Mojo: Abstract out write/writev vs send/sendmsg (for Unix domain sockets).

mojo/embedder/platform_channel_utils_posix.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "mojo/embedder/platform_channel_utils_posix.h"
+
+#include <sys/socket.h>
+#include <sys/uio.h>
+#include <unistd.h>
+
+#include "base/posix/eintr_wrapper.h"
+#include "build/build_config.h"
+
+namespace mojo {
+namespace embedder {
+
+// On Linux, |SIGPIPE| is suppressed by passing |MSG_NOSIGNAL| to
+// |send()|/|sendmsg()|. (There is no way of suppressing |SIGPIPE| on
+// |write()|/|writev().) On Mac, |SIGPIPE| is suppressed by setting the
+// |SO_NOSIGPIPE| option on the socket.
+//
+// Performance notes:
+//  - On Linux, we have to use |send()|/|sendmsg()| rather than
+//    |write()|/|writev()| in order to suppress |SIGPIPE|. This is okay, since
+//    |send()| is (slightly) faster than |write()| (!), while |sendmsg()| is
+//    quite comparable to |writev()|.
+//  - On Mac, we may use |write()|/|writev()|. Here, |write()| is considerably
+//    faster than |send()|, whereas |sendmsg()| is quite comparable to
+//    |writev()|.
+//  - On both platforms, an appropriate |sendmsg()|/|writev()| is considerably
+//    faster than two |send()|s/|write()|s.
+//  - Relative numbers (minimum real times from 10 runs) for one |write()| of
+//    1032 bytes, one |send()| of 1032 bytes, one |writev()| of 32+1000 bytes,
+//    one |sendmsg()| of 32+1000 bytes, two |write()|s of 32 and 1000 bytes, two
+//    |send()|s of 32 and 1000 bytes:
+//    - Linux: 0.81 s, 0.77 s, 0.87 s, 0.89 s, 1.31 s, 1.22 s
+//    - Mac: 2.21 s, 2.91 s, 2.98 s, 3.08 s, 3.59 s, 4.74 s
+
+ssize_t PlatformChannelWrite(PlatformHandle h,
+                             const void* bytes,
+                             size_t num_bytes) {
+#if defined(OS_MACOSX)
+  return HANDLE_EINTR(write(h.fd, bytes, num_bytes));
+#else
+  return send(h.fd, bytes, num_bytes, MSG_NOSIGNAL);
+#endif
+}
+
+ssize_t PlatformChannelWritev(PlatformHandle h,
+                              struct iovec* iov,
+                              size_t num_iov) {
+#if defined(OS_MACOSX)
+  return HANDLE_EINTR(writev(h.fd, iov, static_cast<int>(num_iov)));
+#else
+  struct msghdr msg = {};
+  msg.msg_iov = iov;
+  msg.msg_iovlen = num_iov;
+  return HANDLE_EINTR(sendmsg(h.fd, &msg, MSG_NOSIGNAL));
+#endif
+}
+
+}  // namespace embedder
+}  // namespace mojo