Add a SharedMemSynchronizer class.

media/audio/shared_mem_synchronizer_posix.cc

+// Copyright (c) 2012 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 "media/audio/shared_mem_synchronizer.h"
+
+#include <errno.h>
+#include <sys/poll.h>
+
+#include "base/logging.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/file_descriptor_posix.h"
+
+SharedMemSynchronizer::~SharedMemSynchronizer() {}
+
+SharedMemSynchronizer::SharedMemSynchronizer(IPCHandle handle_1,
+                                             IPCHandle handle_2)
+    : socket_(handle_1.fd) {
+  DCHECK_NE(handle_1.fd, -1);
+  DCHECK_EQ(handle_2.fd, -1);
+  DCHECK(IsValid());
+}
+
+void SharedMemSynchronizer::Signal() {
+  DCHECK(IsValid());
+  char signal = 1;
+  size_t bytes = socket_.Send(&signal, sizeof(signal));
+  DCHECK_EQ(bytes, 1U) << "errno: " << errno;
+}
+
+void SharedMemSynchronizer::Wait() {
+  DCHECK(IsValid());
+  char signal = 0;
+  size_t bytes = socket_.Receive(&signal, sizeof(signal));
+  DCHECK_EQ(bytes, 1U) << "errno: " << errno;
+  DCHECK_EQ(signal, 1);
+}
+
+bool SharedMemSynchronizer::IsValid() const {
+  return socket_.handle() != SocketClass::kInvalidHandle;
+}
+
+bool SharedMemSynchronizer::ShareToProcess(base::ProcessHandle process,
+                                           IPCHandle* handle_1,
+                                           IPCHandle* handle_2) {
+  DCHECK(IsValid());
+  handle_1->fd = socket_.handle();
+  handle_1->auto_close = false;
+  handle_2->fd = -1;
+  return true;
+}
+
+// static
+bool SharedMemSynchronizer::InitializePair(SharedMemSynchronizer* a,
+                                           SharedMemSynchronizer* b) {
+  DCHECK(!a->IsValid());
+  DCHECK(!b->IsValid());
+
+  bool ok = SocketClass::CreatePair(&a->socket_, &b->socket_);
+
+  DLOG_IF(WARNING, !ok) << "failed to create socket: " << errno;
+  DCHECK(!ok || a->IsValid());
+  DCHECK(!ok || b->IsValid());
+  return ok;
+}
+
+// static
+int SharedMemSynchronizer::WaitMultiple(const SynchronizerVector& synchronizers,
+                                        size_t last_signaled) {
+  DCHECK_LT(last_signaled, synchronizers.size());
+
+  for (size_t i = 0; i < synchronizers.size(); ++i) {
+    DCHECK(synchronizers[i]->IsValid());
+  }
+
+  // Below, we always check the |revents| of the first socket in the array
+  // and return the index of that socket if set.  This can cause sockets
+  // that come later in the array to starve when the first sockets are
+  // very busy.  So to avoid the starving problem, we use the |last_signaled|
+  // variable to split up the array so that the last socket to be signaled
+  // becomes the last socket in the array and all the other sockets will have
+  // priority the next time WaitMultiple is called.
+  scoped_array<struct pollfd> sockets(new struct pollfd[synchronizers.size()]);
+  memset(&sockets[0], 0, synchronizers.size() * sizeof(sockets[0]));
+  size_t index = 0;
+  for (size_t i = last_signaled + 1; i < synchronizers.size(); ++i) {
+    struct pollfd& fd = sockets[index++];
+    fd.events = POLLIN;
+    fd.fd = synchronizers[i]->socket_.handle();
+  }
+
+  for (size_t i = 0; i <= last_signaled; ++i) {
+    struct pollfd& fd = sockets[index++];
+    fd.events = POLLIN;
+    fd.fd = synchronizers[i]->socket_.handle();
+  }
+  DCHECK_EQ(index, synchronizers.size());
+
+  int err = poll(&sockets[0], synchronizers.size(), -1);
+  if (err != -1) {
+    for (size_t i = 0; i < synchronizers.size(); ++i) {
+      if (sockets[i].revents) {
+        int ret = (i + last_signaled + 1) % synchronizers.size();

[Ami GONE FROM CHROMIUM, 2012/03/13 20:08:02]

It is asymmetric that ret is an "int" but last_signaled is a "size_t".
 

[tommi (sloooow) - chröme, 2012/03/14 13:32:43]

Done.

+        DCHECK_EQ(sockets[i].fd, synchronizers[ret]->socket_.handle());
+        synchronizers[ret]->Wait();
+        return ret;
+      }
+    }
+  } else {
+    LOG(ERROR) << "poll() failed: " << errno;

[Ami GONE FROM CHROMIUM, 2012/03/13 20:08:02]

The case of poll returning 0 but none of the revents is true is even more of a
poll failure.  I'd omit this else and just stick a 
LOG(FATAL) << "poll() failed impossibly";
instead of the CHECK(false) below.

[tommi (sloooow) - chröme, 2012/03/14 13:32:43]

Done.

+  }
+  // Either poll() failed or we failed to find a single socket that was
+  // signaled.  Either way continuing will result in undefined behavior.
+  CHECK(false);
+  return -1;
+}