網路程式設計定時器三：使用最小堆

前面討論的定時方案都是以固定頻率呼叫心搏函式tick，並在其中一次檢測到期的定時器，然後執行到期定時器上的回撥函式。設計定時器的另一中思路是，將所有定器超時時間最小的一個定時器的超時值作為心搏間隔。這樣，一旦心搏函式tick執行，超時時間最小的定時器必然到期。我們就可以從剩餘定時器中選出超時時間最小的一個，並將這個時間設為下一次心搏間隔。如此反覆，就實現了較為精確的定時。

最小堆適合這種解決方案，下面直接給出最小堆方案的程式碼，並附有測試用例。不過測試用例我仍然只用了alarm來做測試。

#ifndef MIN_HEAP_H
#define MIN_HEAP_H

#include <iostream>
#include <netinet/in.h>
#include <time.h>
#include <assert.h>
#include <string.h>

const int BUFFER_SIZE = 1024;

class heap_timer;
//繫結socket和定時器
struct client_data {
    sockaddr_in addr_;
    int         sockfd_;
    char        buf_[BUFFER_SIZE];
    heap_timer* timer_;
};
//定時器類
class heap_timer {
public:
    heap_timer(int delay) {
        printf("birth time %d, delay: %d\n", time(NULL), delay);
        expire_ = time(NULL) + delay;  //注意，和之前的升序連結串列以及時間輪不同，這次我們在timer中初始化生效時間
    }   
public:
    void (*timeout_callback_)(client_data*);  //定時器的回撥函式
public:
    time_t       expire_;   //定時器生效的絕對時間
    client_data* user_data_;  //使用者資料
};

class time_heap {
public:
    time_heap(int cap) throw (std::exception)  //建構函式之一，初始化一個大小為cap的空堆
        : array_(new heap_timer*[cap]), capacity_(cap), cur_size_(0) {
        memset(array_, 0, sizeof(array_));  //初始化指標
    }
    time_heap(heap_timer** init_array, int size, int capacity) throw (std::exception)  //建構函式之二，使用已有陣列來初始堆
        : array_(new heap_timer*[capacity]), cur_size_(size), capacity_(capacity) {
        assert(capacity >= size);
        memset(array_, 0, sizeof(array_));
        if(cur_size_ != 0){
            for(int i=0; i<cur_size_; ++i)
                array_[i] = init_array[i];  //初始化堆陣列
            for(int i=((cur_size_-1)>>1); i>=0; --i)
                sift_down(i);   //多陣列中的（cur_size_-1)/2 ~ 0 個元素執行下濾操作
        }
    }
    ~time_heap() {   //銷燬時間堆
        for(int i=0; i<cur_size_; ++i)
            delete array_[i];
        delete []array_;
    }
public:
    //新增目標定時器timer
    void add_timer(heap_timer* timer) throw (std::exception) {
        assert(timer != NULL);

        if(cur_size_ >= capacity_)  //如果當前堆陣列容量不夠，擴容
            resize();

        //新插入一個元素，當前堆大小加1，hole是新插入元素的位置
        int hole = cur_size_++;   //hole = cur_size_ - 1
        int parent = 0;
        //對從新插入位置到根節點路徑上所有節點進行上濾操作
        for(; hole > 0; hole = parent){
            parent = (hole - 1) >> 1;
            if(array_[parent]->expire_ <= timer->expire_)
                break;
            array_[hole] = array_[parent];
        }
        array_[hole] = timer;
    }

    //調整，老規矩，先刪除，再新增一次，否則舊定時器還作祟
    void adjust_timer(heap_timer* old_timer, heap_timer* new_timer) {
        del_timer(old_timer);
        add_timer(new_timer);
    }
    //刪除目標定時器   
    void del_timer(heap_timer* timer) {
        assert(timer != NULL);
        //僅僅將目標定時器的回撥函式設定為空，即所謂的延遲銷燬。這將節省真正刪除該定時器造成的開銷，但這樣做容易使堆陣列膨脹
        //說下為什麼延遲銷燬效率高：如果不延遲我們需要查詢O(lgn）+刪除下濾調整O(lgn)；而採用延遲刪除，由於其他定時器可能觸發，其他定時器會呼叫真正的刪除函式pop_heap來刪除自己，所以我們採用了延遲刪除的定時器慢慢會被調整到堆頂，這時候由於時間到達，執行回撥函式（我們已經賦空，tick函式中不會執行空回撥函式)。由於為空，什麼也不做。然後被pop_heap，實際上僅僅消耗刪除調整的代價，無需查詢操作，時間複雜度是O(lgn)。
        timer->timeout_callback_ = NULL;
    }

    heap_timer* top() const { //獲得堆頂定時器
        return empty() ? NULL : array_[0];
    }
    //刪除堆頂定時器
    void pop_timer() {
        if(empty())  //有可能目前時空的，什麼也不做
            return ;
        if(array_[0] != NULL){
            printf("delete user: %d\n", array_[0]->user_data_->sockfd_);
            delete array_[0];
            //將原來的堆頂元素替換為堆陣列中最後一個元素，提高效率
            array_[0] = array_[--cur_size_];
            printf("cur_size_: %d\n", cur_size_);
            sift_down(0); //對新的堆頂元素執行下濾操作
        }
    }
    //心搏函式
    void tick() {
        heap_timer* tmp = array_[0];
        time_t cur = time(NULL);   //迴圈處理堆中到期的定時器
        while(!empty()){
            if(tmp == NULL)
                break;
            printf("current time is: %d\n", cur);
            if(tmp->expire_ > cur)  //如果堆頂定時器沒到期，則退出迴圈
                break;
            if(array_[0]->timeout_callback_ != NULL)  //否則執行沒有被延遲刪除的定時器的任務
                array_[0]->timeout_callback_(array_[0]->user_data_);
            //將堆頂元素刪除，同時生成新的堆頂定時器（array[0]）
            pop_timer();
            tmp = array_[0];
        }
    }

    bool empty() const { return cur_size_ == 0; }
private:
    void sift_down(int hole) {  //下濾操作
        auto left_child = [] (int i) { return 2*i + 1; };
        heap_timer* tmp = array_[hole];
        int child = -1;
        //注意比較的是left_child(hole)而不是hole
        for(; left_child(hole)<cur_size_; hole=child){
            child = left_child(hole);
  if(child < cur_size_-1
                    && array_[child]->expire_ > array_[child+1]->expire_)
                ++child;
            if(array_[child]->expire_ < tmp->expire_)
                array_[hole] = array_[child];
            else
                break;
        }
        array_[hole] = tmp;
    }
    //擴容函式
    void resize() throw (std::exception) {
        heap_timer** tmp = new heap_timer* [capacity_<<1];
        memset(tmp, 0, sizeof(tmp));
        capacity_ = capacity_ << 1;
        for(int i=0; i<cur_size_; ++i)
            tmp[i] = array_[i];
        delete []array_;
        array_ = tmp;
    }
private:
    heap_timer** array_;   //堆陣列
    int          capacity_;   //堆陣列的容量
    int          cur_size_;   //堆陣列當前包含的元素的個數
};

#endif

測試程式碼：

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <assert.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/epoll.h>
#include <pthread.h>
#include <memory>
#include <vector>

#include "min_heap.h"

const int FD_LIMIT = 65535;
const int MAX_EVENT_NUMBER = 1024;
const int TIME_SLOT = 5;
const int INIT_HEAP_SIZE = 2;

static int pipefd[2];
static int epollfd = 0;
static std::shared_ptr<time_heap> timer_lst;

int setnonblocking( int fd )
{
    int old_option = fcntl( fd, F_GETFL );
 int new_option = old_option | O_NONBLOCK;
    fcntl( fd, F_SETFL, new_option );
    return old_option;
}

void addfd(int fd )
{
    epoll_event event;
    event.data.fd = fd;
    event.events = EPOLLIN | EPOLLET;
    epoll_ctl( epollfd, EPOLL_CTL_ADD, fd, &event );
    setnonblocking( fd );
}

void sig_handler( int sig )
{
    int save_errno = errno;
    int msg = sig;
    send( pipefd[1], ( char* )&msg, 1, 0 );
    errno = save_errno;
}

void addsig( int sig )
{
    struct sigaction sa;
    memset( &sa, '\0', sizeof( sa ) );
    sa.sa_handler = sig_handler;
    sa.sa_flags |= SA_RESTART;
    sigfillset( &sa.sa_mask );
    assert( sigaction( sig, &sa, NULL ) != -1 );
}

void timer_handler()
{
    timer_lst->tick();
    alarm( TIME_SLOT );
}

void cb_func( client_data* user_data )
{
    epoll_ctl( epollfd, EPOLL_CTL_DEL, user_data->sockfd_, 0 );
    assert( user_data );
    close( user_data->sockfd_ );
    printf( "close fd %d\n", user_data->sockfd_ );
}

int main( int argc, char* argv[] )
{
    if( argc <= 2 )
    {
        printf( "usage: %s ip_address port_number\n", basename( argv[0] ) );
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi( argv[2] );

    int ret = 0;
    struct sockaddr_in address;
    bzero( &address, sizeof( address ) );
    address.sin_family = AF_INET;
    inet_pton( AF_INET, ip, &address.sin_addr );
 address.sin_port = htons( port );

    int listenfd = socket( PF_INET, SOCK_STREAM, 0 );
    assert( listenfd >= 0 );

    int on = 1;
    ret = setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));
    assert(ret != -1);

    ret = bind( listenfd, ( struct sockaddr* )&address, sizeof( address ) );
    assert( ret != -1 );

    ret = listen( listenfd, 5 );
    assert( ret != -1 );

    epoll_event events[ MAX_EVENT_NUMBER ];
    epollfd = epoll_create( 5 );
    assert( epollfd != -1 );
    addfd(listenfd );

    ret = socketpair( PF_UNIX, SOCK_STREAM, 0, pipefd );
    assert( ret != -1 );
    setnonblocking( pipefd[1] );
    addfd(pipefd[0] );

    // add all the interesting signals here
    addsig( SIGALRM );
    addsig( SIGTERM );
    bool stop_server = false;

    std::vector<client_data> users(FD_LIMIT);
  bool timeout = false;
    alarm( TIME_SLOT );

    //////////////////////////////////////////////////////
    timer_lst.reset(new time_heap(INIT_HEAP_SIZE));
    //////////////////////////////////////////////////////

    while( !stop_server )
    {
        int number = epoll_wait( epollfd, events, MAX_EVENT_NUMBER, -1 );
        if ( ( number < 0 ) && ( errno != EINTR ) )
        {
            printf( "epoll failure\n" );
            break;
        }

        for ( int i = 0; i < number; i++ )
        {
            int sockfd = events[i].data.fd;
            if( sockfd == listenfd )
            {
                struct sockaddr_in client_address;
                socklen_t client_addrlength = sizeof( client_address );
                int connfd = accept( listenfd, ( struct sockaddr* )&client_address, &client_addrlength );
                addfd(connfd);
                users[connfd].addr_ = client_address;
                users[connfd].sockfd_ = connfd;
                printf("user: %d\n", connfd);
                heap_timer *timer = new heap_timer(3 * TIME_SLOT);
                timer_lst->add_timer(timer);
                timer->user_data_ = &users[connfd];
 timer->timeout_callback_ = cb_func;
                users[connfd].timer_ = timer;
            }
            else if( ( sockfd == pipefd[0] ) && ( events[i].events & EPOLLIN ) )
            {
                int sig;
                char signals[1024];
                ret = recv( pipefd[0], signals, sizeof( signals ), 0 );
                if( ret == -1 )
                {
                    // handle the error
                    continue;
                }
                else if( ret == 0 )
                {
                    continue;
                }
                else
                {
                    for( int i = 0; i < ret; ++i )
                    {
                        switch( signals[i] )
                        {
                            case SIGALRM:
                            {
                                timeout = true;
                                break;
                            }
                            case SIGTERM:
                            {
          stop_server = true;
                            }
                        }
                    }
                }
            }
            else if(  events[i].events & EPOLLIN )
            {
                memset( users[sockfd].buf_, '\0', BUFFER_SIZE );
                ret = recv( sockfd, users[sockfd].buf_, BUFFER_SIZE-1, 0 );
                printf( "get %d bytes of client data %s from %d\n", ret, users[sockfd].buf_, sockfd );
                heap_timer* timer = users[sockfd].timer_;
                if( ret < 0 )
                {
                    if( errno != EAGAIN )
                    {
                        cb_func( &users[sockfd] );
                        if( timer )
                        {
                            timer_lst->del_timer( timer );
                        }
                    }
                }
                else if( ret == 0 )
                {
                    cb_func( &users[sockfd] );
                    if( timer )
                    {
                        timer_lst->del_timer( timer );
                    }
                }
  else
                {
                    //send( sockfd, users[sockfd].buf, BUFFER_SIZE-1, 0 );
                    if( timer )
                    {
                       printf("user: %d\n", sockfd);
                       heap_timer* new_timer = new heap_timer(3*TIME_SLOT);
                       timer_lst->adjust_timer(timer, new_timer);
                       new_timer->user_data_ = &users[sockfd];
                       new_timer->timeout_callback_ = cb_func;
                       users[sockfd].timer_ = new_timer;
                    }
                }
            }
            else
            {
                // others
            }
        }

        if( timeout )
        {
            timer_handler();
            timeout = false;
        }
    }

    close( listenfd );
    close( epollfd );
    close( pipefd[1] );
    close( pipefd[0] );
    return 0;
}

對時間堆而言，新增一個定時器的時間複雜度是O(lgn)，刪除一個定時器的時間複雜度是O(1)（採用了延遲刪除），執行一個定時器的時間複雜度是O(1)。因此，時間堆的效率是很高的。