unix: remove incorrect assert

After 41891222bc landed it's possible that uv__udp_sendmsg is called
even if there are no pending write nor write completed requests:

1. User calls uv_udp_send and the request is sent immediately. The
   request is the added to the completed queue and we 'feed' the uv__io
   handle so that we process the completed request in the next
   iteration.
2. User calls uv_udp_send again but the request is not completed
   immediately, so it's queued in the write_queue.
3. The uv__io handle gets a UV__POLLOUT event and uv__udp_sendmsg is
   run, which completes the send request and puts it in the
   write_completed_queue. Afterwards, uv__udp_run_completed is executed
   and the write_completed queue is drained.
4. At this point, the uv__io handle was made pending in step 3, in
   uv__udp_sendmsg, but we no longer have requests to write or to complete,
   so we skip processing.

Makefile.am

@@ -214,6 +214,7 @@ test_run_tests_SOURCES = test/blackhole-server.c \
                          test/test-udp-open.c \
                          test/test-udp-options.c \
                          test/test-udp-send-and-recv.c \
+                         test/test-udp-send-immediate.c \
                          test/test-udp-try-send.c \
                          test/test-walk-handles.c \
                          test/test-watcher-cross-stop.c

src/unix/udp.c

@@ -210,9 +210,6 @@ static void uv__udp_sendmsg(uv_udp_t* handle) {
   struct msghdr h;
   ssize_t size;
 
-  assert(!QUEUE_EMPTY(&handle->write_queue)
-      || !QUEUE_EMPTY(&handle->write_completed_queue));
-
   while (!QUEUE_EMPTY(&handle->write_queue)) {
     q = QUEUE_HEAD(&handle->write_queue);
     assert(q != NULL);

test/test-list.h

@@ -84,6 +84,7 @@ TEST_DECLARE   (tcp_bind6_localhost_ok)
 TEST_DECLARE   (udp_bind)
 TEST_DECLARE   (udp_bind_reuseaddr)
 TEST_DECLARE   (udp_send_and_recv)
+TEST_DECLARE   (udp_send_immediate)
 TEST_DECLARE   (udp_multicast_join)
 TEST_DECLARE   (udp_multicast_join6)
 TEST_DECLARE   (udp_multicast_ttl)
@@ -367,6 +368,7 @@ TASK_LIST_START
   TEST_ENTRY  (udp_bind)
   TEST_ENTRY  (udp_bind_reuseaddr)
   TEST_ENTRY  (udp_send_and_recv)
+  TEST_ENTRY  (udp_send_immediate)
   TEST_ENTRY  (udp_dgram_too_big)
   TEST_ENTRY  (udp_dual_stack)
   TEST_ENTRY  (udp_ipv6_only)

test/test-udp-send-immediate.c

@@ -0,0 +1,148 @@
+/* Copyright Joyent, Inc. and other Node contributors. All rights reserved.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to
+ * deal in the Software without restriction, including without limitation the
+ * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ * sell copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#include "uv.h"
+#include "task.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define CHECK_HANDLE(handle) \
+  ASSERT((uv_udp_t*)(handle) == &server || (uv_udp_t*)(handle) == &client)
+
+static uv_udp_t server;
+static uv_udp_t client;
+
+static int cl_send_cb_called;
+static int sv_recv_cb_called;
+static int close_cb_called;
+
+
+static void alloc_cb(uv_handle_t* handle,
+                     size_t suggested_size,
+                     uv_buf_t* buf) {
+  static char slab[65536];
+  CHECK_HANDLE(handle);
+  ASSERT(suggested_size <= sizeof(slab));
+  buf->base = slab;
+  buf->len = sizeof(slab);
+}
+
+
+static void close_cb(uv_handle_t* handle) {
+  CHECK_HANDLE(handle);
+  ASSERT(1 == uv_is_closing(handle));
+  close_cb_called++;
+}
+
+
+static void cl_send_cb(uv_udp_send_t* req, int status) {
+  ASSERT(req != NULL);
+  ASSERT(status == 0);
+  CHECK_HANDLE(req->handle);
+
+  cl_send_cb_called++;
+}
+
+
+static void sv_recv_cb(uv_udp_t* handle,
+                       ssize_t nread,
+                       const uv_buf_t* rcvbuf,
+                       const struct sockaddr* addr,
+                       unsigned flags) {
+  if (nread < 0) {
+    ASSERT(0 && "unexpected error");
+  }
+
+  if (nread == 0) {
+    /* Returning unused buffer */
+    /* Don't count towards sv_recv_cb_called */
+    ASSERT(addr == NULL);
+    return;
+  }
+
+  CHECK_HANDLE(handle);
+  ASSERT(flags == 0);
+
+  ASSERT(addr != NULL);
+  ASSERT(nread == 4);
+  ASSERT(memcmp("PING", rcvbuf->base, nread) == 0 ||
+         memcmp("PANG", rcvbuf->base, nread) == 0);
+
+  if (++sv_recv_cb_called == 2) {
+    uv_close((uv_handle_t*) &server, close_cb);
+    uv_close((uv_handle_t*) &client, close_cb);
+  }
+}
+
+
+TEST_IMPL(udp_send_immediate) {
+  struct sockaddr_in addr;
+  uv_udp_send_t req1, req2;
+  uv_buf_t buf;
+  int r;
+
+  ASSERT(0 == uv_ip4_addr("0.0.0.0", TEST_PORT, &addr));
+
+  r = uv_udp_init(uv_default_loop(), &server);
+  ASSERT(r == 0);
+
+  r = uv_udp_bind(&server, (const struct sockaddr*) &addr, 0);
+  ASSERT(r == 0);
+
+  r = uv_udp_recv_start(&server, alloc_cb, sv_recv_cb);
+  ASSERT(r == 0);
+
+  ASSERT(0 == uv_ip4_addr("127.0.0.1", TEST_PORT, &addr));
+
+  r = uv_udp_init(uv_default_loop(), &client);
+  ASSERT(r == 0);
+
+  /* client sends "PING", then "PANG" */
+  buf = uv_buf_init("PING", 4);
+
+  r = uv_udp_send(&req1,
+                  &client,
+                  &buf,
+                  1,
+                  (const struct sockaddr*) &addr,
+                  cl_send_cb);
+  ASSERT(r == 0);
+
+  buf = uv_buf_init("PANG", 4);
+
+  r = uv_udp_send(&req2,
+                  &client,
+                  &buf,
+                  1,
+                  (const struct sockaddr*) &addr,
+                  cl_send_cb);
+
+  uv_run(uv_default_loop(), UV_RUN_DEFAULT);
+
+  ASSERT(cl_send_cb_called == 2);
+  ASSERT(sv_recv_cb_called == 2);
+  ASSERT(close_cb_called == 2);
+
+  MAKE_VALGRIND_HAPPY();
+  return 0;
+}

uv.gyp

@@ -394,6 +394,7 @@
         'test/test-udp-open.c',
         'test/test-udp-options.c',
         'test/test-udp-send-and-recv.c',
+        'test/test-udp-send-immediate.c',
         'test/test-udp-multicast-join.c',
         'test/test-udp-multicast-join6.c',
         'test/test-dlerror.c',