUAL (rc, 2);
+
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_poller_remove (poller_, socket_));
+    rc = zmq_poller_size (poller_);
+    TEST_ASSERT_SUCCESS_ERRNO (rc);
+    TEST_ASSERT_EQUAL (rc, 1);
+
+    TEST_ASSERT_SUCCESS_ERRNO (zmq_poller_remove_fd (poller_, plain_socket));
+    TEST_ASSERT_SUCCESS_ERRNO (close (plain_socket));
+    rc = zmq_poller_size (poller_);
+    TEST_ASSERT_SUCCESS_ERRNO (rc);
+    TEST_ASSERT_EQUAL (rc, 0);
+}
+
 void call_poller_add_twice_fails (void *poller_, void *socket_)
 {
     TEST_ASSERT_SUCCESS_ERRNO (
@@ -449,6 +496,7 @@ void call_poller_modify_fd_invalid_events_fails (void *poller_,
     TEST_ASSERT_SUCCESS_ERRNO (close (plain_socket));
 }
 
+TEST_CASE_FUNC_PARAM (call_poller_size, test_with_empty_poller)
 TEST_CASE_FUNC_PARAM (call_poller_add_twice_fails, test_with_empty_poller)
 TEST_CASE_FUNC_PARAM (call_poller_remove_unregistered_fails,
                       test_with_empty_poller)
@@ -666,6 +714,8 @@ int main (void)
     UNITY_BEGIN ();
     RUN_TEST (test_null_poller_pointers_destroy_direct);
     RUN_TEST (test_null_poller_pointers_destroy_indirect);
+    RUN_TEST (test_null_poller_pointers_size_direct);
+    RUN_TEST (test_null_poller_pointers_size_indirect);
     RUN_TEST (test_null_poller_pointers_add_direct);
     RUN_TEST (test_null_poller_pointers_add_indirect);
     RUN_TEST (test_null_poller_pointers_modify_direct);
@@ -690,6 +740,7 @@ int main (void)
     RUN_TEST (test_call_poller_wait_all_null_event_fails_event_count_nonzero);
     RUN_TEST (test_call_poller_wait_all_null_event_fails_event_count_zero);
 
+    RUN_TEST (test_call_poller_size);
     RUN_TEST (test_call_poller_add_twice_fails);
     RUN_TEST (test_call_poller_remove_unregistered_fails);
     RUN_TEST (test_call_poller_modify_unregistered_fails);