libcamera: fence: Introduce Fence

Introduce a Fence class which models a synchronization primitive that
allows to notify the availability of a resource.

The Fence is modeled as a wrapper of a UniqueFD instance where
read events are used to signal the Fence. The class can be later
extended to support additional signalling mechanisms.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/fence.h

@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2021, Google Inc.
+ *
+ * internal/fence.h - Synchronization fence
+ */
+
+#pragma once
+
+#include <libcamera/base/class.h>
+#include <libcamera/base/unique_fd.h>
+
+namespace libcamera {
+
+class Fence
+{
+public:
+	Fence(UniqueFD fd);
+
+	bool isValid() const { return fd_.isValid(); }
+	const UniqueFD &fd() const { return fd_; }
+
+	UniqueFD release() { return std::move(fd_); }
+
+private:
+	LIBCAMERA_DISABLE_COPY_AND_MOVE(Fence)
+
+	UniqueFD fd_;
+};
+
+} /* namespace libcamera */

include/libcamera/meson.build

@@ -6,6 +6,7 @@ libcamera_public_headers = files([
     'camera.h',
     'camera_manager.h',
     'controls.h',
+    'fence.h',
     'framebuffer.h',
     'framebuffer_allocator.h',
     'geometry.h',