src/dawn/native/AsyncTask.h

// Copyright 2021 The Dawn & Tint Authors
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#ifndef SRC_DAWN_NATIVE_ASYNCTASK_H_
#define SRC_DAWN_NATIVE_ASYNCTASK_H_

#include <functional>
#include <memory>
#include <mutex>
#include <utility>
#include <vector>

#include "absl/container/flat_hash_set.h"
#include "dawn/common/MutexProtected.h"
#include "dawn/common/NonCopyable.h"
#include "dawn/common/Ref.h"
#include "dawn/common/RefCounted.h"
#include "dawn/native/Error.h"
#include "partition_alloc/pointers/raw_ptr.h"

namespace dawn::platform {
class WaitableEvent;
class WorkerTaskPool;
}  // namespace dawn::platform

namespace dawn::native {

// TODO(crbug.com/dawn/826): we'll add additional things to AsyncTask in the future, like
// Cancel() and RunNow(). Cancelling helps avoid running the task's body when we are just
// shutting down the device. RunNow() could be used for more advanced scenarios, for example
// always doing ShaderModule initial compilation asynchronously, but being able to steal the
// task if we need it for synchronous pipeline compilation.

enum class AsyncTaskState : uint8_t {
    Pending = 0,
    Running = 1,
    Completed = 2,
};

using AsyncTaskFunction = std::function<void()>;
using AsyncTaskCompletionCallback = std::function<void()>;

class AsyncTask : public RefCounted {
  public:
    explicit AsyncTask(AsyncTaskFunction task);

    AsyncTaskState GetState() const { return mState; }

    void Wait();

    void AddCompletionCallback(AsyncTaskCompletionCallback completionCallback);

  private:
    // Friends with the task manager to privately manage when tasks are executed.
    friend class AsyncTaskManager;
    void Run();

    AsyncTaskFunction mTask;

    // Use a mutex to guard changes to mCompletionCallbacks, mWaitableEvent and transitioning mState
    // to Completed.
    // mState is atomic for a lockless getter and Pending -> Running transition.
    std::mutex mMutex;
    std::atomic<AsyncTaskState> mState = AsyncTaskState::Pending;
    std::vector<AsyncTaskCompletionCallback> mCompletionCallbacks;

    // Hold onto the waitable platform event until the task has completed. Released before the
    // destruction of the AsyncTask to be as light weight as possible.
    std::unique_ptr<dawn::platform::WaitableEvent> mWaitableEvent;
};

class ErrorGeneratingAsyncTask : public AsyncTask {
  public:
    explicit ErrorGeneratingAsyncTask(std::function<MaybeError()> task);

    bool IsSuccess() const;
    bool IsError() const;
    InternalErrorType GetErrorType() const;
    std::unique_ptr<ErrorData> AcquireError();

  private:
    std::unique_ptr<ErrorData> mErrorData;
};

class AsyncTaskManager {
  public:
    explicit AsyncTaskManager(dawn::platform::WorkerTaskPool* workerTaskPool);
    ~AsyncTaskManager();

    template <typename TaskType, class... Args>
    Ref<TaskType> PostTask(Args&&... args) {
        Ref<TaskType> asyncTask = AcquireRef(new TaskType(std::forward<Args>(args)...));
        PostConstructedTask(asyncTask);
        return asyncTask;
    }

    void WaitAllPendingTasks();
    bool HasPendingTasks();

  private:
    struct WaitableTask : NonCopyable {
        Ref<AsyncTask> asyncTask;
        raw_ptr<AsyncTaskManager> taskManager;
    };

    void PostConstructedTask(Ref<AsyncTask> asyncTask);

    static void RunTask(void* task);
    void HandleTaskCompletion(WaitableTask* task);

    using PendingTasksSet = absl::flat_hash_set<std::unique_ptr<WaitableTask>>;
    MutexProtected<PendingTasksSet> mPendingTasks;

    raw_ptr<dawn::platform::WorkerTaskPool> mWorkerTaskPool;
};

}  // namespace dawn::native

#endif  // SRC_DAWN_NATIVE_ASYNCTASK_H_