From 2b55f8f5eafdb17fa606c8f572de48e35b74e425 Mon Sep 17 00:00:00 2001
From: Nav <nav@navnav.me>
Date: Mon, 28 Mar 2022 01:02:52 +0100
Subject: [PATCH] New RAII wrapper for Linux epoll instances

---
 CMakeLists.txt                |  1 +
 src/Helpers/EpollInstance.cpp | 77 +++++++++++++++++++++++++++++++++++
 src/Helpers/EpollInstance.hpp | 70 +++++++++++++++++++++++++++++++
 3 files changed, 148 insertions(+)
 create mode 100644 src/Helpers/EpollInstance.cpp
 create mode 100644 src/Helpers/EpollInstance.hpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index cb43fb70..9e1ffd87 100755
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -66,6 +66,7 @@ add_executable(Bloom
     # Helpers & other
     src/Logger/Logger.cpp
     src/Helpers/Paths.cpp
+    src/Helpers/EpollInstance.cpp
     src/VersionNumber.cpp
     src/Generated/resources.cpp
 
diff --git a/src/Helpers/EpollInstance.cpp b/src/Helpers/EpollInstance.cpp
new file mode 100644
index 00000000..d2adea14
--- /dev/null
+++ b/src/Helpers/EpollInstance.cpp
@@ -0,0 +1,77 @@
+#include "EpollInstance.hpp"
+
+#include <string>
+#include <cerrno>
+#include <unistd.h>
+#include <array>
+
+#include "src/Exceptions/Exception.hpp"
+
+namespace Bloom
+{
+    using Exceptions::Exception;
+
+    EpollInstance::EpollInstance() {
+        this->fileDescriptor = ::epoll_create(1);
+
+        if (this->fileDescriptor < 0) {
+            throw Exception(
+                "Failed to create epoll instance - error number " + std::to_string(errno)
+                    + " returned."
+            );
+        }
+    }
+
+    void EpollInstance::addEntry(int fileDescriptor, std::uint16_t eventMask) {
+        struct epoll_event event = {
+            .events = eventMask,
+            .data = {
+                .fd = fileDescriptor
+            }
+        };
+
+        if (::epoll_ctl(this->fileDescriptor.value(), EPOLL_CTL_ADD, fileDescriptor, &event) != 0) {
+            throw Exception(
+                "Failed to add entry to epoll instance - error number " + std::to_string(errno) + " returned."
+            );
+        }
+    }
+
+    void EpollInstance::removeEntry(int fileDescriptor) {
+        if (::epoll_ctl(this->fileDescriptor.value(), EPOLL_CTL_DEL, fileDescriptor, NULL) != 0) {
+            throw Exception(
+                "Failed to remove entry from epoll instance - error number " + std::to_string(errno)
+                    + " returned."
+            );
+        }
+    }
+
+    std::optional<int> EpollInstance::waitForEvent(std::optional<std::chrono::milliseconds> timeout) {
+        std::array<struct epoll_event, 1> events = {};
+
+        const auto eventCount = ::epoll_wait(
+            this->fileDescriptor.value(),
+            events.data(),
+            1,
+            timeout.has_value() ? static_cast<int>(timeout->count()) : -1
+        );
+
+        if (eventCount < 1) {
+            return std::nullopt;
+        }
+
+        return static_cast<int>(events.at(0).data.fd);
+    }
+
+    EpollInstance::EpollInstance(EpollInstance&& other) noexcept
+        : fileDescriptor(other.fileDescriptor)
+    {
+        other.fileDescriptor = std::nullopt;
+    }
+
+    EpollInstance::~EpollInstance() noexcept {
+        if (this->fileDescriptor.value_or(-1) >= 0) {
+            ::close(this->fileDescriptor.value());
+        }
+    }
+}
diff --git a/src/Helpers/EpollInstance.hpp b/src/Helpers/EpollInstance.hpp
new file mode 100644
index 00000000..278a1678
--- /dev/null
+++ b/src/Helpers/EpollInstance.hpp
@@ -0,0 +1,70 @@
+#pragma once
+
+#include <sys/epoll.h>
+#include <cstdint>
+#include <optional>
+#include <chrono>
+
+namespace Bloom
+{
+    enum class EpollEvent: std::uint16_t
+    {
+        READ_READY = EPOLL_EVENTS::EPOLLIN,
+        WRITE_READY = EPOLL_EVENTS::EPOLLOUT,
+    };
+
+    /**
+     * The EpollInstance class is an RAII wrapper for a single Linux epoll instance.
+     *
+     * See https://man7.org/linux/man-pages/man7/epoll.7.html for more on the Linux epoll API.
+     */
+    class EpollInstance
+    {
+    public:
+        EpollInstance();
+
+        /**
+         * Adds an entry to the epoll instance.
+         *
+         * @param fileDescriptor
+         * @param eventMask
+         */
+        void addEntry(int fileDescriptor, std::uint16_t eventMask);
+
+        /**
+         * Removes an entry from the epoll instance.
+         *
+         * @param fileDescriptor
+         */
+        void removeEntry(int fileDescriptor);
+
+        /**
+         * Waits on the epoll instance until an event occurs for any of the registered files.
+         *
+         * @param timeout
+         *  Millisecond timeout. If not provided, no timeout will be applied and this function will block until an
+         *  event occurs.
+         *
+         * @return
+         *  The file descriptor of the file for which the event occurred, or std::nullopt if a timeout was reached.
+         */
+        std::optional<int> waitForEvent(std::optional<std::chrono::milliseconds> timeout = std::nullopt);
+
+        /*
+         * EpollInstance objects should not be copied.
+         */
+        EpollInstance(EpollInstance& other) = delete;
+        EpollInstance& operator = (EpollInstance& other) = delete;
+
+        /*
+         * TODO: Implement this. For now, use the move constructor.
+         */
+        EpollInstance& operator = (EpollInstance&& other) = delete;
+
+        EpollInstance(EpollInstance&& other) noexcept;
+        ~EpollInstance() noexcept;
+
+    private:
+        std::optional<int> fileDescriptor;
+    };
+}