libcamera: Add FileDescriptor to help pass numerical fds around

Add a helper to make it easier to pass file descriptors around. The
helper class duplicates the fd which decouples it from the original fd
which could be closed by its owner while the new FileDescriptor remains
valid.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/file_descriptor.h

@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * file_descriptor.h - File descriptor wrapper
+ */
+#ifndef __LIBCAMERA_FILE_DESCRIPTOR_H__
+#define __LIBCAMERA_FILE_DESCRIPTOR_H__
+
+#include <memory>
+
+namespace libcamera {
+
+class FileDescriptor final
+{
+public:
+	explicit FileDescriptor(int fd = -1);
+	FileDescriptor(const FileDescriptor &other);
+	FileDescriptor(FileDescriptor &&other);
+	~FileDescriptor();
+
+	FileDescriptor &operator=(const FileDescriptor &other);
+	FileDescriptor &operator=(FileDescriptor &&other);
+
+	bool isValid() const { return fd_ != nullptr; }
+	int fd() const { return fd_ ? fd_->fd() : -1; }
+	FileDescriptor dup() const;
+
+private:
+	class Descriptor
+	{
+	public:
+		Descriptor(int fd);
+		~Descriptor();
+
+		int fd() const { return fd_; }
+
+	private:
+		int fd_;
+	};
+
+	std::shared_ptr<Descriptor> fd_;
+};
+
+} /* namespace libcamera */
+
+#endif /* __LIBCAMERA_FILE_DESCRIPTOR_H__ */

include/libcamera/meson.build

@@ -6,6 +6,7 @@ libcamera_api = files([
     'controls.h',
     'event_dispatcher.h',
     'event_notifier.h',
+    'file_descriptor.h',
     'geometry.h',
     'logging.h',
     'object.h',