stream: process more than one write req per loop tick (#3728)

Fixes: https://github.com/libuv/libuv/issues/3668

CMakeLists.txt

@@ -584,6 +584,7 @@ if(LIBUV_BUILD_TESTS)
        test/test-tcp-rst.c
        test/test-tcp-shutdown-after-write.c
        test/test-tcp-try-write.c
+       test/test-tcp-write-in-a-row.c
        test/test-tcp-try-write-error.c
        test/test-tcp-unexpected-read.c
        test/test-tcp-write-after-connect.c

Makefile.am

@@ -277,6 +277,7 @@ test_run_tests_SOURCES = test/blackhole-server.c \
                          test/test-tcp-writealot.c \
                          test/test-tcp-write-fail.c \
                          test/test-tcp-try-write.c \
+                         test/test-tcp-write-in-a-row.c \
                          test/test-tcp-try-write-error.c \
                          test/test-tcp-write-queue-order.c \
                          test/test-test-macros.c \

src/unix/stream.c

@@ -841,9 +841,16 @@ static void uv__write(uv_stream_t* stream) {
   QUEUE* q;
   uv_write_t* req;
   ssize_t n;
+  int count;
 
   assert(uv__stream_fd(stream) >= 0);
 
+  /* Prevent loop starvation when the consumer of this stream read as fast as
+   * (or faster than) we can write it. This `count` mechanism does not need to
+   * change even if we switch to edge-triggered I/O.
+   */
+  count = 32;
+
   for (;;) {
     if (QUEUE_EMPTY(&stream->write_queue))
       return;
@@ -862,10 +869,13 @@ static void uv__write(uv_stream_t* stream) {
       req->send_handle = NULL;
       if (uv__write_req_update(stream, req, n)) {
         uv__write_req_finish(req);
-        return;  /* TODO(bnoordhuis) Start trying to write the next request. */
+        if (count-- > 0)
+          continue; /* Start trying to write the next request. */
+
+        return;
       }
     } else if (n != UV_EAGAIN)
-      break;
+      goto error;
 
     /* If this is a blocking stream, try again. */
     if (stream->flags & UV_HANDLE_BLOCKING_WRITES)
@@ -880,6 +890,7 @@ static void uv__write(uv_stream_t* stream) {
     return;
   }
 
+error:
   req->error = n;
   uv__write_req_finish(req);
   uv__io_stop(stream->loop, &stream->io_watcher, POLLOUT);

test/test-list.h

@@ -105,6 +105,7 @@ TEST_DECLARE   (tcp_write_after_connect)
 TEST_DECLARE   (tcp_writealot)
 TEST_DECLARE   (tcp_write_fail)
 TEST_DECLARE   (tcp_try_write)
+TEST_DECLARE   (tcp_write_in_a_row)
 TEST_DECLARE   (tcp_try_write_error)
 TEST_DECLARE   (tcp_write_queue_order)
 TEST_DECLARE   (tcp_open)
@@ -670,6 +671,7 @@ TASK_LIST_START
   TEST_HELPER (tcp_write_fail, tcp4_echo_server)
 
   TEST_ENTRY  (tcp_try_write)
+  TEST_ENTRY  (tcp_write_in_a_row)
   TEST_ENTRY  (tcp_try_write_error)
 
   TEST_ENTRY  (tcp_write_queue_order)

test/test-tcp-write-in-a-row.c

@@ -0,0 +1,141 @@
+/* Copyright libuv project 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 <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "task.h"
+#include "uv.h"
+
+static uv_tcp_t server;
+static uv_tcp_t client;
+static uv_tcp_t incoming;
+static int connect_cb_called;
+static int close_cb_called;
+static int connection_cb_called;
+static int write_cb_called;
+static uv_write_t small_write;
+static uv_write_t big_write;
+
+/* 10 MB, which is large than the send buffer size and the recv buffer */
+static char data[1024 * 1024 * 10];
+
+static void close_cb(uv_handle_t* handle) {
+  close_cb_called++;
+}
+
+static void write_cb(uv_write_t* w, int status) {
+  /* the small write should finish immediately after the big write */
+  ASSERT_EQ(0, uv_stream_get_write_queue_size((uv_stream_t*) &client));
+
+  write_cb_called++;
+
+  if (write_cb_called == 2) {
+    /* we are done */
+    uv_close((uv_handle_t*) &client, close_cb);
+    uv_close((uv_handle_t*) &incoming, close_cb);
+    uv_close((uv_handle_t*) &server, close_cb);
+  }
+}
+
+static void connect_cb(uv_connect_t* _, int status) {
+  int r;
+  uv_buf_t buf;
+  size_t write_queue_size0, write_queue_size1;
+
+  ASSERT_EQ(0, status);
+  connect_cb_called++;
+
+  /* fire a big write */
+  buf = uv_buf_init(data, sizeof(data));
+  r = uv_write(&small_write, (uv_stream_t*) &client, &buf, 1, write_cb);
+  ASSERT_EQ(0, r);
+
+  /* check that the write process gets stuck */
+  write_queue_size0 = uv_stream_get_write_queue_size((uv_stream_t*) &client);
+  ASSERT_GT(write_queue_size0, 0);
+
+  /* fire a small write, which should be queued */
+  buf = uv_buf_init("A", 1);
+  r = uv_write(&big_write, (uv_stream_t*) &client, &buf, 1, write_cb);
+  ASSERT_EQ(0, r);
+
+  write_queue_size1 = uv_stream_get_write_queue_size((uv_stream_t*) &client);
+  ASSERT_EQ(write_queue_size1, write_queue_size0 + 1);
+}
+
+static void alloc_cb(uv_handle_t* handle, size_t size, uv_buf_t* buf) {
+  static char base[1024];
+
+  buf->base = base;
+  buf->len = sizeof(base);
+}
+
+static void read_cb(uv_stream_t* tcp, ssize_t nread, const uv_buf_t* buf) {}
+
+static void connection_cb(uv_stream_t* tcp, int status) {
+  ASSERT_EQ(0, status);
+  connection_cb_called++;
+
+  ASSERT_EQ(0, uv_tcp_init(tcp->loop, &incoming));
+  ASSERT_EQ(0, uv_accept(tcp, (uv_stream_t*) &incoming));
+  ASSERT_EQ(0, uv_read_start((uv_stream_t*) &incoming, alloc_cb, read_cb));
+}
+
+static void start_server(void) {
+  struct sockaddr_in addr;
+
+  ASSERT_EQ(0, uv_ip4_addr("0.0.0.0", TEST_PORT, &addr));
+
+  ASSERT_EQ(0, uv_tcp_init(uv_default_loop(), &server));
+  ASSERT_EQ(0, uv_tcp_bind(&server, (struct sockaddr*) &addr, 0));
+  ASSERT_EQ(0, uv_listen((uv_stream_t*) &server, 128, connection_cb));
+}
+
+TEST_IMPL(tcp_write_in_a_row) {
+#if defined(_WIN32)
+  RETURN_SKIP("tcp_write_in_a_row does not work on Windows");
+#endif
+
+  uv_connect_t connect_req;
+  struct sockaddr_in addr;
+
+  start_server();
+
+  ASSERT_EQ(0, uv_ip4_addr("127.0.0.1", TEST_PORT, &addr));
+
+  ASSERT_EQ(0, uv_tcp_init(uv_default_loop(), &client));
+  ASSERT_EQ(0, uv_tcp_connect(&connect_req,
+                              &client,
+                              (struct sockaddr*) &addr,
+                              connect_cb));
+
+  ASSERT_EQ(0, uv_run(uv_default_loop(), UV_RUN_DEFAULT));
+
+  ASSERT_EQ(1, connect_cb_called);
+  ASSERT_EQ(3, close_cb_called);
+  ASSERT_EQ(1, connection_cb_called);
+  ASSERT_EQ(2, write_cb_called);
+
+  MAKE_VALGRIND_HAPPY();
+  return 0;
+}