产生原因之一

该连接超过空闲等待时间(wait_timeout，默认8小时)，服务端主动关闭该连接，此时客户端再执行query，则mysql client返回该错误。

研究

设置mysql wait_timeout为5s，方便复现
```
set global wait_timeout=5;
```

客户端首先建立好连接

18:07:19.831124 IP localhost.46740 > localhost.mysql: Flags [S], seq 424450064, win 43690, options [mss 65495,sackOK,TS val 357860862 ecr 0,nop,wscale 7], length 0
18:07:19.831158 IP localhost.mysql > localhost.46740: Flags [S.], seq 2970334829, ack 424450065, win 43690, options [mss 65495,sackOK,TS val 357860862 ecr 357860862,nop,wscale 7], length 0
18:07:19.831179 IP localhost.46740 > localhost.mysql: Flags [.], ack 1, win 342, options [nop,nop,TS val 357860862 ecr 357860862], length 0
...

5s后，发现服务端主动断开连接(关闭了)，服务端关闭了写通道

18:07:24.902120 IP localhost.mysql > localhost.46740: Flags [F.], seq 233, ack 704, win 350, options [nop,nop,TS val 357865933 ecr 357860968], length 0
18:07:24.943661 IP localhost.46740 > localhost.mysql: Flags [.], ack 234, win 342, options [nop,nop,TS val 357865974 ecr 357865933], length 0

[I] wallace@centos:/m/s/W/p/B/p/b/B/judge_server> ss -aon|grep 3306
tcp    CLOSE-WAIT 1      0      127.0.0.1:46739              127.0.0.1:3306                timer:(keepalive,120min,0)
tcp    LISTEN     0      80       :::3306                 :::*                  
tcp    FIN-WAIT-2 0      0      ::ffff:127.0.0.1:3306               ::ffff:127.0.0.1:46739               timer:(timewait,56sec,0)

观看连接状态，发现服务端socket状态已变为`FIN-WAIT-2`，60s后超时关闭，服务端socket关闭连接后；
此后客户端进入了半连接状态，若客户端继续发送数据，则会收到`RST`，客户端读返回-1，errno=ECONNRESET，
`mysql connector c`会将该错误作为`CR_SERVER_LOST`

客户端发送数据

18:13:23.289923 IP localhost.46740 > localhost.mysql: Flags [P.], seq 704:737, ack 234, win 342, options [nop,nop,TS val 358224321 ecr 357865933], length 33
18:13:23.289935 IP localhost.mysql > localhost.46740: Flags [R], seq 2970335063, win 0, length 0

gdb源码跟踪(mysql-connector-c-6.1.5-src)

sql-common/client.c:957

ulong
cli_safe_read_with_ok(MYSQL *mysql, my_bool read_ok)
{
  NET *net= &mysql->net;
  ulong len=0;

  MYSQL_TRACE(READ_PACKET, mysql, ());

  if (net->vio != 0)
    len=my_net_read(net); // <= 此调用失败

  if (len == packet_error || len == 0) // len == packet_error 成立
  {
    DBUG_PRINT("error",("Wrong connection or packet. fd: %s  len: %lu",
            vio_description(net->vio),len));
#ifdef MYSQL_SERVER
    if (net->vio && (net->last_errno == ER_NET_READ_INTERRUPTED))
      return (packet_error);
#endif /*MYSQL_SERVER*/
    end_server(mysql);
    set_mysql_error(mysql, net->last_errno == ER_NET_PACKET_TOO_LARGE ?
                    CR_NET_PACKET_TOO_LARGE: CR_SERVER_LOST, unknown_sqlstate); // 设置错误代码为：CR_SERVER_LOST
    return (packet_error);
  }

Breakpoint 3, cli_safe_read_with_ok (mysql=0xb99e80, read_ok=0 '\000') at /media/sf_E_DRIVE/Soft/mysql-connector/mysql-connector-c-6.1.5-src/sql-common/client.c:976
976     set_mysql_error(mysql, net->last_errno == ER_NET_PACKET_TOO_LARGE ?
(gdb) bt
#0  cli_safe_read_with_ok (mysql=0xb99e80, read_ok=0 '\000') at /media/sf_E_DRIVE/Soft/mysql-connector/mysql-connector-c-6.1.5-src/sql-common/client.c:976
#1  0x00000000004183da in cli_safe_read (mysql=0xb99e80) at /media/sf_E_DRIVE/Soft/mysql-connector/mysql-connector-c-6.1.5-src/sql-common/client.c:1047
#2  0x0000000000421f0b in cli_read_query_result (mysql=0xb99e80) at /media/sf_E_DRIVE/Soft/mysql-connector/mysql-connector-c-6.1.5-src/sql-common/client.c:4711
#3  0x000000000042275c in mysql_real_query (mysql=0xb99e80, query=0x59c2db "SELECT age, name FROM test.a", length=28)
    at /media/sf_E_DRIVE/Soft/mysql-connector/mysql-connector-c-6.1.5-src/sql-common/client.c:4802
#4  0x000000000040ad7b in mysql::Connection::Query (this=0xba01d0, sql=...) at Beme4wdServer/lily/mysql/connection.cpp:89

参考

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