pipeline: rpi: Add SharedMemObject class

Add new SharedMemObject class that wraps a memfd memory allocation and
constructs a templated object in the memory. With appropriate locking,
this object can then be shared across different processes using the
associated allocation file handle.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/pipeline/rpi/common/shared_mem_object.h

@@ -0,0 +1,128 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2023, Raspberry Pi Ltd
+ *
+ * shared_mem_object.h - Helper class for shared memory allocations
+ */
+#pragma once
+
+#include <cstddef>
+#include <fcntl.h>
+#include <string>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <utility>
+
+#include <libcamera/base/class.h>
+#include <libcamera/base/shared_fd.h>
+
+namespace libcamera {
+
+namespace RPi {
+
+template<class T>
+class SharedMemObject
+{
+public:
+	static constexpr std::size_t SIZE = sizeof(T);
+
+	SharedMemObject()
+		: obj_(nullptr)
+	{
+	}
+
+	template<class... Args>
+	SharedMemObject(const std::string &name, Args &&...args)
+		: name_(name), obj_(nullptr)
+	{
+		void *mem;
+		int ret;
+
+		ret = memfd_create(name_.c_str(), MFD_CLOEXEC);
+		if (ret < 0)
+			return;
+
+		fd_ = SharedFD(std::move(ret));
+		if (!fd_.isValid())
+			return;
+
+		ret = ftruncate(fd_.get(), SIZE);
+		if (ret < 0)
+			return;
+
+		mem = mmap(nullptr, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED,
+			   fd_.get(), 0);
+		if (mem == MAP_FAILED)
+			return;
+
+		obj_ = new (mem) T(std::forward<Args>(args)...);
+	}
+
+	SharedMemObject(SharedMemObject<T> &&rhs)
+	{
+		this->name_ = std::move(rhs.name_);
+		this->fd_ = std::move(rhs.fd_);
+		this->obj_ = rhs.obj_;
+		rhs.obj_ = nullptr;
+	}
+
+	~SharedMemObject()
+	{
+		if (obj_) {
+			obj_->~T();
+			munmap(obj_, SIZE);
+		}
+	}
+
+	/* Make SharedMemObject non-copyable for now. */
+	LIBCAMERA_DISABLE_COPY(SharedMemObject)
+
+	SharedMemObject<T> &operator=(SharedMemObject<T> &&rhs)
+	{
+		this->name_ = std::move(rhs.name_);
+		this->fd_ = std::move(rhs.fd_);
+		this->obj_ = rhs.obj_;
+		rhs.obj_ = nullptr;
+		return *this;
+	}
+
+	T *operator->()
+	{
+		return obj_;
+	}
+
+	const T *operator->() const
+	{
+		return obj_;
+	}
+
+	T &operator*()
+	{
+		return *obj_;
+	}
+
+	const T &operator*() const
+	{
+		return *obj_;
+	}
+
+	const SharedFD &fd() const
+	{
+		return fd_;
+	}
+
+	explicit operator bool() const
+	{
+		return !!obj_;
+	}
+
+private:
+	std::string name_;
+	SharedFD fd_;
+	T *obj_;
+};
+
+} /* namespace RPi */
+
+} /* namespace libcamera */