Add a SharedMemSynchronizer class.

media/audio/shared_mem_synchronizer.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"
+
+#if defined(OS_POSIX)
+#include <errno.h>
+#include <sys/poll.h>
+#endif
+
+#include "base/logging.h"
+#include "base/memory/scoped_ptr.h"
+
+#if defined(OS_POSIX)
+#include "base/file_descriptor_posix.h"
+#endif
+
+SharedMemSynchronizer::WaitForMultiple::WaitForMultiple(
+    SharedMemSynchronizer* synchronizers, size_t count)
+    : synchronizers_(synchronizers), count_(count), last_(count - 1) {
+  DCHECK_GT(count, 0U);
+}
+
+int SharedMemSynchronizer::WaitForMultiple::Wait() {
+  int ret = WaitMultiple(synchronizers_, count_, last_);
+  last_ = (ret >= 0) ? static_cast<size_t>(ret) : count_ - 1;

[Ami GONE FROM CHROMIUM, 2012/03/08 17:04:36]

When will the conditional be false, given the
DCHECK_NE(ret, -1);
at the bottom of WaitMultiple() ?

[tommi (sloooow) - chröme, 2012/03/09 12:15:49]

That DCHECK is to catch programmer error so hopefully never.

[Ami GONE FROM CHROMIUM, 2012/03/09 16:36:47]

That's what the DCHECK connotes to me, as well.
But then the error-handling code here makes me think that it's a case needing
handling.
Why not just promote the DCHECK to a CHECK and remove this test here?

(I don't see why returning count_-1 is helpful; at this point SMS::WFM::W() will
already have returned -1 to the user, which is unspecified behavior and the
caller would be reasonable to not handle the case explicitly, just like it's
unlikely to explicitly handle the case of a ret value of count_ * 100 from here)

[tommi (sloooow) - chröme, 2012/03/13 12:26:13]

Done.  Added CHECKs to WaitMultiple.

+  return ret;
+}
+
+SharedMemSynchronizer::SharedMemSynchronizer() {}
+
+#if defined(OS_POSIX)
+
+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));

[Ami GONE FROM CHROMIUM, 2012/03/08 17:04:36]

Is this depending on the socket having a buffer so that this doesn't block even
if there's no Wait() happening elsewhere?
(what happens if many Signal()'s are sent and none are waited for?)

[tommi (sloooow) - chröme, 2012/03/09 12:15:49]

Yes, the socket will buffer the signal to satisfy a single Wait() on the other
end.
That's not a supported scenario and will result in undefined behavior (not the
same on Windows and posix).  On Windows you'll hit a DCHECK if you try.

[Ami GONE FROM CHROMIUM, 2012/03/09 16:36:47]

I suspect on posix you'll get a hang as the pipe's buffer is filled and the
write syscall blocks.

[tommi (sloooow) - chröme, 2012/03/13 12:26:13]

It doesn't block actually and there should only be at most 1 byte in the buffer
at all times.  There's a single threaded test for this,
SharedMemSynchronizer.Basic.

+  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(SharedMemSynchronizer* synchronizers,
+                                        size_t count,
+                                        size_t last_signaled) {
+  DCHECK_LT(last_signaled < count);
+
+  for (size_t i = 0; i < count; ++i) {
+    DCHECK(synchronizers[i].IsValid());
+  }
+
+  int ret = -1;
+
+  // 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[count]);
+  memset(&sockets[0], 0, count * sizeof(sockets[0]));
+  size_t index = 0;
+  for (size_t i = last_signaled + 1; i < count; ++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, count);
+
+  int err = poll(&sockets[0], count, -1);
+  if (err != -1) {
+    for (size_t i = 0; i < count; ++i) {
+      if (sockets[i].revents) {
+        ret = (i + last_signaled + 1) % count;
+        DCHECK_EQ(sockets[i].fd, synchronizers[ret].socket_.handle());
+        synchronizers[ret].Wait();
+        break;

[Ami GONE FROM CHROMIUM, 2012/03/08 17:04:36]

could just return ret; here and replace the remainder of the function with an
unconditional NOTREACHED/return -1.

[tommi (sloooow) - chröme, 2012/03/09 12:15:49]

This has fewer returns and produces smaller code.  Is this less readable to you?

[Ami GONE FROM CHROMIUM, 2012/03/09 16:36:47]

Yes.

[tommi (sloooow) - chröme, 2012/03/13 12:26:13]

Done.

+      }
+    }
+  } else {
+    NOTREACHED() << "poll() failed errno: " << errno;
+  }
+
+  DCHECK_NE(ret, -1);
+  return ret;
+}
+
+#endif  // !defined(OS_POSIX)