redis源碼筆記-ae.c

ae.c是redis事件框架的具體實現，這篇blog對這份源碼進行簡單說明。其中談到了作者已經標記的一些未來可能做的改進。

ae.c

#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>

#include "ae.h"
#include "zmalloc.h"
#include "config.h"

/* Include the best multiplexing layer supported by this system.
 * The following should be ordered by performances, descending. */
    //為了支持不同的平台，redis用相同的接口封裝了系統提供的多路復用層代碼。接口共提供如下函數：aeApiCreate\aeApiFree\aeApiAddEvent\aeApiDelEvent\aeApiPoll\aeApiName函數，以及一個struct aeApiState
    //通過包含不同的頭文件，選擇不同的底層實現
    //按如下順序，效率遞減： epoll > kqueue > select
#ifdef HAVE_EPOLL
#include "ae_epoll.c"
#else
    #ifdef HAVE_KQUEUE
    #include "ae_kqueue.c"
    #else
    #include "ae_select.c"
    #endif
#endif

    //初始化函數，創建事件循環，函數內部alloc一個結構，用於表示事件狀態，供后續其他函數作為參數使用
aeEventLoop *aeCreateEventLoop(void) {
    aeEventLoop *eventLoop;
    int i;

    eventLoop = zmalloc(sizeof(*eventLoop));
    if (!eventLoop) return NULL;
        //時間event用鏈表存儲
    eventLoop->timeEventHead = NULL;
    eventLoop->timeEventNextId = 0;
        //表示是否停止事件循環
    eventLoop->stop = 0;
        //maxfd只由ae_select.c使用，后續有些相關的處理，如果使用epoll的話，其實可以進行簡化
    eventLoop->maxfd = -1;
        //每次調用epoll\select前調用的函數，由框架使用者注冊
    eventLoop->beforesleep = NULL;
    if (aeApiCreate(eventLoop) == -1) {
        zfree(eventLoop);
        return NULL;
    }
    /* Events with mask == AE_NONE are not set. So let's initialize the
     * vector with it. */
        
        //將所有的文件描述符的mask設置為無效值，作為初始化
    for (i = 0; i < AE_SETSIZE; i++)
        eventLoop->events[i].mask = AE_NONE;
    return eventLoop;
}

    //底層實現執行釋放操作后，釋放state的內存
void aeDeleteEventLoop(aeEventLoop *eventLoop) {
    aeApiFree(eventLoop);
    zfree(eventLoop);
}

    //停止事件循環,redis作為一個無限循環的server，在redis.c中並沒有任何一處調用此函數
void aeStop(aeEventLoop *eventLoop) {
    eventLoop->stop = 1;
}

    //創建文件fd事件，加入事件循環監控列表，使得后續epoll\select時將會測試這個文件描述符的可讀性（可寫性）
int aeCreateFileEvent(aeEventLoop *eventLoop, int fd, int mask,
        aeFileProc *proc, void *clientData)
{
    if (fd >= AE_SETSIZE) return AE_ERR;
    aeFileEvent *fe = &eventLoop->events[fd];

    if (aeApiAddEvent(eventLoop, fd, mask) == -1)
        return AE_ERR;
    fe->mask |= mask;
        //注冊函數
    if (mask & AE_READABLE) fe->rfileProc = proc;
    if (mask & AE_WRITABLE) fe->wfileProc = proc;
    fe->clientData = clientData;
        //更新maxfd
    if (fd > eventLoop->maxfd)
        eventLoop->maxfd = fd;
    return AE_OK;
}

void aeDeleteFileEvent(aeEventLoop *eventLoop, int fd, int mask)
{
    if (fd >= AE_SETSIZE) return;
    aeFileEvent *fe = &eventLoop->events[fd];

    if (fe->mask == AE_NONE) return;
    fe->mask = fe->mask & (~mask);
    if (fd == eventLoop->maxfd && fe->mask == AE_NONE) {
        /* Update the max fd */
        int j;

        for (j = eventLoop->maxfd-1; j >= 0; j--)
            if (eventLoop->events[j].mask != AE_NONE) break;
        eventLoop->maxfd = j;
    }
    aeApiDelEvent(eventLoop, fd, mask);
}

    //返回值為該文件描述符關注的事件類型（可讀、可寫）
int aeGetFileEvents(aeEventLoop *eventLoop, int fd) {
    if (fd >= AE_SETSIZE) return 0;
    aeFileEvent *fe = &eventLoop->events[fd];

    return fe->mask;
}

static void aeGetTime(long *seconds, long *milliseconds)
{
    struct timeval tv;

    gettimeofday(&tv, NULL);
    *seconds = tv.tv_sec;
    *milliseconds = tv.tv_usec/1000;
}

static void aeAddMillisecondsToNow(long long milliseconds, long *sec, long *ms) {
    long cur_sec, cur_ms, when_sec, when_ms;

    aeGetTime(&cur_sec, &cur_ms);
    when_sec = cur_sec + milliseconds/1000;
    when_ms = cur_ms + milliseconds%1000;
    if (when_ms >= 1000) {
        when_sec ++;
        when_ms -= 1000;
    }
    *sec = when_sec;
    *ms = when_ms;
}

long long aeCreateTimeEvent(aeEventLoop *eventLoop, long long milliseconds,
        aeTimeProc *proc, void *clientData,
        aeEventFinalizerProc *finalizerProc)
{
    long long id = eventLoop->timeEventNextId++;
    aeTimeEvent *te;

    te = zmalloc(sizeof(*te));
    if (te == NULL) return AE_ERR;
    te->id = id;
        //time event執行時間為絕對時間點
    aeAddMillisecondsToNow(milliseconds,&te->when_sec,&te->when_ms);
    te->timeProc = proc;
    te->finalizerProc = finalizerProc;
    te->clientData = clientData;
        //time event鏈表為無序表，直接插入到鏈表頭
    te->next = eventLoop->timeEventHead;
    eventLoop->timeEventHead = te;
    return id;
}

    //從鏈表中刪除，是以id作為key查找的（順序遍歷）
int aeDeleteTimeEvent(aeEventLoop *eventLoop, long long id)
{
    aeTimeEvent *te, *prev = NULL;

    te = eventLoop->timeEventHead;
    while(te) {
        if (te->id == id) {
            if (prev == NULL)
                eventLoop->timeEventHead = te->next;
            else
                prev->next = te->next;
            if (te->finalizerProc)
                te->finalizerProc(eventLoop, te->clientData);
            zfree(te);
            return AE_OK;
        }
        prev = te;
        te = te->next;
    }
    return AE_ERR; /* NO event with the specified ID found */
}

/* Search the first timer to fire.
161  * This operation is useful to know how many time the select can be
162  * put in sleep without to delay any event. （沒大看懂）,也許是給一個阻塞時間的上限值
 * If there are no timers NULL is returned.
 *
 * Note that's O(N) since time events are unsorted.
 * Possible optimizations (not needed by Redis so far, but...):
 * 1) Insert the event in order, so that the nearest is just the head.
 *    Much better but still insertion or deletion of timers is O(N).
 * 2) Use a skiplist to have this operation as O(1) and insertion as O(log(N)).
 */
static aeTimeEvent *aeSearchNearestTimer(aeEventLoop *eventLoop)
{
    aeTimeEvent *te = eventLoop->timeEventHead;
    aeTimeEvent *nearest = NULL;

    while(te) {
        if (!nearest || te->when_sec < nearest->when_sec ||
                (te->when_sec == nearest->when_sec &&
                 te->when_ms < nearest->when_ms))
            nearest = te;
        te = te->next;
    }
    return nearest;
}

    //對time event進行處理
/* Process time events */
static int processTimeEvents(aeEventLoop *eventLoop) {

        //處理的事件數
    int processed = 0;
    aeTimeEvent *te;
    long long maxId;

    te = eventLoop->timeEventHead;
    maxId = eventLoop->timeEventNextId-1;
    while(te) {
        long now_sec, now_ms;
        long long id;

        if (te->id > maxId) {
            te = te->next;
            continue;
        }
        aeGetTime(&now_sec, &now_ms);
        if (now_sec > te->when_sec ||
            (now_sec == te->when_sec && now_ms >= te->when_ms))
        {
            int retval;

            id = te->id;
                //如果執行時間到或已超出，則執行對應的時間處理函數
            retval = te->timeProc(eventLoop, id, te->clientData);
            processed++;
            /* After an event is processed our time event list may
             * no longer be the same, so we restart from head. //因為時間處理函數timeProc可能改變此鏈表
213              * Still we make sure to don't process events registered
214              * by event handlers itself in order to don't loop forever.？
             * To do so we saved the max ID we want to handle.
             *
             * FUTURE OPTIMIZATIONS:
             * Note that this is NOT great algorithmically. Redis uses
             * a single time event so it's not a problem but the right
             * way to do this is to add the new elements on head, and
             * to flag deleted elements in a special way for later
             * deletion (putting references to the nodes to delete into
             * another linked list). */
           
                //如果這個時間處理函數不再繼續執行，則從time event的鏈表中刪除事件；否則，retval為繼續執行的時間間隔（單位為ms），在當前的timeevent struct的時間值上進行增加
            if (retval != AE_NOMORE) {
                aeAddMillisecondsToNow(retval,&te->when_sec,&te->when_ms);
            } else {
                aeDeleteTimeEvent(eventLoop, id);
            }
            te = eventLoop->timeEventHead;
        } else {
            te = te->next;
        }
    }
    return processed;
}

/* Process every pending(懸而未決) time event, then every pending file event
 * (that may be registered by time event callbacks just processed).
 * Without special flags the function sleeps until some file event
 * fires, or when the next time event occurrs (if any).
 *
 * If flags is 0, the function does nothing and returns.
 * if flags has AE_ALL_EVENTS set, all the kind of events are processed.
 * if flags has AE_FILE_EVENTS set, file events are processed.
 * if flags has AE_TIME_EVENTS set, time events are processed.
 * if flags has AE_DONT_WAIT set the function returns ASAP until all
 * the events that's possible to process without to wait are processed.
 *
 * The function returns the number of events processed. */
int aeProcessEvents(aeEventLoop *eventLoop, int flags)
{
    int processed = 0, numevents;

    /* Nothing to do? return ASAP */
    if (!(flags & AE_TIME_EVENTS) && !(flags & AE_FILE_EVENTS)) return 0;

    /* Note that we want call select() even if there are no
     * file events to process as long as we want to process time
     * events, in order to sleep until the next time event is ready
     * to fire. */
    if (eventLoop->maxfd != -1 ||
        ((flags & AE_TIME_EVENTS) && !(flags & AE_DONT_WAIT))) {
        int j;
        aeTimeEvent *shortest = NULL;
        struct timeval tv, *tvp;

        if (flags & AE_TIME_EVENTS && !(flags & AE_DONT_WAIT))
            shortest = aeSearchNearestTimer(eventLoop);
        if (shortest) {
            long now_sec, now_ms;

            /* Calculate the time missing for the nearest
             * timer to fire. */
            aeGetTime(&now_sec, &now_ms);
            tvp = &tv;
            tvp->tv_sec = shortest->when_sec - now_sec;
277             if (shortest->when_ms < now_ms) {
278                 tvp->tv_usec = ((shortest->when_ms+1000) - now_ms)*1000;
279                 tvp->tv_sec --;
280             } else {
281                 tvp->tv_usec = (shortest->when_ms - now_ms)*1000;
282             }
283             if (tvp->tv_sec < 0) tvp->tv_sec = 0;
284             if (tvp->tv_usec < 0) tvp->tv_usec = 0;  疑似有bug，比如當前時間為 1s +2ms  ，shortest時間為0s+1ms的case
        } else {
            /* If we have to check for events but need to return
             * ASAP because of AE_DONT_WAIT we need to se the timeout
             * to zero */
            if (flags & AE_DONT_WAIT) {
                tv.tv_sec = tv.tv_usec = 0;
                tvp = &tv;
            } else {
                /* Otherwise we can block */
                tvp = NULL; /* wait forever */
            }
        }

            //指定這個tvp是說這個tvp到期時，至少由time event可以執行
        numevents = aeApiPoll(eventLoop, tvp);
        for (j = 0; j < numevents; j++) {
            aeFileEvent *fe = &eventLoop->events[eventLoop->fired[j].fd];
            int mask = eventLoop->fired[j].mask;
            int fd = eventLoop->fired[j].fd;
            int rfired = 0;

        /* note the fe->mask & mask & ... code: maybe an already processed
             * event removed an element that fired and we still didn't
             * processed, so we check if the event is still valid. */
            if (fe->mask & mask & AE_READABLE) {
                rfired = 1;
                fe->rfileProc(eventLoop,fd,fe->clientData,mask);
            }
                //避免同一個注冊函數被調用兩次
            if (fe->mask & mask & AE_WRITABLE) {
                if (!rfired || fe->wfileProc != fe->rfileProc)
                    fe->wfileProc(eventLoop,fd,fe->clientData,mask);
            }
            processed++;
        }
    }
    /* Check time events */
    if (flags & AE_TIME_EVENTS)
        processed += processTimeEvents(eventLoop);

    return processed; /* return the number of processed file/time events */
}

    //對select的一個簡單封裝，沒有太大意義
/* Wait for millseconds until the given file descriptor becomes
 * writable/readable/exception */
int aeWait(int fd, int mask, long long milliseconds) {
    struct timeval tv;
    fd_set rfds, wfds, efds;
    int retmask = 0, retval;

    tv.tv_sec = milliseconds/1000;
    tv.tv_usec = (milliseconds%1000)*1000;
    FD_ZERO(&rfds);
    FD_ZERO(&wfds);
    FD_ZERO(&efds);

    if (mask & AE_READABLE) FD_SET(fd,&rfds);
    if (mask & AE_WRITABLE) FD_SET(fd,&wfds);
    if ((retval = select(fd+1, &rfds, &wfds, &efds, &tv)) > 0) {
        if (FD_ISSET(fd,&rfds)) retmask |= AE_READABLE;
        if (FD_ISSET(fd,&wfds)) retmask |= AE_WRITABLE;
        return retmask;
    } else {
        return retval;
    }
}

void aeMain(aeEventLoop *eventLoop) {
    eventLoop->stop = 0;
    while (!eventLoop->stop) {
        if (eventLoop->beforesleep != NULL)
            eventLoop->beforesleep(eventLoop);
        aeProcessEvents(eventLoop, AE_ALL_EVENTS);
    }
}

char *aeGetApiName(void) {
    return aeApiName();
}

void aeSetBeforeSleepProc(aeEventLoop *eventLoop, aeBeforeSleepProc *beforesleep) {
    eventLoop->beforesleep = beforesleep;
}