/*
 * Copyright (c) 2009-2012, Salvatore Sanfilippo <antirez at gmail dot com>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include "redis.h"
#include "lzf.h"    /* LZF compression library */
#include "zipmap.h"
#include "endianconv.h"

#include <math.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <sys/wait.h>
#include <arpa/inet.h>
#include <sys/stat.h>

static int rdbWriteRaw(rio *rdb, void *p, size_t len) {
    if (rdb && rioWrite(rdb,p,len) == 0)
        return -1;
    return len;
}

int rdbSaveType(rio *rdb, unsigned char type) {
    return rdbWriteRaw(rdb,&type,1);
}

/* Load a "type" in RDB format, that is a one byte unsigned integer.
 * This function is not only used to load object types, but also special
 * "types" like the end-of-file type, the EXPIRE type, and so forth. */
int rdbLoadType(rio *rdb) {
    unsigned char type;
    if (rioRead(rdb,&type,1) == 0) return -1;
    return type;
}

time_t rdbLoadTime(rio *rdb) {
    int32_t t32;
    if (rioRead(rdb,&t32,4) == 0) return -1;
    return (time_t)t32;
}

int rdbSaveMillisecondTime(rio *rdb, long long t) {
    int64_t t64 = (int64_t) t;
    return rdbWriteRaw(rdb,&t64,8);
}

long long rdbLoadMillisecondTime(rio *rdb) {
    int64_t t64;
    if (rioRead(rdb,&t64,8) == 0) return -1;
    return (long long)t64;
}

/* Saves an encoded length. The first two bits in the first byte are used to
 * hold the encoding type. See the REDIS_RDB_* definitions for more information
 * on the types of encoding. */
int rdbSaveLen(rio *rdb, uint32_t len) {
    unsigned char buf[2];
    size_t nwritten;

    if (len < (1<<6)) {
        /* Save a 6 bit len */
        buf[0] = (len&0xFF)|(REDIS_RDB_6BITLEN<<6);
        if (rdbWriteRaw(rdb,buf,1) == -1) return -1;
        nwritten = 1;
    } else if (len < (1<<14)) {
        /* Save a 14 bit len */
        buf[0] = ((len>>8)&0xFF)|(REDIS_RDB_14BITLEN<<6);
        buf[1] = len&0xFF;
        if (rdbWriteRaw(rdb,buf,2) == -1) return -1;
        nwritten = 2;
    } else {
        /* Save a 32 bit len */
        buf[0] = (REDIS_RDB_32BITLEN<<6);
        if (rdbWriteRaw(rdb,buf,1) == -1) return -1;
        len = htonl(len);
        if (rdbWriteRaw(rdb,&len,4) == -1) return -1;
        nwritten = 1+4;
    }
    return nwritten;
}

/* Load an encoded length. The "isencoded" argument is set to 1 if the length
 * is not actually a length but an "encoding type". See the REDIS_RDB_ENC_*
 * definitions in rdb.h for more information. */
uint32_t rdbLoadLen(rio *rdb, int *isencoded) {
    unsigned char buf[2];
    uint32_t len;
    int type;

    if (isencoded) *isencoded = 0;
    if (rioRead(rdb,buf,1) == 0) return REDIS_RDB_LENERR;
    type = (buf[0]&0xC0)>>6;
    if (type == REDIS_RDB_ENCVAL) {
        /* Read a 6 bit encoding type. */
        if (isencoded) *isencoded = 1;
        return buf[0]&0x3F;
    } else if (type == REDIS_RDB_6BITLEN) {
        /* Read a 6 bit len. */
        return buf[0]&0x3F;
    } else if (type == REDIS_RDB_14BITLEN) {
        /* Read a 14 bit len. */
        if (rioRead(rdb,buf+1,1) == 0) return REDIS_RDB_LENERR;
        return ((buf[0]&0x3F)<<8)|buf[1];
    } else {
        /* Read a 32 bit len. */
        if (rioRead(rdb,&len,4) == 0) return REDIS_RDB_LENERR;
        return ntohl(len);
    }
}

/* Encodes the "value" argument as integer when it fits in the supported ranges
 * for encoded types. If the function successfully encodes the integer, the
 * representation is stored in the buffer pointer to by "enc" and the string
 * length is returned. Otherwise 0 is returned. */
int rdbEncodeInteger(long long value, unsigned char *enc) {
    if (value >= -(1<<7) && value <= (1<<7)-1) {
        enc[0] = (REDIS_RDB_ENCVAL<<6)|REDIS_RDB_ENC_INT8;
        enc[1] = value&0xFF;
        return 2;
    } else if (value >= -(1<<15) && value <= (1<<15)-1) {
        enc[0] = (REDIS_RDB_ENCVAL<<6)|REDIS_RDB_ENC_INT16;
        enc[1] = value&0xFF;
        enc[2] = (value>>8)&0xFF;
        return 3;
    } else if (value >= -((long long)1<<31) && value <= ((long long)1<<31)-1) {
        enc[0] = (REDIS_RDB_ENCVAL<<6)|REDIS_RDB_ENC_INT32;
        enc[1] = value&0xFF;
        enc[2] = (value>>8)&0xFF;
        enc[3] = (value>>16)&0xFF;
        enc[4] = (value>>24)&0xFF;
        return 5;
    } else {
        return 0;
    }
}

/* Loads an integer-encoded object with the specified encoding type "enctype".
 * If the "encode" argument is set the function may return an integer-encoded
 * string object, otherwise it always returns a raw string object. */
robj *rdbLoadIntegerObject(rio *rdb, int enctype, int encode) {
    unsigned char enc[4];
    long long val;

    if (enctype == REDIS_RDB_ENC_INT8) {
        if (rioRead(rdb,enc,1) == 0) return NULL;
        val = (signed char)enc[0];
    } else if (enctype == REDIS_RDB_ENC_INT16) {
        uint16_t v;
        if (rioRead(rdb,enc,2) == 0) return NULL;
        v = enc[0]|(enc[1]<<8);
        val = (int16_t)v;
    } else if (enctype == REDIS_RDB_ENC_INT32) {
        uint32_t v;
        if (rioRead(rdb,enc,4) == 0) return NULL;
        v = enc[0]|(enc[1]<<8)|(enc[2]<<16)|(enc[3]<<24);
        val = (int32_t)v;
    } else {
        val = 0; /* anti-warning */
        redisPanic("Unknown RDB integer encoding type");
    }
    if (encode)
        return createStringObjectFromLongLong(val);
    else
        return createObject(REDIS_STRING,sdsfromlonglong(val));
}

/* String objects in the form "2391" "-100" without any space and with a
 * range of values that can fit in an 8, 16 or 32 bit signed value can be
 * encoded as integers to save space */
int rdbTryIntegerEncoding(char *s, size_t len, unsigned char *enc) {
    long long value;
    char *endptr, buf[32];

    /* Check if it's possible to encode this value as a number */
    value = strtoll(s, &endptr, 10);
    if (endptr[0] != '\0') return 0;
    ll2string(buf,32,value);

    /* If the number converted back into a string is not identical
     * then it's not possible to encode the string as integer */
    if (strlen(buf) != len || memcmp(buf,s,len)) return 0;

    return rdbEncodeInteger(value,enc);
}

// 保存压缩字符串
int rdbSaveLzfStringObject(rio *rdb, unsigned char *s, size_t len) {
    size_t comprlen, outlen;
    unsigned char byte;
    int n, nwritten = 0;
    void *out;

    /* We require at least four bytes compression for this to be worth it */
    if (len <= 4) return 0;
    outlen = len-4;
    if ((out = zmalloc(outlen+1)) == NULL) return 0;
    comprlen = lzf_compress(s, len, out, outlen);
    if (comprlen == 0) {
        zfree(out);
        return 0;
    }
    /* Data compressed! Let's save it on disk */
    byte = (REDIS_RDB_ENCVAL<<6)|REDIS_RDB_ENC_LZF;
    if ((n = rdbWriteRaw(rdb,&byte,1)) == -1) goto writeerr;
    nwritten += n;

    if ((n = rdbSaveLen(rdb,comprlen)) == -1) goto writeerr;
    nwritten += n;

    if ((n = rdbSaveLen(rdb,len)) == -1) goto writeerr;
    nwritten += n;

    if ((n = rdbWriteRaw(rdb,out,comprlen)) == -1) goto writeerr;
    nwritten += n;

    zfree(out);
    return nwritten;

writeerr:
    zfree(out);
    return -1;
}

// 加载压缩字符串
robj *rdbLoadLzfStringObject(rio *rdb) {
    unsigned int len, clen;
    unsigned char *c = NULL;
    sds val = NULL;

    if ((clen = rdbLoadLen(rdb,NULL)) == REDIS_RDB_LENERR) return NULL;
    if ((len = rdbLoadLen(rdb,NULL)) == REDIS_RDB_LENERR) return NULL;
    if ((c = zmalloc(clen)) == NULL) goto err;
    if ((val = sdsnewlen(NULL,len)) == NULL) goto err;
    if (rioRead(rdb,c,clen) == 0) goto err;
    if (lzf_decompress(c,clen,val,len) == 0) goto err;
    zfree(c);
    return createObject(REDIS_STRING,val);
err:
    zfree(c);
    sdsfree(val);
    return NULL;
}

// 长度 | 字符串数据
/* Save a string object as [len][data] on disk. If the object is a string
 * representation of an integer value we try to save it in a special form */
int rdbSaveRawString(rio *rdb, unsigned char *s, size_t len) {
    int enclen;
    int n, nwritten = 0;

    // 尝试将他解析为一个整数，11 位刚好是整数的长度
    /* Try integer encoding */
    if (len <= 11) {
        unsigned char buf[5];
        if ((enclen = rdbTryIntegerEncoding((char*)s,len,buf)) > 0) {
            // 解析整数成功，直接写入整数，退出
            if (rdbWriteRaw(rdb,buf,enclen) == -1) return -1;
            return enclen;
        }
    }

    // 压缩字符串
    /* Try LZF compression - under 20 bytes it's unable to compress even
     * aaaaaaaaaaaaaaaaaa so skip it */
    if (server.rdb_compression && len > 20) {
        n = rdbSaveLzfStringObject(rdb,s,len);
        if (n == -1) return -1;
        if (n > 0) return n;
        /* Return value of 0 means data can't be compressed, save the old way */
    }

    // 如果尝试编码为数字和压缩字符串失败，将逐项保存，长度，字符串数据
    /* Store verbatim */
    if ((n = rdbSaveLen(rdb,len)) == -1) return -1;
    nwritten += n;
    if (len > 0) {
        if (rdbWriteRaw(rdb,s,len) == -1) return -1;
        nwritten += len;
    }
    return nwritten;
}

/* Save a long long value as either an encoded string or a string. */
int rdbSaveLongLongAsStringObject(rio *rdb, long long value) {
    unsigned char buf[32];
    int n, nwritten = 0;

    // 转换成字符串
    int enclen = rdbEncodeInteger(value,buf);
    if (enclen > 0) {
        return rdbWriteRaw(rdb,buf,enclen);
    } else {
        /* Encode as string */
        enclen = ll2string((char*)buf,32,value);
        redisAssert(enclen < 32);
        if ((n = rdbSaveLen(rdb,enclen)) == -1) return -1;
        nwritten += n;
        if ((n = rdbWriteRaw(rdb,buf,enclen)) == -1) return -1;
        nwritten += n;
    }
    return nwritten;
}

/* Like rdbSaveStringObjectRaw() but handle encoded objects */
int rdbSaveStringObject(rio *rdb, robj *obj) {
    /* Avoid to decode the object, then encode it again, if the
     * object is already integer encoded. */
    if (obj->encoding == REDIS_ENCODING_INT) {
        return rdbSaveLongLongAsStringObject(rdb,(long)obj->ptr);
    } else {
        redisAssertWithInfo(NULL,obj,obj->encoding == REDIS_ENCODING_RAW);
        return rdbSaveRawString(rdb,obj->ptr,sdslen(obj->ptr));
    }
}

robj *rdbGenericLoadStringObject(rio *rdb, int encode) {
    int isencoded;
    uint32_t len;
    sds val;

    len = rdbLoadLen(rdb,&isencoded);

    // isencoded ==1 表示 6 bit length
    if (isencoded) {
        switch(len) {
        case REDIS_RDB_ENC_INT8:
        case REDIS_RDB_ENC_INT16:
        case REDIS_RDB_ENC_INT32:
            return rdbLoadIntegerObject(rdb,len,encode);

        // 压缩
        case REDIS_RDB_ENC_LZF:
            return rdbLoadLzfStringObject(rdb);
        default:
            redisPanic("Unknown RDB encoding type");
        }
    }

    if (len == REDIS_RDB_LENERR) return NULL;

    // 读取字符串
    val = sdsnewlen(NULL,len);
    if (len && rioRead(rdb,val,len) == 0) {
        sdsfree(val);
        return NULL;
    }

    // 返回没有解码的数据
    return createObject(REDIS_STRING,val);
}

robj *rdbLoadStringObject(rio *rdb) {
    return rdbGenericLoadStringObject(rdb,0);
}

robj *rdbLoadEncodedStringObject(rio *rdb) {
    return rdbGenericLoadStringObject(rdb,1);
}

/* Save a double value. Doubles are saved as strings prefixed by an unsigned
 * 8 bit integer specifying the length of the representation.
 * This 8 bit integer has special values in order to specify the following
 * conditions:
 * 253: not a number
 * 254: + inf
 * 255: - inf
 */
int rdbSaveDoubleValue(rio *rdb, double val) {
    unsigned char buf[128];
    int len;

    if (isnan(val)) {
        buf[0] = 253;
        len = 1;
    } else if (!isfinite(val)) {
        len = 1;
        buf[0] = (val < 0) ? 255 : 254;
    } else {
#if (DBL_MANT_DIG >= 52) && (LLONG_MAX == 0x7fffffffffffffffLL)
        /* Check if the float is in a safe range to be casted into a
         * long long. We are assuming that long long is 64 bit here.
         * Also we are assuming that there are no implementations around where
         * double has precision < 52 bit.
         *
         * Under this assumptions we test if a double is inside an interval
         * where casting to long long is safe. Then using two castings we
         * make sure the decimal part is zero. If all this is true we use
         * integer printing function that is much faster. */
        double min = -4503599627370495; /* (2^52)-1 */
        double max = 4503599627370496; /* -(2^52) */
        if (val > min && val < max && val == ((double)((long long)val)))
            ll2string((char*)buf+1,sizeof(buf),(long long)val);
        else
#endif
            snprintf((char*)buf+1,sizeof(buf)-1,"%.17g",val);
        buf[0] = strlen((char*)buf+1);
        len = buf[0]+1;
    }
    return rdbWriteRaw(rdb,buf,len);
}

/* For information about double serialization check rdbSaveDoubleValue() */
int rdbLoadDoubleValue(rio *rdb, double *val) {
    char buf[128];
    unsigned char len;

    if (rioRead(rdb,&len,1) == 0) return -1;
    switch(len) {
    case 255: *val = R_NegInf; return 0;
    case 254: *val = R_PosInf; return 0;
    case 253: *val = R_Nan; return 0;
    default:
        if (rioRead(rdb,buf,len) == 0) return -1;
        buf[len] = '\0';
        sscanf(buf, "%lg", val);
        return 0;
    }
}

/* Save the object type of object "o". */
int rdbSaveObjectType(rio *rdb, robj *o) {
    switch (o->type) {
    case REDIS_STRING:
        return rdbSaveType(rdb,REDIS_RDB_TYPE_STRING);
    case REDIS_LIST:
        if (o->encoding == REDIS_ENCODING_ZIPLIST)
            return rdbSaveType(rdb,REDIS_RDB_TYPE_LIST_ZIPLIST);
        else if (o->encoding == REDIS_ENCODING_LINKEDLIST)
            return rdbSaveType(rdb,REDIS_RDB_TYPE_LIST);
        else
            redisPanic("Unknown list encoding");
    case REDIS_SET:
        if (o->encoding == REDIS_ENCODING_INTSET)
            return rdbSaveType(rdb,REDIS_RDB_TYPE_SET_INTSET);
        else if (o->encoding == REDIS_ENCODING_HT)
            return rdbSaveType(rdb,REDIS_RDB_TYPE_SET);
        else
            redisPanic("Unknown set encoding");
    case REDIS_ZSET:
        if (o->encoding == REDIS_ENCODING_ZIPLIST)
            return rdbSaveType(rdb,REDIS_RDB_TYPE_ZSET_ZIPLIST);
        else if (o->encoding == REDIS_ENCODING_SKIPLIST)
            return rdbSaveType(rdb,REDIS_RDB_TYPE_ZSET);
        else
            redisPanic("Unknown sorted set encoding");
    case REDIS_HASH:
        if (o->encoding == REDIS_ENCODING_ZIPLIST)
            return rdbSaveType(rdb,REDIS_RDB_TYPE_HASH_ZIPLIST);
        else if (o->encoding == REDIS_ENCODING_HT)
            return rdbSaveType(rdb,REDIS_RDB_TYPE_HASH);
        else
            redisPanic("Unknown hash encoding");
    default:
        redisPanic("Unknown object type");
    }
    return -1; /* avoid warning */
}

/* Use rdbLoadType() to load a TYPE in RDB format, but returns -1 if the
 * type is not specifically a valid Object Type. */
int rdbLoadObjectType(rio *rdb) {
    int type;
    if ((type = rdbLoadType(rdb)) == -1) return -1;
    if (!rdbIsObjectType(type)) return -1;
    return type;
}

/* Save a Redis object. Returns -1 on error, 0 on success. */
int rdbSaveObject(rio *rdb, robj *o) {
    int n, nwritten = 0;

    if (o->type == REDIS_STRING) {
        /* Save a string value */
        if ((n = rdbSaveStringObject(rdb,o)) == -1) return -1;
        nwritten += n;
    } else if (o->type == REDIS_LIST) {
        /* Save a list value */
        if (o->encoding == REDIS_ENCODING_ZIPLIST) {
            size_t l = ziplistBlobLen((unsigned char*)o->ptr);

            if ((n = rdbSaveRawString(rdb,o->ptr,l)) == -1) return -1;
            nwritten += n;
        } else if (o->encoding == REDIS_ENCODING_LINKEDLIST) {
            list *list = o->ptr;
            listIter li;
            listNode *ln;

            if ((n = rdbSaveLen(rdb,listLength(list))) == -1) return -1;
            nwritten += n;

            listRewind(list,&li);
            while((ln = listNext(&li))) {
                robj *eleobj = listNodeValue(ln);
                if ((n = rdbSaveStringObject(rdb,eleobj)) == -1) return -1;
                nwritten += n;
            }
        } else {
            redisPanic("Unknown list encoding");
        }
    } else if (o->type == REDIS_SET) {
        /* Save a set value */
        if (o->encoding == REDIS_ENCODING_HT) {
            dict *set = o->ptr;
            dictIterator *di = dictGetIterator(set);
            dictEntry *de;

            if ((n = rdbSaveLen(rdb,dictSize(set))) == -1) return -1;
            nwritten += n;

            while((de = dictNext(di)) != NULL) {
                robj *eleobj = dictGetKey(de);
                if ((n = rdbSaveStringObject(rdb,eleobj)) == -1) return -1;
                nwritten += n;
            }
            dictReleaseIterator(di);
        } else if (o->encoding == REDIS_ENCODING_INTSET) {
            size_t l = intsetBlobLen((intset*)o->ptr);

            if ((n = rdbSaveRawString(rdb,o->ptr,l)) == -1) return -1;
            nwritten += n;
        } else {
            redisPanic("Unknown set encoding");
        }
    } else if (o->type == REDIS_ZSET) {
        /* Save a sorted set value */
        if (o->encoding == REDIS_ENCODING_ZIPLIST) {
            size_t l = ziplistBlobLen((unsigned char*)o->ptr);

            if ((n = rdbSaveRawString(rdb,o->ptr,l)) == -1) return -1;
            nwritten += n;
        } else if (o->encoding == REDIS_ENCODING_SKIPLIST) {
            zset *zs = o->ptr;
            dictIterator *di = dictGetIterator(zs->dict);
            dictEntry *de;

            if ((n = rdbSaveLen(rdb,dictSize(zs->dict))) == -1) return -1;
            nwritten += n;

            while((de = dictNext(di)) != NULL) {
                robj *eleobj = dictGetKey(de);
                double *score = dictGetVal(de);

                if ((n = rdbSaveStringObject(rdb,eleobj)) == -1) return -1;
                nwritten += n;
                if ((n = rdbSaveDoubleValue(rdb,*score)) == -1) return -1;
                nwritten += n;
            }
            dictReleaseIterator(di);
        } else {
            redisPanic("Unknown sorted set encoding");
        }
    } else if (o->type == REDIS_HASH) {
        /* Save a hash value */
        if (o->encoding == REDIS_ENCODING_ZIPLIST) {
            size_t l = ziplistBlobLen((unsigned char*)o->ptr);

            if ((n = rdbSaveRawString(rdb,o->ptr,l)) == -1) return -1;
            nwritten += n;

        } else if (o->encoding == REDIS_ENCODING_HT) {
            dictIterator *di = dictGetIterator(o->ptr);
            dictEntry *de;

            if ((n = rdbSaveLen(rdb,dictSize((dict*)o->ptr))) == -1) return -1;
            nwritten += n;

            while((de = dictNext(di)) != NULL) {
                robj *key = dictGetKey(de);
                robj *val = dictGetVal(de);

                if ((n = rdbSaveStringObject(rdb,key)) == -1) return -1;
                nwritten += n;
                if ((n = rdbSaveStringObject(rdb,val)) == -1) return -1;
                nwritten += n;
            }
            dictReleaseIterator(di);

        } else {
            redisPanic("Unknown hash encoding");
        }

    } else {
        redisPanic("Unknown object type");
    }
    return nwritten;
}

/* Return the length the object will have on disk if saved with
 * the rdbSaveObject() function. Currently we use a trick to get
 * this length with very little changes to the code. In the future
 * we could switch to a faster solution. */
off_t rdbSavedObjectLen(robj *o) {
    int len = rdbSaveObject(NULL,o);
    redisAssertWithInfo(NULL,o,len != -1);
    return len;
}

/* Save a key-value pair, with expire time, type, key, value.
 * On error -1 is returned.
 * On success if the key was actually saved 1 is returned, otherwise 0
 * is returned (the key was already expired). */
int rdbSaveKeyValuePair(rio *rdb, robj *key, robj *val,
                        long long expiretime, long long now)
{
    // 过期时间
    /* Save the expire time */
    if (expiretime != -1) {
        /* If this key is already expired skip it */
        if (expiretime < now) return 0;
        if (rdbSaveType(rdb,REDIS_RDB_OPCODE_EXPIRETIME_MS) == -1) return -1;
        if (rdbSaveMillisecondTime(rdb,expiretime) == -1) return -1;
    }

    /* Save type, key, value */
    // 数据类型
    if (rdbSaveObjectType(rdb,val) == -1) return -1;

    // 键
    if (rdbSaveStringObject(rdb,key) == -1) return -1;

    // 值
    if (rdbSaveObject(rdb,val) == -1) return -1;
    return 1;
}

// 备份主程序
/* Save the DB on disk. Return REDIS_ERR on error, REDIS_OK on success */
int rdbSave(char *filename) {
    dictIterator *di = NULL;
    dictEntry *de;
    char tmpfile[256];
    char magic[10];
    int j;
    long long now = mstime();
    FILE *fp;
    rio rdb;
    uint64_t cksum;

    // 打开文件，准备写
    snprintf(tmpfile,256,"temp-%d.rdb", (int) getpid());
    fp = fopen(tmpfile,"w");
    if (!fp) {
        redisLog(REDIS_WARNING, "Failed opening .rdb for saving: %s",
            strerror(errno));
        return REDIS_ERR;
    }

    // 初始化 rdb 结构体。rdb 结构体内指定了读写文件的函数，已写/读字符统计等数据
    rioInitWithFile(&rdb,fp);

    if (server.rdb_checksum) // 校验和
        rdb.update_cksum = rioGenericUpdateChecksum;

    // 先写入版本号
    snprintf(magic,sizeof(magic),"REDIS%04d",REDIS_RDB_VERSION);
    if (rdbWriteRaw(&rdb,magic,9) == -1) goto werr;

    for (j = 0; j < server.dbnum; j++) {
        // server 中保存的数据
        redisDb *db = server.db+j;

        // 字典
        dict *d = db->dict;
        if (dictSize(d) == 0) continue;

        // 字典迭代器
        di = dictGetSafeIterator(d);
        if (!di) {
            fclose(fp);
            return REDIS_ERR;
        }

        // 写入 RDB 操作码
        /* Write the SELECT DB opcode */
        if (rdbSaveType(&rdb,REDIS_RDB_OPCODE_SELECTDB) == -1) goto werr;

        // 写入数据库序号
        if (rdbSaveLen(&rdb,j) == -1) goto werr;

        // 写入数据集中每一个数据项
        /* Iterate this DB writing every entry */
        while((de = dictNext(di)) != NULL) {
            sds keystr = dictGetKey(de);
            robj key,
                *o = dictGetVal(de);
            long long expire;

            // 将 keystr 封装在 robj 里
            initStaticStringObject(key,keystr);

            // 获取过期时间
            expire = getExpire(db,&key);

            // 开始写入磁盘
            if (rdbSaveKeyValuePair(&rdb,&key,o,expire,now) == -1) goto werr;
        }
        dictReleaseIterator(di);
    }
    di = NULL; /* So that we don't release it again on error. */

    // RDB 结束码
    /* EOF opcode */
    if (rdbSaveType(&rdb,REDIS_RDB_OPCODE_EOF) == -1) goto werr;

    // 校验和
    /* CRC64 checksum. It will be zero if checksum computation is disabled, the
     * loading code skips the check in this case. */
    cksum = rdb.cksum;
    memrev64ifbe(&cksum);
    rioWrite(&rdb,&cksum,8);

    // 同步到磁盘
    /* Make sure data will not remain on the OS's output buffers */
    fflush(fp);
    fsync(fileno(fp));
    fclose(fp);

    // 修改临时文件名为指定文件名
    /* Use RENAME to make sure the DB file is changed atomically only
     * if the generate DB file is ok. */
    if (rename(tmpfile,filename) == -1) {
        redisLog(REDIS_WARNING,"Error moving temp DB file on the final destination: %s", strerror(errno));
        unlink(tmpfile);
        return REDIS_ERR;
    }
    redisLog(REDIS_NOTICE,"DB saved on disk");
    server.dirty = 0;

    // 记录成功执行保存的时间
    server.lastsave = time(NULL);

    // 记录执行的结果状态为成功
    server.lastbgsave_status = REDIS_OK;
    return REDIS_OK;

werr:
    // 清理工作，关闭文件描述符等
    fclose(fp);
    unlink(tmpfile);
    redisLog(REDIS_WARNING,"Write error saving DB on disk: %s", strerror(errno));
    if (di) dictReleaseIterator(di);
    return REDIS_ERR;
}

// bgsaveCommand(),serverCron(),syncCommand(),updateSlavesWaitingBgsave() 会调用 rdbSaveBackground()
int rdbSaveBackground(char *filename) {
    pid_t childpid;
    long long start;

    // 已经有后台程序了，拒绝再次执行
    if (server.rdb_child_pid != -1) return REDIS_ERR;

    server.dirty_before_bgsave = server.dirty;

    // 记录这次尝试执行持久化操作的时间
    server.lastbgsave_try = time(NULL);

    start = ustime();
    if ((childpid = fork()) == 0) {
        int retval;

        // 取消监听
        /* Child */
        closeListeningSockets(0);
        redisSetProcTitle("redis-rdb-bgsave");

        // 执行备份主程序
        retval = rdbSave(filename);

        // 脏数据，其实就是子进程所消耗的内存大小
        if (retval == REDIS_OK) {
            // 获取脏数据大小
            size_t private_dirty = zmalloc_get_private_dirty();

            // 记录脏数据
            if (private_dirty) {
                redisLog(REDIS_NOTICE,
                    "RDB: %zu MB of memory used by copy-on-write",
                    private_dirty/(1024*1024));
            }
        }

        // 退出子进程
        exitFromChild((retval == REDIS_OK) ? 0 : 1);
    } else {
        /* Parent */
        // 计算 fork 消耗的时间
        server.stat_fork_time = ustime()-start;

        // fork 出错
        if (childpid == -1) {
            // 记录执行的结果状态为失败
            server.lastbgsave_status = REDIS_ERR;
            redisLog(REDIS_WARNING,"Can't save in background: fork: %s",
                strerror(errno));
            return REDIS_ERR;
        }
        redisLog(REDIS_NOTICE,"Background saving started by pid %d",childpid);

        // 记录保存的起始时间
        server.rdb_save_time_start = time(NULL);

        // 子进程 ID
        server.rdb_child_pid = childpid;
        updateDictResizePolicy();
        return REDIS_OK;
    }
    return REDIS_OK; /* unreached */
}

void rdbRemoveTempFile(pid_t childpid) {
    char tmpfile[256];

    snprintf(tmpfile,256,"temp-%d.rdb", (int) childpid);
    unlink(tmpfile);
}

/* Load a Redis object of the specified type from the specified file.
 * On success a newly allocated object is returned, otherwise NULL. */
robj *rdbLoadObject(int rdbtype, rio *rdb) {
    robj *o, *ele, *dec;
    size_t len;
    unsigned int i;

    if (rdbtype == REDIS_RDB_TYPE_STRING) {
        /* Read string value */
        if ((o = rdbLoadEncodedStringObject(rdb)) == NULL) return NULL;

        // 尝试解码
        o = tryObjectEncoding(o);
    } else if (rdbtype == REDIS_RDB_TYPE_LIST) {
        /* Read list value */
        if ((len = rdbLoadLen(rdb,NULL)) == REDIS_RDB_LENERR) return NULL;

        /* Use a real list when there are too many entries */
        if (len > server.list_max_ziplist_entries) {
            o = createListObject();
        } else {
            o = createZiplistObject();
        }

        /* Load every single element of the list */
        while(len--) {
            // load element
            if ((ele = rdbLoadEncodedStringObject(rdb)) == NULL) return NULL;

            /* If we are using a ziplist and the value is too big, convert
             * the object to a real list. */
            if (o->encoding == REDIS_ENCODING_ZIPLIST &&
                ele->encoding == REDIS_ENCODING_RAW &&
                sdslen(ele->ptr) > server.list_max_ziplist_value)
                    listTypeConvert(o,REDIS_ENCODING_LINKEDLIST);

            if (o->encoding == REDIS_ENCODING_ZIPLIST) {
                dec = getDecodedObject(ele);
                o->ptr = ziplistPush(o->ptr,dec->ptr,sdslen(dec->ptr),REDIS_TAIL);
                decrRefCount(dec);
                decrRefCount(ele);
            } else {
                ele = tryObjectEncoding(ele);
                listAddNodeTail(o->ptr,ele);
            }
        }
    } else if (rdbtype == REDIS_RDB_TYPE_SET) {
        /* Read list/set value */
        if ((len = rdbLoadLen(rdb,NULL)) == REDIS_RDB_LENERR) return NULL;

        /* Use a regular set when there are too many entries. */
        if (len > server.set_max_intset_entries) {
            o = createSetObject();
            /* It's faster to expand the dict to the right size asap in order
             * to avoid rehashing */
            if (len > DICT_HT_INITIAL_SIZE)
                dictExpand(o->ptr,len);
        } else {
            o = createIntsetObject();
        }

        /* Load every single element of the list/set */
        for (i = 0; i < len; i++) {
            long long llval;
            if ((ele = rdbLoadEncodedStringObject(rdb)) == NULL) return NULL;
            ele = tryObjectEncoding(ele);

            if (o->encoding == REDIS_ENCODING_INTSET) {
                /* Fetch integer value from element */
                if (isObjectRepresentableAsLongLong(ele,&llval) == REDIS_OK) {
                    o->ptr = intsetAdd(o->ptr,llval,NULL);
                } else {
                    setTypeConvert(o,REDIS_ENCODING_HT);
                    dictExpand(o->ptr,len);
                }
            }

            /* This will also be called when the set was just converted
             * to regular hash table encoded set */
            if (o->encoding == REDIS_ENCODING_HT) {
                dictAdd((dict*)o->ptr,ele,NULL);
            } else {
                decrRefCount(ele);
            }
        }
    } else if (rdbtype == REDIS_RDB_TYPE_ZSET) {
        /* Read list/set value */
        size_t zsetlen;
        size_t maxelelen = 0;
        zset *zs;

        if ((zsetlen = rdbLoadLen(rdb,NULL)) == REDIS_RDB_LENERR) return NULL;
        o = createZsetObject();
        zs = o->ptr;

        /* Load every single element of the list/set */
        while(zsetlen--) {
            robj *ele;
            double score;
            zskiplistNode *znode;

            if ((ele = rdbLoadEncodedStringObject(rdb)) == NULL) return NULL;
            ele = tryObjectEncoding(ele);
            if (rdbLoadDoubleValue(rdb,&score) == -1) return NULL;

            /* Don't care about integer-encoded strings. */
            if (ele->encoding == REDIS_ENCODING_RAW &&
                sdslen(ele->ptr) > maxelelen)
                    maxelelen = sdslen(ele->ptr);

            znode = zslInsert(zs->zsl,score,ele);
            dictAdd(zs->dict,ele,&znode->score);
            incrRefCount(ele); /* added to skiplist */
        }

        /* Convert *after* loading, since sorted sets are not stored ordered. */
        if (zsetLength(o) <= server.zset_max_ziplist_entries &&
            maxelelen <= server.zset_max_ziplist_value)
                zsetConvert(o,REDIS_ENCODING_ZIPLIST);
    } else if (rdbtype == REDIS_RDB_TYPE_HASH) {
        size_t len;
        int ret;

        len = rdbLoadLen(rdb, NULL);
        if (len == REDIS_RDB_LENERR) return NULL;

        o = createHashObject();

        /* Too many entries? Use a hash table. */
        if (len > server.hash_max_ziplist_entries)
            hashTypeConvert(o, REDIS_ENCODING_HT);

        /* Load every field and value into the ziplist */
        while (o->encoding == REDIS_ENCODING_ZIPLIST && len > 0) {
            robj *field, *value;

            len--;
            /* Load raw strings */
            field = rdbLoadStringObject(rdb);
            if (field == NULL) return NULL;
            redisAssert(field->encoding == REDIS_ENCODING_RAW);
            value = rdbLoadStringObject(rdb);
            if (value == NULL) return NULL;
            redisAssert(field->encoding == REDIS_ENCODING_RAW);

            /* Add pair to ziplist */
            o->ptr = ziplistPush(o->ptr, field->ptr, sdslen(field->ptr), ZIPLIST_TAIL);
            o->ptr = ziplistPush(o->ptr, value->ptr, sdslen(value->ptr), ZIPLIST_TAIL);
            /* Convert to hash table if size threshold is exceeded */
            if (sdslen(field->ptr) > server.hash_max_ziplist_value ||
                sdslen(value->ptr) > server.hash_max_ziplist_value)
            {
                decrRefCount(field);
                decrRefCount(value);
                hashTypeConvert(o, REDIS_ENCODING_HT);
                break;
            }
            decrRefCount(field);
            decrRefCount(value);
        }

        /* Load remaining fields and values into the hash table */
        while (o->encoding == REDIS_ENCODING_HT && len > 0) {
            robj *field, *value;

            len--;
            /* Load encoded strings */
            field = rdbLoadEncodedStringObject(rdb);
            if (field == NULL) return NULL;
            value = rdbLoadEncodedStringObject(rdb);
            if (value == NULL) return NULL;

            field = tryObjectEncoding(field);
            value = tryObjectEncoding(value);

            /* Add pair to hash table */
            ret = dictAdd((dict*)o->ptr, field, value);
            redisAssert(ret == REDIS_OK);
        }

        /* All pairs should be read by now */
        redisAssert(len == 0);

    } else if (rdbtype == REDIS_RDB_TYPE_HASH_ZIPMAP  ||
               rdbtype == REDIS_RDB_TYPE_LIST_ZIPLIST ||
               rdbtype == REDIS_RDB_TYPE_SET_INTSET   ||
               rdbtype == REDIS_RDB_TYPE_ZSET_ZIPLIST ||
               rdbtype == REDIS_RDB_TYPE_HASH_ZIPLIST)
    {
        robj *aux = rdbLoadStringObject(rdb);

        if (aux == NULL) return NULL;
        o = createObject(REDIS_STRING,NULL); /* string is just placeholder */
        o->ptr = zmalloc(sdslen(aux->ptr));
        memcpy(o->ptr,aux->ptr,sdslen(aux->ptr));
        decrRefCount(aux);

        /* Fix the object encoding, and make sure to convert the encoded
         * data type into the base type if accordingly to the current
         * configuration there are too many elements in the encoded data
         * type. Note that we only check the length and not max element
         * size as this is an O(N) scan. Eventually everything will get
         * converted. */
        switch(rdbtype) {
            case REDIS_RDB_TYPE_HASH_ZIPMAP:
                /* Convert to ziplist encoded hash. This must be deprecated
                 * when loading dumps created by Redis 2.4 gets deprecated. */
                {
                    unsigned char *zl = ziplistNew();
                    unsigned char *zi = zipmapRewind(o->ptr);
                    unsigned char *fstr, *vstr;
                    unsigned int flen, vlen;
                    unsigned int maxlen = 0;

                    while ((zi = zipmapNext(zi, &fstr, &flen, &vstr, &vlen)) != NULL) {
                        if (flen > maxlen) maxlen = flen;
                        if (vlen > maxlen) maxlen = vlen;
                        zl = ziplistPush(zl, fstr, flen, ZIPLIST_TAIL);
                        zl = ziplistPush(zl, vstr, vlen, ZIPLIST_TAIL);
                    }

                    zfree(o->ptr);
                    o->ptr = zl;
                    o->type = REDIS_HASH;
                    o->encoding = REDIS_ENCODING_ZIPLIST;

                    if (hashTypeLength(o) > server.hash_max_ziplist_entries ||
                        maxlen > server.hash_max_ziplist_value)
                    {
                        hashTypeConvert(o, REDIS_ENCODING_HT);
                    }
                }
                break;
            case REDIS_RDB_TYPE_LIST_ZIPLIST:
                o->type = REDIS_LIST;
                o->encoding = REDIS_ENCODING_ZIPLIST;
                if (ziplistLen(o->ptr) > server.list_max_ziplist_entries)
                    listTypeConvert(o,REDIS_ENCODING_LINKEDLIST);
                break;
            case REDIS_RDB_TYPE_SET_INTSET:
                o->type = REDIS_SET;
                o->encoding = REDIS_ENCODING_INTSET;
                if (intsetLen(o->ptr) > server.set_max_intset_entries)
                    setTypeConvert(o,REDIS_ENCODING_HT);
                break;
            case REDIS_RDB_TYPE_ZSET_ZIPLIST:
                o->type = REDIS_ZSET;
                o->encoding = REDIS_ENCODING_ZIPLIST;
                if (zsetLength(o) > server.zset_max_ziplist_entries)
                    zsetConvert(o,REDIS_ENCODING_SKIPLIST);
                break;
            case REDIS_RDB_TYPE_HASH_ZIPLIST:
                o->type = REDIS_HASH;
                o->encoding = REDIS_ENCODING_ZIPLIST;
                if (hashTypeLength(o) > server.hash_max_ziplist_entries)
                    hashTypeConvert(o, REDIS_ENCODING_HT);
                break;
            default:
                redisPanic("Unknown encoding");
                break;
        }
    } else {
        redisPanic("Unknown object type");
    }
    return o;
}

/* Mark that we are loading in the global state and setup the fields
 * needed to provide loading stats. */
void startLoading(FILE *fp) {
    struct stat sb;

    /* Load the DB */
    server.loading = 1;
    server.loading_start_time = time(NULL);
    if (fstat(fileno(fp), &sb) == -1) {
        server.loading_total_bytes = 1; /* just to avoid division by zero */
    } else {
        server.loading_total_bytes = sb.st_size;
    }
}

/* Refresh the loading progress info */
void loadingProgress(off_t pos) {
    server.loading_loaded_bytes = pos;
    if (server.stat_peak_memory < zmalloc_used_memory())
        server.stat_peak_memory = zmalloc_used_memory();
}

/* Loading finished */
void stopLoading(void) {
    server.loading = 0;
}

/* Track loading progress in order to serve client's from time to time
   and if needed calculate rdb checksum  */
void rdbLoadProgressCallback(rio *r, const void *buf, size_t len) {
    if (server.rdb_checksum)
        rioGenericUpdateChecksum(r, buf, len);
    if (server.loading_process_events_interval_bytes &&
        (r->processed_bytes + len)/server.loading_process_events_interval_bytes > r->processed_bytes/server.loading_process_events_interval_bytes)
    {
        if (server.masterhost && server.repl_state == REDIS_REPL_TRANSFER)
            replicationSendNewlineToMaster();
        loadingProgress(r->processed_bytes);
        aeProcessEvents(server.el, AE_FILE_EVENTS|AE_DONT_WAIT);
    }
}

int rdbLoad(char *filename) {
    uint32_t dbid;
    int type, rdbver;
    redisDb *db = server.db+0;
    char buf[1024];
    long long expiretime, now = mstime();
    FILE *fp;
    rio rdb;

    if ((fp = fopen(filename,"r")) == NULL) return REDIS_ERR;

    rioInitWithFile(&rdb,fp);
    rdb.update_cksum = rdbLoadProgressCallback;
    rdb.max_processing_chunk = server.loading_process_events_interval_bytes;

    // 读取 redis 版本号
    if (rioRead(&rdb,buf,9) == 0) goto eoferr;
    buf[9] = '\0';
    if (memcmp(buf,"REDIS",5) != 0) {
        fclose(fp);
        redisLog(REDIS_WARNING,"Wrong signature trying to load DB from file");
        errno = EINVAL;
        return REDIS_ERR;
    }

    // redis 的版本号
    rdbver = atoi(buf+5);
    if (rdbver < 1 || rdbver > REDIS_RDB_VERSION) {
        fclose(fp);
        redisLog(REDIS_WARNING,"Can't handle RDB format version %d",rdbver);
        errno = EINVAL;
        return REDIS_ERR;
    }

    startLoading(fp);
    while(1) {
        robj *key, *val;
        expiretime = -1;

        // 数据类型
        /* Read type. */
        if ((type = rdbLoadType(&rdb)) == -1) goto eoferr;

        if (type == REDIS_RDB_OPCODE_EXPIRETIME) {
            if ((expiretime = rdbLoadTime(&rdb)) == -1) goto eoferr;
            /* We read the time so we need to read the object type again. */
            if ((type = rdbLoadType(&rdb)) == -1) goto eoferr;
            /* the EXPIRETIME opcode specifies time in seconds, so convert
             * into milliseconds. */
            expiretime *= 1000;
        } else if (type == REDIS_RDB_OPCODE_EXPIRETIME_MS) {
            /* Milliseconds precision expire times introduced with RDB
             * version 3. */
            if ((expiretime = rdbLoadMillisecondTime(&rdb)) == -1) goto eoferr;
            /* We read the time so we need to read the object type again. */
            if ((type = rdbLoadType(&rdb)) == -1) goto eoferr;
        }

        // 读到了数据库的结尾
        if (type == REDIS_RDB_OPCODE_EOF)
            break;

        /* Handle SELECT DB opcode as a special case */
        if (type == REDIS_RDB_OPCODE_SELECTDB) {
            if ((dbid = rdbLoadLen(&rdb,NULL)) == REDIS_RDB_LENERR)
                goto eoferr;
            if (dbid >= (unsigned)server.dbnum) {
                redisLog(REDIS_WARNING,"FATAL: Data file was created with a Redis server configured to handle more than %d databases. Exiting\n", server.dbnum);
                exit(1);
            }
            db = server.db+dbid;
            continue;
        }

        // 键
        /* Read key */
        if ((key = rdbLoadStringObject(&rdb)) == NULL) goto eoferr;

        // 值
        /* Read value */
        if ((val = rdbLoadObject(type,&rdb)) == NULL) goto eoferr;

        // 如果已经过期，直接放弃数据
        /* Check if the key already expired. This function is used when loading
         * an RDB file from disk, either at startup, or when an RDB was
         * received from the master. In the latter case, the master is
         * responsible for key expiry. If we would expire keys here, the
         * snapshot taken by the master may not be reflected on the slave. */
        if (server.masterhost == NULL && expiretime != -1 && expiretime < now) {
            decrRefCount(key);
            decrRefCount(val);
            continue;
        }

        // 添加到内存数据库中
        /* Add the new object in the hash table */
        dbAdd(db,key,val);

        // 设置过期时间
        /* Set the expire time if needed */
        if (expiretime != -1) setExpire(db,key,expiretime);

        decrRefCount(key);
    }

    /* Verify the checksum if RDB version is >= 5 */
    if (rdbver >= 5 && server.rdb_checksum) {
        uint64_t cksum, expected = rdb.cksum;

        // 校验和
        if (rioRead(&rdb,&cksum,8) == 0) goto eoferr;
        memrev64ifbe(&cksum);
        if (cksum == 0) {
            redisLog(REDIS_WARNING,"RDB file was saved with checksum disabled: no check performed.");
        } else if (cksum != expected) {
            redisLog(REDIS_WARNING,"Wrong RDB checksum. Aborting now.");
            exit(1);
        }
    }

    fclose(fp);
    stopLoading();
    return REDIS_OK;

eoferr: /* unexpected end of file is handled here with a fatal exit */
    redisLog(REDIS_WARNING,"Short read or OOM loading DB. Unrecoverable error, aborting now.");
    exit(1);
    return REDIS_ERR; /* Just to avoid warning */
}

/* A background saving child (BGSAVE) terminated its work. Handle this. */
void backgroundSaveDoneHandler(int exitcode, int bysignal) {
    // 正常情况
    if (!bysignal && exitcode == 0) {
        redisLog(REDIS_NOTICE,
            "Background saving terminated with success");
        server.dirty = server.dirty - server.dirty_before_bgsave;
        server.lastsave = time(NULL);
        server.lastbgsave_status = REDIS_OK;
    // RDB 的某些错误
    } else if (!bysignal && exitcode != 0) {
        redisLog(REDIS_WARNING, "Background saving error");
        server.lastbgsave_status = REDIS_ERR;

    // 被信号叫停
    } else {
        redisLog(REDIS_WARNING,
            "Background saving terminated by signal %d", bysignal);
        rdbRemoveTempFile(server.rdb_child_pid);
        /* SIGUSR1 is whitelisted, so we have a way to kill a child without
         * tirggering an error conditon. */
        if (bysignal != SIGUSR1)
            server.lastbgsave_status = REDIS_ERR;
    }
    server.rdb_child_pid = -1;

    // 计算备份所用的时间
    server.rdb_save_time_last = time(NULL)-server.rdb_save_time_start;
    server.rdb_save_time_start = -1;

    // 可能从机正在等待 BGSAVE 进程的终止
    /* Possibly there are slaves waiting for a BGSAVE in order to be served
     * (the first stage of SYNC is a bulk transfer of dump.rdb) */
    updateSlavesWaitingBgsave(exitcode == 0 ? REDIS_OK : REDIS_ERR);
}

// 立即执行备份命令，非后台备份
void saveCommand(redisClient *c) {
    if (server.rdb_child_pid != -1) {
        addReplyError(c,"Background save already in progress");
        return;
    }
    if (rdbSave(server.rdb_filename) == REDIS_OK) {
        addReply(c,shared.ok);
    } else {
        addReply(c,shared.err);
    }
}

// 后台备份命令
void bgsaveCommand(redisClient *c) {
    if (server.rdb_child_pid != -1) {
        addReplyError(c,"Background save already in progress");
    } else if (server.aof_child_pid != -1) {
        addReplyError(c,"Can't BGSAVE while AOF log rewriting is in progress");
    } else if (rdbSaveBackground(server.rdb_filename) == REDIS_OK) {
        addReplyStatus(c,"Background saving started");
    } else {
        addReply(c,shared.err);
    }
}
