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>

2 files changed, 204 insertions, 0 deletions

diff --git a/src/libcamera/file_descriptor.cpp b/src/libcamera/file_descriptor.cpp
new file mode 100644
index 00000000..88385476
--- /dev/null
+++ b/src/libcamera/file_descriptor.cpp
@@ -0,0 +1,203 @@
+/* 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 provides RAII-style lifetime management of file
+ * descriptors with an efficient mechanism for ownership sharing. At its core,
+ * an internal Descriptor object wraps a file descriptor (expressed as a signed
+ * integer) with an RAII-style interface. The Descriptor is then implicitly
+ * shared with all FileDescriptor instances constructed as copies.
+ *
+ * When constructed from a numerical file descriptor, the FileDescriptor
+ * instance duplicates the file descriptor and wraps the duplicate as a
+ * Descriptor. The copy constructor and assignment operator create copies that
+ * share the Descriptor, while the move versions of those methods additionally
+ * make the other FileDescriptor invalid. When the last FileDescriptor that
+ * references a Descriptor is destroyed, the file descriptor is closed.
+ *
+ * The numerical file descriptor is available through the fd() method. As
+ * constructing a FileDescriptor from a numerical file descriptor duplicates
+ * the file descriptor, the value returned by fd() will be different than the
+ * value passed to the constructor. All FileDescriptor instances created as
+ * copies of a FileDescriptor will report the same fd() value. Callers can
+ * perform operations on the fd(), but shall never close it manually.
+ */
+
+/**
+ * \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 all FileDescriptor
+ * instances that reference it are destroyed.
+ *
+ * If the \a fd is negative, the FileDescriptor is constructed as invalid and
+ * the fd() method will return -1.
+ */
+FileDescriptor::FileDescriptor(int fd)
+{
+	if (fd < 0)
+		return;
+
+	fd_ = std::make_shared<Descriptor>(fd);
+	if (fd_->fd() < 0)
+		fd_.reset();
+}
+
+/**
+ * \brief Copy constructor, create a FileDescriptor from a copy of \a other
+ * \param[in] other The other FileDescriptor
+ *
+ * Copying a FileDescriptor implicitly shares ownership of the wrapped file
+ * descriptor. The original FileDescriptor is left untouched, and the caller is
+ * responsible for destroying it when appropriate. The wrapped file descriptor
+ * will be closed automatically when all FileDescriptor instances that
+ * reference it are destroyed.
+ */
+FileDescriptor::FileDescriptor(const FileDescriptor &other)
+	: fd_(other.fd_)
+{
+}
+
+/**
+ * \brief Move constructor, create a FileDescriptor by taking over \a other
+ * \param[in] other The other FileDescriptor
+ *
+ * Moving a FileDescriptor moves the reference to the wrapped descriptor owned
+ * by \a other to the new FileDescriptor. The \a other FileDescriptor is
+ * invalidated and its fd() method will return -1. The wrapped file descriptor
+ * will be closed automatically when all FileDescriptor instances that
+ * reference it are destroyed.
+ */
+FileDescriptor::FileDescriptor(FileDescriptor &&other)
+	: fd_(std::move(other.fd_))
+{
+}
+
+/**
+ * \brief Destroy the FileDescriptor instance
+ *
+ * Destroying a FileDescriptor instance releases its reference to the wrapped
+ * descriptor, if any. When the last instance that references a wrapped
+ * descriptor is destroyed, the file descriptor is automatically closed.
+ */
+FileDescriptor::~FileDescriptor()
+{
+}
+
+/**
+ * \brief Copy assignment operator, replace the wrapped file descriptor with a
+ * copy of \a other
+ * \param[in] other The other FileDescriptor
+ *
+ * Copying a FileDescriptor creates a new reference to the wrapped file
+ * descriptor owner by \a other. If \a other is invalid, *this will also be
+ * invalid. The original FileDescriptor is left untouched, and the caller is
+ * responsible for destroying it when appropriate. The wrapped file descriptor
+ * will be closed automatically when all FileDescriptor instances that
+ * reference it are destroyed.
+ *
+ * \return A reference to this FileDescriptor
+ */
+FileDescriptor &FileDescriptor::operator=(const FileDescriptor &other)
+{
+	fd_ = other.fd_;
+
+	return *this;
+}
+
+/**
+ * \brief Move assignment operator, replace the wrapped file descriptor by
+ * taking over \a other
+ * \param[in] other The other FileDescriptor
+ *
+ * Moving a FileDescriptor moves the reference to the wrapped descriptor owned
+ * by \a other to the new FileDescriptor. If \a other is invalid, *this will
+ * also be invalid. The \a other FileDescriptor is invalidated and its fd()
+ * method will return -1. The wrapped file descriptor will be closed
+ * automatically when all FileDescriptor instances that reference it are
+ * destroyed.
+ *
+ * \return A reference to this FileDescriptor
+ */
+FileDescriptor &FileDescriptor::operator=(FileDescriptor &&other)
+{
+	fd_ = std::move(other.fd_);
+
+	return *this;
+}
+
+/**
+ * \fn FileDescriptor::isValid()
+ * \brief Check if the FileDescriptor instance is valid
+ * \return True if the FileDescriptor is valid, false otherwise
+ */
+
+/**
+ * \fn FileDescriptor::fd()
+ * \brief Retrieve the numerical file descriptor
+ * \return The numerical file descriptor, which may be -1 if the FileDescriptor
+ * instance is invalid
+ */
+
+/**
+ * \brief Duplicate a FileDescriptor
+ *
+ * Duplicating a FileDescriptor creates a duplicate of the wrapped file
+ * descriptor and returns a new FileDescriptor instance that wraps the
+ * duplicate. The fd() method of the original and duplicate instances will
+ * return different values. The duplicate instance will not be affected by
+ * destruction of the original instance or its copies.
+ *
+ * \return A new FileDescriptor instance wrapping a duplicate of the original
+ * file descriptor
+ */
+FileDescriptor FileDescriptor::dup() const
+{
+	return FileDescriptor(fd());
+}
+
+FileDescriptor::Descriptor::Descriptor(int fd)
+{
+	/* 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);
+	}
+}
+
+FileDescriptor::Descriptor::~Descriptor()
+{
+	if (fd_ != -1)
+		close(fd_);
+}
+
+} /* namespace libcamera */
diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
index c4f965bd..722c5bc1 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',