问题描述：
同事使用mysqlbinlog工具的--read-from-remote-server --raw选项，从远程实例实时拉取二进制日志时，发现得到的二进制日志文件大小与远程实例上的源文件大小不相同，并且使用mysqlbinlog解析时会报错。
测试环境版本信息如下：
MySQL版本：5.7.17 log MySQL Community Server (GPL) 通用tar包安装
Mysqlbinlog版本：5.7.17 自带版本，mysqlbinlog Ver 3.4 for linux-glibc2.5 at x86_64
操作系统版本：CentOS Linux release 7.6.1810 (Core)
分析过程：
1、对比拉取到的二进制日志文件、源文件的大小，发现拉取到的二进制日志文件比源文件小，并且该文件大小：7315456/4096，刚好可以整除，源库上进行多次更新后，多次观察，发现该数值都可以被4096整除。
因为Linux块大小是4096 Bytes，所以，我先做个假设：mysqlbinlog工具以4096字节为单位进行写入
[root@192_168_129_128 tmp]# ll /usr/local/mysql-5.7.17-linux-glibc2.5-x86_64/data/mybinlog.000006 ; ll mybinlog.000006 -rw-r----- 1 mysql mysql 7318723 May 12 16:55 /usr/local/mysql-5.7.17-linux-glibc2.5-x86_64/data/mybinlog.000006 -rw-r----- 1 root root 7315456 May 12 16:55 mybinlog.000006
2、我在8.0.19版本中测试，问题却不能复现。
3、写一个简单的shell脚本，将mysqlbinlog拉取进程放到后台执行，获取到上一步的pid之后，使用pstack命令和strace命令，分别查看该进程的函数调用和系统调用
#!/bin/bash nohup mysqlbinlog --read-from-remote-server --host=192.168.129.128 --user=dba --password=123456 --raw --to-last-log binlog.000002 --stop-never & pid=`ps -ef | grep mysqlbinlog | grep -v ‘grep‘ | awk ‘{print $2}‘` pstack $pid > pstack.log strace -f -t -p $pid -o strace.log
pstack命令的输出结果如下，虽然没有直接的线索，但是后续可以利用这些函数栈名，去源码中找线索：
#0 0x00007faf58f60a2d in recv () from /lib64/libpthread.so.0 #1 0x000000000045e9f9 in inline_mysql_socket_recv (flags=0, n=5826, buf=0x1d959d1, mysql_socket=..., src_line=123, src_file=0x550918 "/export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/vio/viosocket.c") at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/include/mysql/psi/mysql_socket.h:823 #2 vio_read (vio=0x1d90e60, buf=0x1d959d1 "27399199-10542799900-96491182343-85303866742-80196460272-89060617578-51177070778-10421134155;80308169113-21291571753-18876715410-91134905277-85771492482\360j\003", size=5826) at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/vio/viosocket.c:123 #3 0x0000000000434873 in net_read_raw_loop (count=5826, net=0x1d8e550) at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/sql/net_serv.cc:672 #4 net_read_packet (net=0x1d8e550, complen=0x7fff6d023f88) at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/sql/net_serv.cc:859 #5 0x0000000000434a7c in my_net_read (net=0x1d8e550) at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/sql/net_serv.cc:899 #6 0x000000000043985b in cli_safe_read_with_ok (mysql=0x1d8e550, parse_ok=0 ‘\000‘, is_data_packet=0x0) at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/sql-common/client.c:1040 #7 0x0000000000426b36 in dump_remote_log_entries (logname=0x7fff6d0277e9 "binlog.000002", print_event_info=0x7fff6d0250f0) at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/client/mysqlbinlog.cc:2664 #8 dump_single_log (logname=0x7fff6d0277e9 "binlog.000002", print_event_info=0x7fff6d0250f0) at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/client/mysqlbinlog.cc:2334 #9 dump_multiple_logs (argc=1, argv=<optimized out>) at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/client/mysqlbinlog.cc:2374 #10 0x0000000000427821 in main (argc=1, argv=0x1d58468) at /export/home/pb2/build/sb_0-21378219-1480347226.17/mysql-5.7.17/client/mysqlbinlog.cc:3436
strace命令的输出结果如下：
....................省略若干行.................... 20528 11:24:25 recvfrom(3, "6813690-49945547265-44609719076-"..., 6605, 0, NULL, NULL) = 6605 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3424256 20528 11:24:25 write(4, "553290165-79173089006-0778194955"..., 4096) = 4096 20528 11:24:25 write(4, "2\360E\t\0\0p\3\0\0w71247594556-464035229"..., 4096) = 4096 20528 11:24:25 recvfrom(3, " \0\0\211\0K\200\266^\20/\201\0\0\37\0\0\0\320d4\0\0\0\21\1\0\0\0\0\0\0"..., 16384, 0, NULL, NULL) = 1523 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 lseek(4, 0, SEEK_CUR) = 3432448 20528 11:24:25 recvfrom(3,
strace到这里就没有后续的输出了。
其中，recvfrom系统调用，是经socket接收数据；lseek是一个用于改变读写一个文件时读写指针位置的一个系统调用；write把buf中nbyte写入文件描述符handle所指的文档，成功时返回写的字节数，错误时返回-1.
分析strace日志中的多条write记录，发现每次write的写入字节数都是4096，看起来与前面的假设吻合。
但是，每达到4096字节才写入磁盘，那么拉取到的binlog几乎不可能与源库一致。而mysqlbinlog的help显示，--read-from-remote-server参数，实质上是调用MySQL Server的BINLOG-DUMP-NON-GTIDS接口，远程实例应该是实时发送binlog events才对。
-R, --read-from-remote-server Read binary logs from a MySQL server. This is an alias for read-from-remote-master=BINLOG-DUMP-NON-GTIDS.
好吧，从pstack日志中得到的函数名，我们去源码中逐个查看，最终与本问题相关的是client/mysqlbinlog.cc中的dump_remote_log_entries()函数，如下：
/** Requests binlog dump from a remote server and prints the events it receives. @param[in,out] print_event_info Parameters and context state determining how to print. @param[in] logname Name of input binlog. @retval ERROR_STOP An error occurred - the program should terminate. @retval OK_CONTINUE No error, the program should continue. @retval OK_STOP No error, but the end of the specified range of events to process has been reached and the program should terminate. */ static Exit_status dump_remote_log_entries(PRINT_EVENT_INFO *print_event_info, const char* logname) { ....................省略若干行.................... for (;;) { const char *error_msg= NULL; Log_event *ev= NULL; Log_event_type type= binary_log::UNKNOWN_EVENT; len= cli_safe_read(mysql, NULL); if (len == packet_error) { error("Got error reading packet from server: %s", mysql_error(mysql)); DBUG_RETURN(ERROR_STOP); } if (len < 8 && net->read_pos[0] == 254) break; // end of data DBUG_PRINT("info",( "len: %lu net->read_pos[5]: %d\n", len, net->read_pos[5])); /* In raw mode We only need the full event details if it is a ROTATE_EVENT or FORMAT_DESCRIPTION_EVENT */ ....................省略若干行.................... if (raw_mode || (type != binary_log::LOAD_EVENT)) { ....................省略若干行.................... if (raw_mode) { DBUG_EXECUTE_IF("simulate_result_file_write_error", DBUG_SET("+d,simulate_fwrite_error");); if (my_fwrite(result_file, net->read_pos + 1 , len - 1, MYF(MY_NABP))) /*可以看到，cli_safe_read()方法读取到的binlog event，都用调用my_fwrite函数进行写入，my_fwrite是对fwrite()系统调用的封装 这里并没有以4096字节为单位写入，而是读多少就写入多少 这就无法解释了，代码逻辑显示每次拿到数据之后，都会写入磁盘，为什么实际上却不是呢？*/ { error("Could not write into log file ‘%s‘", log_file_name); retval= ERROR_STOP; } if (ev) reset_temp_buf_and_delete(ev); } ....................省略若干行.................... if (retval != OK_CONTINUE) DBUG_RETURN(retval); } else { ....................省略若干行.................... old_off+= len-1; } DBUG_RETURN(OK_CONTINUE); }
请看我用中文注释在上述源码中的分析。
代码逻辑没有问题，那就有可能是BUG了。。。
结论：
通过上述分析，怀疑遇到了BUG，于是去官方的Release Notes中查找，最终在https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-19.html 中找到如下说明：
Replication:
mysqlbinlog, if invoked with the --raw option, does not flush the output file until the process terminates. But if also invoked with the --stop-never option,
the process never terminates, thus nothing is ever written to the output file. Now the output is flushed after each event. (Bug #24609402)
但是这个BUG在BUG system中找不到，应该是需要有权限的MOS账号才能查看。