FileDescriptor.h

/*
 *  Phusion Passenger - http://www.modrails.com/
 *  Copyright (c) 2010 Phusion
 *
 *  "Phusion Passenger" is a trademark of Hongli Lai & Ninh Bui.
 *
 *  Permission is hereby granted, free of charge, to any person obtaining a copy
 *  of this software and associated documentation files (the "Software"), to deal
 *  in the Software without restriction, including without limitation the rights
 *  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 *  copies of the Software, and to permit persons to whom the Software is
 *  furnished to do so, subject to the following conditions:
 *
 *  The above copyright notice and this permission notice shall be included in
 *  all copies or substantial portions of the Software.
 *
 *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 *  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 *  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 *  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 *  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 *  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 *  THE SOFTWARE.
 */
#ifndef _PASSENGER_FILE_DESCRIPTOR_H_
#define _PASSENGER_FILE_DESCRIPTOR_H_

#include <boost/shared_ptr.hpp>
#include <boost/make_shared.hpp>
#include <oxt/system_calls.hpp>

#include <utility>
#include <unistd.h>
#include <cerrno>

#include <Exceptions.h>

namespace Passenger {

using namespace std;
using namespace boost;
using namespace oxt;


void safelyClose(int fd);


/**
 * Wrapper class around a file descriptor integer, for RAII behavior.
 *
 * A FileDescriptor object behaves just like an int, so that you can pass it to
 * system calls such as read(). It performs reference counting. When the last
 * copy of a FileDescriptor has been destroyed, the underlying file descriptor
 * will be automatically closed. In this case, any close() system call errors
 * are silently ignored. If you are interested in whether the close() system
 * call succeeded, then you should call FileDescriptor::close().
 *
 * This class is *not* thread-safe. It is safe to call system calls on the
 * underlying file descriptor from multiple threads, but it's not safe to
 * call FileDescriptor::close() from multiple threads if all those
 * FileDescriptor objects point to the same underlying file descriptor.
 */
class FileDescriptor {
private:
        struct SharedData {
                int fd;
                
                SharedData(int fd) {
                        this->fd = fd;
                }
                
                ~SharedData() {
                        if (fd >= 0) {
                                this_thread::disable_syscall_interruption dsi;
                                syscalls::close(fd);
                        }
                }
                
                void close(bool checkErrors = true) {
                        if (fd >= 0) {
                                this_thread::disable_syscall_interruption dsi;
                                int theFd = fd;
                                fd = -1;
                                if (checkErrors) {
                                        safelyClose(theFd);
                                } else {
                                        syscalls::close(fd);
                                }
                        }
                }
                
                void detach() {
                        fd = -1;
                }
        };

        /** Shared pointer for reference counting on this file descriptor */
        shared_ptr<SharedData> data;

public:
        /** 
         * Creates a new empty FileDescriptor instance that has no underlying
         * file descriptor.
         *
         * @post *this == -1
         */
        FileDescriptor() { }
        
        /**
         * Creates a new FileDescriptor instance with the given fd as a handle.
         *
         * @post *this == fd
         */
        FileDescriptor(int fd) {
                if (fd >= 0) {
                        /* Make sure that the 'new' operator doesn't overwrite
                         * errno so that we can write code like this:
                         *
                         *    FileDescriptor fd = open(...);
                         *    if (fd == -1) {
                         *       print_error(errno);
                         *    }
                         */
                        int e = errno;
                        data = make_shared<SharedData>(fd);
                        errno = e;
                }
        }
        
        /**
         * Close the underlying file descriptor. If it was already closed, then
         * nothing will happen. If there are multiple copies of this FileDescriptor
         * then the underlying file descriptor will be closed for every one of them.
         *
         * @params checkErrors Whether a SystemException should be thrown in case
         *                     closing the file descriptor fails. If false, errors
         *                     are silently ignored.
         * @throws SystemException Something went wrong while closing
         *                         the file descriptor. Only thrown if
         *                         checkErrors is true.
         * @post *this == -1
         */
        void close(bool checkErrors = true) {
                if (data != NULL) {
                        data->close(checkErrors);
                        data.reset();
                }
        }
        
        /**
         * Detach from the underlying file descriptor without closing it.
         * This FileDescriptor and all copies will no longer affect the
         * underlying file descriptors.
         *
         * @return The underlying file descriptor, or -1 if already closed.
         * @post *this == -1
         */
        int detach() {
                if (data != NULL) {
                        int fd = data->fd;
                        data->detach();
                        data.reset();
                        return fd;
                } else {
                        return -1;
                }
        }
        
        /**
         * Overloads the integer cast operator so that it will return the underlying
         * file descriptor handle as an integer.
         *
         * Returns -1 if FileDescriptor::close() was called.
         */
        operator int () const {
                if (data == NULL) {
                        return -1;
                } else {
                        return data->fd;
                }
        }
        
        FileDescriptor &operator=(int fd) {
                /* Make sure that the 'new' and 'delete' operators don't
                 * overwrite errno so that we can write code like this:
                 *
                 *    FileDescriptor fd;
                 *    fd = open(...);
                 *    if (fd == -1) {
                 *       print_error(errno);
                 *    }
                 */
                int e = errno;
                if (fd >= 0) {
                        data = make_shared<SharedData>(fd);
                } else {
                        data.reset();
                }
                errno = e;
                return *this;
        }
        
        FileDescriptor &operator=(const FileDescriptor &other) {
                /* Make sure that the 'delete' operator implicitly invoked by
                 * shared_ptr doesn't overwrite errno so that we can write code
                 * like this:
                 *
                 *    FileDescriptor fd;
                 *    fd = other_file_descriptor_object;
                 *    if (fd == -1) {
                 *       print_error(errno);
                 *    }
                 */
                int e = errno;
                data = other.data;
                errno = e;
                return *this;
        }
};

/**
 * A structure containing two FileDescriptor objects. Behaves like a pair
 * and like a two-element array.
 */
class FileDescriptorPair: public pair<FileDescriptor, FileDescriptor> {
public:
        FileDescriptorPair() { }
        
        FileDescriptorPair(const FileDescriptor &a, const FileDescriptor &b)
                : pair<FileDescriptor, FileDescriptor>(a, b)
                { }
        
        FileDescriptor &operator[](int index) {
                if (index == 0) {
                        return first;
                } else if (index == 1) {
                        return second;
                } else {
                        throw ArgumentException("Index must be either 0 of 1");
                }
        }
};

// Convenience aliases.
typedef FileDescriptorPair Pipe;
typedef FileDescriptorPair SocketPair;

/**
 * A synchronization mechanism that's implemented with file descriptors,
 * and as such can be used in combination with select() and friends.
 *
 * One can wait for an event on an EventFd by select()ing it on read events.
 * Another thread can signal the EventFd by calling notify().
 */
class EventFd {
private:
        int reader;
        int writer;
        
public:
        EventFd() {
                int fds[2];
                
                if (syscalls::pipe(fds) == -1) {
                        int e = errno;
                        throw SystemException("Cannot create a pipe", e);
                }
                reader = fds[0];
                writer = fds[1];
        }
        
        ~EventFd() {
                this_thread::disable_syscall_interruption dsi;
                syscalls::close(reader);
                syscalls::close(writer);
        }
        
        void notify() {
                ssize_t ret = syscalls::write(writer, "x", 1);
                if (ret == -1 && errno != EAGAIN) {
                        int e = errno;
                        throw SystemException("Cannot write notification data", e);
                }
        }
        
        int fd() const {
                return reader;
        }
};

} // namespace Passenger

#endif /* _PASSENGER_FILE_DESCRIPTOR_H_ */

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