Gossip about redis source code 79

Redis Of AOF operation , It's really common , And in all kinds of written interviews , I often meet ：

/* Return the current size of the AOF rewrite buffer. */
unsigned long aofRewriteBufferSize(void) {
    listNode *ln;
    listIter li;
    unsigned long size = 0;

    listRewind(server.aof_rewrite_buf_blocks,&li);
    while((ln = listNext(&li))) {
        aofrwblock *block = listNodeValue(ln);
        size += block->used;
    }
    return size;
}

/* Event handler used to send data to the child process doing the AOF
 * rewrite. We send pieces of our AOF differences buffer so that the final
 * write when the child finishes the rewrite will be small. */
void aofChildWriteDiffData(aeEventLoop *el, int fd, void *privdata, int mask) {
    listNode *ln;
    aofrwblock *block;
    ssize_t nwritten;
    UNUSED(el);
    UNUSED(fd);
    UNUSED(privdata);
    UNUSED(mask);

    while(1) {
        ln = listFirst(server.aof_rewrite_buf_blocks);
        block = ln ? ln->value : NULL;
        if (server.aof_stop_sending_diff || !block) {
            aeDeleteFileEvent(server.el,server.aof_pipe_write_data_to_child,
                              AE_WRITABLE);
            return;
        }
        if (block->used > 0) {
            nwritten = write(server.aof_pipe_write_data_to_child,
                             block->buf,block->used);
            if (nwritten <= 0) return;
            memmove(block->buf,block->buf+nwritten,block->used-nwritten);
            block->used -= nwritten;
            block->free += nwritten;
        }
        if (block->used == 0) listDelNode(server.aof_rewrite_buf_blocks,ln);
    }
}

/* Append data to the AOF rewrite buffer, allocating new blocks if needed. */
void aofRewriteBufferAppend(unsigned char *s, unsigned long len) {
    listNode *ln = listLast(server.aof_rewrite_buf_blocks);
    aofrwblock *block = ln ? ln->value : NULL;

    while(len) {
        /* If we already got at least an allocated block, try appending
         * at least some piece into it. */
        if (block) {
            unsigned long thislen = (block->free < len) ? block->free : len;
            if (thislen) {  /* The current block is not already full. */
                memcpy(block->buf+block->used, s, thislen);
                block->used += thislen;
                block->free -= thislen;
                s += thislen;
                len -= thislen;
            }
        }

        if (len) { /* First block to allocate, or need another block. */
            int numblocks;

            block = zmalloc(sizeof(*block));
            block->free = AOF_RW_BUF_BLOCK_SIZE;
            block->used = 0;
            listAddNodeTail(server.aof_rewrite_buf_blocks,block);

            /* Log every time we cross more 10 or 100 blocks, respectively
             * as a notice or warning. */
            numblocks = listLength(server.aof_rewrite_buf_blocks);
            if (((numblocks+1) % 10) == 0) {
                int level = ((numblocks+1) % 100) == 0 ? LL_WARNING :
                                                         LL_NOTICE;
                serverLog(level,"Background AOF buffer size: %lu MB",
                    aofRewriteBufferSize()/(1024*1024));
            }
        }
    }

    /* Install a file event to send data to the rewrite child if there is
     * not one already. */
    if (!server.aof_stop_sending_diff &&
        aeGetFileEvents(server.el,server.aof_pipe_write_data_to_child) == 0)
    {
        aeCreateFileEvent(server.el, server.aof_pipe_write_data_to_child,
            AE_WRITABLE, aofChildWriteDiffData, NULL);
    }
}

/* Write the buffer (possibly composed of multiple blocks) into the specified
 * fd. If a short write or any other error happens -1 is returned,
 * otherwise the number of bytes written is returned. */
ssize_t aofRewriteBufferWrite(int fd) {
    listNode *ln;
    listIter li;
    ssize_t count = 0;

    listRewind(server.aof_rewrite_buf_blocks,&li);
    while((ln = listNext(&li))) {
        aofrwblock *block = listNodeValue(ln);
        ssize_t nwritten;

        if (block->used) {
            nwritten = write(fd,block->buf,block->used);
            if (nwritten != (ssize_t)block->used) {
                if (nwritten == 0) errno = EIO;
                return -1;
            }
            count += nwritten;
        }
    }
    return count;
}

/* ----------------------------------------------------------------------------
 * AOF file implementation
 * ------------------------------------------------------------------------- */

/* Return true if an AOf fsync is currently already in progress in a
 * BIO thread. */
int aofFsyncInProgress(void) {
    return bioPendingJobsOfType(BIO_AOF_FSYNC) != 0;
}

/* Starts a background task that performs fsync() against the specified
 * file descriptor (the one of the AOF file) in another thread. */
void aof_background_fsync(int fd) {
    bioCreateFsyncJob(fd);
}

/* Kills an AOFRW child process if exists */
void killAppendOnlyChild(void) {
    int statloc;
    /* No AOFRW child? return. */
    if (server.child_type != CHILD_TYPE_AOF) return;
    /* Kill AOFRW child, wait for child exit. */
    serverLog(LL_NOTICE,"Killing running AOF rewrite child: %ld",
        (long) server.child_pid);
    if (kill(server.child_pid,SIGUSR1) != -1) {
        while(waitpid(-1, &statloc, 0) != server.child_pid);
    }
    /* Reset the buffer accumulating changes while the child saves. */
    aofRewriteBufferReset();
    aofRemoveTempFile(server.child_pid);
    resetChildState();
    server.aof_rewrite_time_start = -1;
    /* Close pipes used for IPC between the two processes. */
    aofClosePipes();
}

/* Called when the user switches from "appendonly yes" to "appendonly no"
 * at runtime using the CONFIG command. */
void stopAppendOnly(void) {
    serverAssert(server.aof_state != AOF_OFF);
    flushAppendOnlyFile(1);
    if (redis_fsync(server.aof_fd) == -1) {
        serverLog(LL_WARNING,"Fail to fsync the AOF file: %s",strerror(errno));
    } else {
        server.aof_fsync_offset = server.aof_current_size;
        server.aof_last_fsync = server.unixtime;
    }
    close(server.aof_fd);

    server.aof_fd = -1;
    server.aof_selected_db = -1;
    server.aof_state = AOF_OFF;
    server.aof_rewrite_scheduled = 0;
    killAppendOnlyChild();
    sdsfree(server.aof_buf);
    server.aof_buf = sdsempty();
}

/* Called when the user switches from "appendonly no" to "appendonly yes"
 * at runtime using the CONFIG command. */
int startAppendOnly(void) {
    char cwd[MAXPATHLEN]; /* Current working dir path for error messages. */
    int newfd;

    newfd = open(server.aof_filename,O_WRONLY|O_APPEND|O_CREAT,0644);
    serverAssert(server.aof_state == AOF_OFF);
    if (newfd == -1) {
        char *cwdp = getcwd(cwd,MAXPATHLEN);

        serverLog(LL_WARNING,
            "Redis needs to enable the AOF but can't open the "
            "append only file %s (in server root dir %s): %s",
            server.aof_filename,
            cwdp ? cwdp : "unknown",
            strerror(errno));
        return C_ERR;
    }
    if (hasActiveChildProcess() && server.child_type != CHILD_TYPE_AOF) {
        server.aof_rewrite_scheduled = 1;
        serverLog(LL_WARNING,"AOF was enabled but there is already another background operation. An AOF background was scheduled to start when possible.");
    } else {
        /* If there is a pending AOF rewrite, we need to switch it off and
         * start a new one: the old one cannot be reused because it is not
         * accumulating the AOF buffer. */
        if (server.child_type == CHILD_TYPE_AOF) {
            serverLog(LL_WARNING,"AOF was enabled but there is already an AOF rewriting in background. Stopping background AOF and starting a rewrite now.");
            killAppendOnlyChild();
        }
        if (rewriteAppendOnlyFileBackground() == C_ERR) {
            close(newfd);
            serverLog(LL_WARNING,"Redis needs to enable the AOF but can't trigger a background AOF rewrite operation. Check the above logs for more info about the error.");
            return C_ERR;
        }
    }
    /* We correctly switched on AOF, now wait for the rewrite to be complete
     * in order to append data on disk. */
    server.aof_state = AOF_WAIT_REWRITE;
    server.aof_last_fsync = server.unixtime;
    server.aof_fd = newfd;

    /* If AOF fsync error in bio job, we just ignore it and log the event. */
    int aof_bio_fsync_status;
    atomicGet(server.aof_bio_fsync_status, aof_bio_fsync_status);
    if (aof_bio_fsync_status == C_ERR) {
        serverLog(LL_WARNING,
            "AOF reopen, just ignore the AOF fsync error in bio job");
        atomicSet(server.aof_bio_fsync_status,C_OK);
    }

    /* If AOF was in error state, we just ignore it and log the event. */
    if (server.aof_last_write_status == C_ERR) {
        serverLog(LL_WARNING,"AOF reopen, just ignore the last error.");
        server.aof_last_write_status = C_OK;
    }
    return C_OK;
}

/* This is a wrapper to the write syscall in order to retry on short writes
 * or if the syscall gets interrupted. It could look strange that we retry
 * on short writes given that we are writing to a block device: normally if
 * the first call is short, there is a end-of-space condition, so the next
 * is likely to fail. However apparently in modern systems this is no longer
 * true, and in general it looks just more resilient to retry the write. If
 * there is an actual error condition we'll get it at the next try. */
ssize_t aofWrite(int fd, const char *buf, size_t len) {
    ssize_t nwritten = 0, totwritten = 0;

    while(len) {
        nwritten = write(fd, buf, len);

        if (nwritten < 0) {
            if (errno == EINTR) continue;
            return totwritten ? totwritten : -1;
        }

        len -= nwritten;
        buf += nwritten;
        totwritten += nwritten;
    }

    return totwritten;
}

/* Write the append only file buffer on disk.
 *
 * Since we are required to write the AOF before replying to the client,
 * and the only way the client socket can get a write is entering when the
 * the event loop, we accumulate all the AOF writes in a memory
 * buffer and write it on disk using this function just before entering
 * the event loop again.
 *
 * About the 'force' argument:
 *
 * When the fsync policy is set to 'everysec' we may delay the flush if there
 * is still an fsync() going on in the background thread, since for instance
 * on Linux write(2) will be blocked by the background fsync anyway.
 * When this happens we remember that there is some aof buffer to be
 * flushed ASAP, and will try to do that in the serverCron() function.
 *
 * However if force is set to 1 we'll write regardless of the background
 * fsync. */
#define AOF_WRITE_LOG_ERROR_RATE 30 /* Seconds between errors logging. */
void flushAppendOnlyFile(int force) {
    ssize_t nwritten;
    int sync_in_progress = 0;
    mstime_t latency;

    if (sdslen(server.aof_buf) == 0) {
        /* Check if we need to do fsync even the aof buffer is empty,
         * because previously in AOF_FSYNC_EVERYSEC mode, fsync is
         * called only when aof buffer is not empty, so if users
         * stop write commands before fsync called in one second,
         * the data in page cache cannot be flushed in time. */
        if (server.aof_fsync == AOF_FSYNC_EVERYSEC &&
            server.aof_fsync_offset != server.aof_current_size &&
            server.unixtime > server.aof_last_fsync &&
            !(sync_in_progress = aofFsyncInProgress())) {
            goto try_fsync;
        } else {
            return;
        }
    }

    if (server.aof_fsync == AOF_FSYNC_EVERYSEC)
        sync_in_progress = aofFsyncInProgress();

    if (server.aof_fsync == AOF_FSYNC_EVERYSEC && !force) {
        /* With this append fsync policy we do background fsyncing.
         * If the fsync is still in progress we can try to delay
         * the write for a couple of seconds. */
        if (sync_in_progress) {
            if (server.aof_flush_postponed_start == 0) {
                /* No previous write postponing, remember that we are
                 * postponing the flush and return. */
                server.aof_flush_postponed_start = server.unixtime;
                return;
            } else if (server.unixtime - server.aof_flush_postponed_start < 2) {
                /* We were already waiting for fsync to finish, but for less
                 * than two seconds this is still ok. Postpone again. */
                return;
            }
            /* Otherwise fall trough, and go write since we can't wait
             * over two seconds. */
            server.aof_delayed_fsync++;
            serverLog(LL_NOTICE,"Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.");
        }
    }
    /* We want to perform a single write. This should be guaranteed atomic
     * at least if the filesystem we are writing is a real physical one.
     * While this will save us against the server being killed I don't think
     * there is much to do about the whole server stopping for power problems
     * or alike */

    if (server.aof_flush_sleep && sdslen(server.aof_buf)) {
        usleep(server.aof_flush_sleep);
    }

    latencyStartMonitor(latency);
    nwritten = aofWrite(server.aof_fd,server.aof_buf,sdslen(server.aof_buf));
    latencyEndMonitor(latency);
    /* We want to capture different events for delayed writes:
     * when the delay happens with a pending fsync, or with a saving child
     * active, and when the above two conditions are missing.
     * We also use an additional event name to save all samples which is
     * useful for graphing / monitoring purposes. */
    if (sync_in_progress) {
        latencyAddSampleIfNeeded("aof-write-pending-fsync",latency);
    } else if (hasActiveChildProcess()) {
        latencyAddSampleIfNeeded("aof-write-active-child",latency);
    } else {
        latencyAddSampleIfNeeded("aof-write-alone",latency);
    }
    latencyAddSampleIfNeeded("aof-write",latency);

    /* We performed the write so reset the postponed flush sentinel to zero. */
    server.aof_flush_postponed_start = 0;

    if (nwritten != (ssize_t)sdslen(server.aof_buf)) {
        static time_t last_write_error_log = 0;
        int can_log = 0;

        /* Limit logging rate to 1 line per AOF_WRITE_LOG_ERROR_RATE seconds. */
        if ((server.unixtime - last_write_error_log) > AOF_WRITE_LOG_ERROR_RATE) {
            can_log = 1;
            last_write_error_log = server.unixtime;
        }

        /* Log the AOF write error and record the error code. */
        if (nwritten == -1) {
            if (can_log) {
                serverLog(LL_WARNING,"Error writing to the AOF file: %s",
                    strerror(errno));
                server.aof_last_write_errno = errno;
            }
        } else {
            if (can_log) {
                serverLog(LL_WARNING,"Short write while writing to "
                                       "the AOF file: (nwritten=%lld, "
                                       "expected=%lld)",
                                       (long long)nwritten,
                                       (long long)sdslen(server.aof_buf));
            }

            if (ftruncate(server.aof_fd, server.aof_current_size) == -1) {
                if (can_log) {
                    serverLog(LL_WARNING, "Could not remove short write "
                             "from the append-only file.  Redis may refuse "
                             "to load the AOF the next time it starts.  "
                             "ftruncate: %s", strerror(errno));
                }
            } else {
                /* If the ftruncate() succeeded we can set nwritten to
                 * -1 since there is no longer partial data into the AOF. */
                nwritten = -1;
            }
            server.aof_last_write_errno = ENOSPC;
        }

        /* Handle the AOF write error. */
        if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
            /* We can't recover when the fsync policy is ALWAYS since the reply
             * for the client is already in the output buffers (both writes and
             * reads), and the changes to the db can't be rolled back. Since we
             * have a contract with the user that on acknowledged or observed
             * writes are is synced on disk, we must exit. */
            serverLog(LL_WARNING,"Can't recover from AOF write error when the AOF fsync policy is 'always'. Exiting...");
            exit(1);
        } else {
            /* Recover from failed write leaving data into the buffer. However
             * set an error to stop accepting writes as long as the error
             * condition is not cleared. */
            server.aof_last_write_status = C_ERR;

            /* Trim the sds buffer if there was a partial write, and there
             * was no way to undo it with ftruncate(2). */
            if (nwritten > 0) {
                server.aof_current_size += nwritten;
                sdsrange(server.aof_buf,nwritten,-1);
            }
            return; /* We'll try again on the next call... */
        }
    } else {
        /* Successful write(2). If AOF was in error state, restore the
         * OK state and log the event. */
        if (server.aof_last_write_status == C_ERR) {
            serverLog(LL_WARNING,
                "AOF write error looks solved, Redis can write again.");
            server.aof_last_write_status = C_OK;
        }
    }
    server.aof_current_size += nwritten;

    /* Re-use AOF buffer when it is small enough. The maximum comes from the
     * arena size of 4k minus some overhead (but is otherwise arbitrary). */
    if ((sdslen(server.aof_buf)+sdsavail(server.aof_buf)) < 4000) {
        sdsclear(server.aof_buf);
    } else {
        sdsfree(server.aof_buf);
        server.aof_buf = sdsempty();
    }

try_fsync:
    /* Don't fsync if no-appendfsync-on-rewrite is set to yes and there are
     * children doing I/O in the background. */
    if (server.aof_no_fsync_on_rewrite && hasActiveChildProcess())
        return;

    /* Perform the fsync if needed. */
    if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
        /* redis_fsync is defined as fdatasync() for Linux in order to avoid
         * flushing metadata. */
        latencyStartMonitor(latency);
        /* Let's try to get this data on the disk. To guarantee data safe when
         * the AOF fsync policy is 'always', we should exit if failed to fsync
         * AOF (see comment next to the exit(1) after write error above). */
        if (redis_fsync(server.aof_fd) == -1) {
            serverLog(LL_WARNING,"Can't persist AOF for fsync error when the "
              "AOF fsync policy is 'always': %s. Exiting...", strerror(errno));
            exit(1);
        }
        latencyEndMonitor(latency);
        latencyAddSampleIfNeeded("aof-fsync-always",latency);
        server.aof_fsync_offset = server.aof_current_size;
        server.aof_last_fsync = server.unixtime;
    } else if ((server.aof_fsync == AOF_FSYNC_EVERYSEC &&
                server.unixtime > server.aof_last_fsync)) {
        if (!sync_in_progress) {
            aof_background_fsync(server.aof_fd);
            server.aof_fsync_offset = server.aof_current_size;
        }
        server.aof_last_fsync = server.unixtime;
    }
}