Function rewriteAppendOnlyFileBackground

app/redis-6.2.6/src/aof.c:1763–1804

This is how rewriting of the append only file in background works: * * 1) The user calls BGREWRITEAOF * 2) Redis calls this function, that forks(): * 2a) the child rewrite the append only file in a temp file. * 2b) the parent accumulates differences in server.aof_rewrite_buf. * 3) When the child finished '2a' exists. * 4) The parent will trap the exit code, if it's OK, will append the

 *    The the new file is reopened as the new append only file. Profit!
 */
int rewriteAppendOnlyFileBackground(void) {
    pid_t childpid;

    if (hasActiveChildProcess()) return C_ERR;
    if (aofCreatePipes() != C_OK) return C_ERR;
    if ((childpid = redisFork(CHILD_TYPE_AOF)) == 0) {
        char tmpfile[256];

        /* Child */
        redisSetProcTitle("redis-aof-rewrite");
        redisSetCpuAffinity(server.aof_rewrite_cpulist);
        snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int) getpid());
        if (rewriteAppendOnlyFile(tmpfile) == C_OK) {
            sendChildCowInfo(CHILD_INFO_TYPE_AOF_COW_SIZE, "AOF rewrite");
            exitFromChild(0);
        } else {
            exitFromChild(1);
        }
    } else {
        /* Parent */
        if (childpid == -1) {
            serverLog(LL_WARNING,
                "Can't rewrite append only file in background: fork: %s",
                strerror(errno));
            aofClosePipes();
            return C_ERR;
        }
        serverLog(LL_NOTICE,
            "Background append only file rewriting started by pid %ld",(long) childpid);
        server.aof_rewrite_scheduled = 0;
        server.aof_rewrite_time_start = time(NULL);

        /* We set appendseldb to -1 in order to force the next call to the
         * feedAppendOnlyFile() to issue a SELECT command, so the differences
         * accumulated by the parent into server.aof_rewrite_buf will start
         * with a SELECT statement and it will be safe to merge. */
        server.aof_selected_db = -1;
        replicationScriptCacheFlush();
        return C_OK;
    }
    return C_OK; /* unreached */
}

void bgrewriteaofCommand(client *c) {
    if (server.child_type == CHILD_TYPE_AOF) {