implement audio device to pull directly from renderer

app/render/previewaudiodevice.cpp

@@ -0,0 +1,111 @@
+/***
+
+  Olive - Non-Linear Video Editor
+  Copyright (C) 2021 Olive Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#include "previewaudiodevice.h"
+
+namespace olive {
+
+PreviewAudioDevice::PreviewAudioDevice(QObject *parent)
+{
+  // These pointers are always valid
+  using_ = &internal_buffer_[0];
+  pushing_ = &internal_buffer_[1];
+
+  // Default to swap being true because we'll have nothing in the main buffer at first
+  swap_requested_ = true;
+}
+
+PreviewAudioDevice::~PreviewAudioDevice()
+{
+  close();
+}
+
+bool PreviewAudioDevice::isSequential() const
+{
+  return true;
+}
+
+qint64 PreviewAudioDevice::readData(char *data, qint64 maxSize)
+{
+  if (swap_requested_) {
+    SwapBuffers(kFullLock);
+    swap_requested_ = false;
+  }
+
+  // This function should NEVER touch the buffer in `pushing_`
+  qint64 copy_length = qMin(maxSize, qint64(using_->size()));
+
+  if (copy_length) {
+    memcpy(data, using_->constData(), copy_length);
+    *using_ = using_->mid(copy_length);
+
+    if (using_->isEmpty() && !SwapBuffers(kTryLock)) {
+      // Ask push function to swap if it can. If it can't, we'll catch it next read.
+      swap_requested_ = true;
+    }
+  }
+
+  return copy_length;
+}
+
+qint64 PreviewAudioDevice::writeData(const char *, qint64)
+{
+  // No writing to this device
+  return -1;
+}
+
+void PreviewAudioDevice::Push(const QByteArray &b)
+{
+  // This function should NEVER touch the buffer in `using_`
+  QMutexLocker locker(&lock_);
+  pushing_->append(b);
+
+  // If swap requested, do this now
+  if (swap_requested_) {
+    SwapBuffers(kDontLock);
+    swap_requested_ = false;
+  }
+}
+
+bool PreviewAudioDevice::SwapBuffers(LockMethod m)
+{
+  switch (m) {
+  case kDontLock:
+    break;
+  case kTryLock:
+    if (!lock_.tryLock()) {
+      return false;
+    }
+    break;
+  case kFullLock:
+    lock_.lock();
+    break;
+  }
+
+  std::swap(using_, pushing_);
+
+  if (m != kDontLock) {
+    lock_.unlock();
+  }
+
+  return true;
+}
+
+}