[libcamera-devel] [PATCH 2/3] libcamera: Add FileDescriptor to help pass numerical fds around

[Niklas Söderlund]

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 at ragnatech.se>
---
 include/libcamera/file_descriptor.h |  33 ++++++
 include/libcamera/meson.build       |   1 +
 src/libcamera/file_descriptor.cpp   | 158 ++++++++++++++++++++++++++++
 src/libcamera/meson.build           |   1 +
 4 files changed, 193 insertions(+)
 create mode 100644 include/libcamera/file_descriptor.h
 create mode 100644 src/libcamera/file_descriptor.cpp

diff --git a/include/libcamera/file_descriptor.h b/include/libcamera/file_descriptor.h
new file mode 100644
index 0000000000000000..e05f111d128e0ab1
--- /dev/null
+++ b/include/libcamera/file_descriptor.h
@@ -0,0 +1,33 @@
+/* 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__
+
+namespace libcamera {
+
+class FileDescriptor final
+{
+public:
+	explicit FileDescriptor(int fd = -1);
+	explicit FileDescriptor(const FileDescriptor &other);
+	explicit FileDescriptor(FileDescriptor &&other);
+	~FileDescriptor();
+
+	FileDescriptor &operator=(const FileDescriptor &other);
+	FileDescriptor &operator=(FileDescriptor &&other);
+
+	int fd() const { return fd_; }
+
+private:
+	void duplicate(int fd);
+
+	int fd_;
+};
+
+} /* namespace libcamera */
+
+#endif /* __LIBCAMERA_FILE_DESCRIPTOR_H__ */
diff --git a/include/libcamera/meson.build b/include/libcamera/meson.build
index 99abf06099407c1f..543e6773cc5158a0 100644
--- a/include/libcamera/meson.build
+++ b/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',
diff --git a/src/libcamera/file_descriptor.cpp b/src/libcamera/file_descriptor.cpp
new file mode 100644
index 0000000000000000..fd87753b456fc3b7
--- /dev/null
+++ b/src/libcamera/file_descriptor.cpp
@@ -0,0 +1,158 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * file_descriptor.cpp - File descriptor wrapper
+ */
+
+#include <libcamera/file_descriptor.h>
+
+#include <string.h>
+#include <unistd.h>
+#include <utility>
+
+#include "log.h"
+
+/**
+ * \file file_descriptor.h
+ * \brief File descriptor wrapper
+ */
+
+namespace libcamera {
+
+LOG_DEFINE_CATEGORY(FileDescriptor)
+
+/**
+ * \class FileDescriptor
+ * \brief RAII-style wrapper for file descriptors
+ *
+ * The FileDescriptor class wraps a file descriptor (expressed as a signed
+ * integer) to provide an RAII-style mechanism for owning the file descriptor.
+ * When constructed, the FileDescriptor instance duplicates a given file
+ * descriptor and takes ownership of the duplicate as a resource. The copy
+ * constructor and assignment operator duplicate the file descriptor, while the
+ * move version of those methods move the resource and make the original
+ * FileDescriptor invalid. When the FileDescriptor is deleted, it closes the
+ * file descriptor it owns, if any.
+ */
+
+/**
+ * \brief Create a FileDescriptor wrapping a copy of a given \a fd
+ * \param[in] fd File descriptor
+ *
+ * Constructing a FileDescriptor from a numerical file descriptor duplicates the
+ * \a fd and takes ownership of the copy. The original \a fd is left untouched,
+ * and the caller is responsible for closing it when appropriate. The duplicated
+ * file descriptor will be closed automatically when the FileDescriptor instance
+ * is destroyed.
+ */
+FileDescriptor::FileDescriptor(int fd)
+	: fd_(-1)
+{
+	duplicate(fd);
+}
+
+/**
+ * \brief Copy constructor, create a FileDescriptor from a copy of \a other
+ * \param[in] other The other FileDescriptor
+ *
+ * Constructing a FileDescriptor from another FileDescriptor duplicates the
+ * wrapped numerical file descriptor and takes ownership of the copy. The
+ * original FileDescriptor is left untouched, and the caller is responsible for
+ * closing it when appropriate. The duplicated file descriptor will be closed
+ * automatically when the FileDescriptor instance is destroyed.
+ */
+FileDescriptor::FileDescriptor(const FileDescriptor &other)
+	: FileDescriptor(other.fd_)
+{
+}
+
+/**
+ * \brief Move constructor, create a FileDescriptor by taking over \a other
+ * \param[in] other The other FileDescriptor
+ *
+ * Constructing a FileDescriptor by taking over a wrapped numerical file
+ * descriptor and taking ownership of it. The \a other FileDescriptor is
+ * invalidated and set to -1. The taken over file descriptor will be closed
+ * automatically when the FileDescriptor instance is destroyed.
+ */
+FileDescriptor::FileDescriptor(FileDescriptor &&other)
+	: fd_(utils::exchange(other.fd_, -1))
+{
+}
+
+FileDescriptor::~FileDescriptor()
+{
+	if (fd_ != -1)
+		close(fd_);
+}
+
+/**
+ * \brief Copy assignment operator, replace the wrapped file descriptor with a
+ * duplicate from \a other
+ * \param[in] other The other FileDescriptor
+ *
+ * Close the currently wrapped numerical file descriptor if any and duplicate
+ * the file descriptor from \a other. The original FileDescriptor is left
+ * untouched, and the caller is responsible for closing it when appropriate.
+ * The duplicated file descriptor will be closed automatically when the
+ * FileDescriptor instance is destroyed.
+ *
+ * \return A reference to the FileDescriptor
+ */
+FileDescriptor &FileDescriptor::operator=(const FileDescriptor &other)
+{
+	if (fd_ != -1)
+		close(fd_);
+
+	duplicate(other.fd_);
+
+	return *this;
+}
+
+/**
+ * \brief Move assignment operator, replace the wrapped file descriptor by
+ * taking over \a other
+ * \param[in] other The other FileDescriptor
+ *
+ * Close the currently wrapped numerical file descriptor if any and take over
+ * the file descriptor from \a other. The \a other FileDescriptor is invalidated
+ * and set to -1. The taken over file descriptor will be closed automatically
+ * when the FileDescriptor instance is destroyed.
+ *
+ * \return A reference to the FileDescriptor
+ */
+FileDescriptor &FileDescriptor::operator=(FileDescriptor &&other)
+{
+	if (fd_ != -1)
+		close(fd_);
+
+	fd_ = utils::exchange(other.fd_, -1);
+
+	return *this;
+}
+
+/**
+ * \fn FileDescriptor::fd()
+ * \brief Retrieve the numerical file descriptor
+ * \return The numerical file descriptor, which may be -1 if the FileDescriptor
+ * instance doesn't own a file descriptor
+ */
+
+void FileDescriptor::duplicate(int fd)
+{
+	if (fd < 0) {
+		fd_ = -1;
+		return;
+	}
+
+	/* Failing to dup() a fd should not happen and is fatal. */
+	fd_ = dup(fd);
+	if (fd_ == -1) {
+		int ret = -errno;
+		LOG(FileDescriptor, Fatal)
+			<< "Failed to dup() fd: " << strerror(-ret);
+	}
+}
+
+} /* namespace libcamera */
diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
index c4f965bd7413b37e..722c5bc15afe52ef 100644
--- a/src/libcamera/meson.build
+++ b/src/libcamera/meson.build
@@ -14,6 +14,7 @@ libcamera_sources = files([
     'event_dispatcher.cpp',
     'event_dispatcher_poll.cpp',
     'event_notifier.cpp',
+    'file_descriptor.cpp',
     'formats.cpp',
     'geometry.cpp',
     'ipa_context_wrapper.cpp',
-- 
2.24.0