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

2426 diff mbox series

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>
---
 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

Hi Niklas,

Thank you for the patch.

On Mon, Dec 16, 2019 at 01:10:28PM +0100, Niklas Söderlund wrote:
> 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>
> ---
>  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

s/version/versions/

> + * 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.

s/the FileDescriptor instance/this FileDescriptor instance/ to avoid any
ambiguity with \a other ? Same for the move constructor below.

> + */
> +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

s/Constructing/Construct/

> + * 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))
> +{

You could also have written this

	fd_ = other.fd_;
	other.fd_ = -1;

and similarly below in operator=(), to avoid introducing
std::exchange(). Up to you.

> +}
> +

How about documenting the destructor too ?

/**
 * \brief Destroy the managed file descriptor
 *
 * If the managed file descriptor, as returned by fd(), is not equal to -1, the
 * file descriptor is closed.
 */

Then we could remove the "The file descriptor will be closed
automatically when the FileDescriptor instance is destroyed." sentence
from all constructors, if desired.

> +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

s/currently wrapped numerical file descriptor/wrapped file descriptor/ ?
s/ if any/, if any,/ of s/if any/(if any)/

Both comments apply to the move assignment operator below.

> + * the file descriptor from \a other. The original FileDescriptor is left

s/original/\a other/

> + * untouched, and the caller is responsible for closing it when appropriate.

s/closing/destroying/ as FileDescriptor has no close method ?

> + * The duplicated file descriptor will be closed automatically when the
> + * FileDescriptor instance is destroyed.
> + *
> + * \return A reference to the FileDescriptor

s/the FileDescriptor/this FileDescriptor/

> + */
> +FileDescriptor &FileDescriptor::operator=(const FileDescriptor &other)
> +{
> +	if (fd_ != -1)
> +		close(fd_);

I would assign fd_ to -1 here to avoid depending on duplicate() doing
so.

> +
> +	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

s/the FileDescriptor/this 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;
> +	}

To make this method more self-contained, I think it should either
close(fd_) if fd_ != -1 (removing the close from the copy assignment
operator), or rely on fd_ being == -1 when called, thus removing the fd_
= -1 assignment above. Otherwise duplicate() requires fd_ to be closed
but tolerates it being != -1, which is confusing.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

> +
> +	/* 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',