Mysql的Debug模式实现

前一段领导开发了一个内核的模块，测试的过程中，发现导致MYSQL客户端无法连接服务器。

经过查询文档，追根溯源，终于找到了MYSQL实现链接客户端的代码，在源文件sql-common/client.c里的 CLI_MYSQL_REAL_CONNECT 函数。

但是代码很长，一时半会儿肯定看不明白。这个时候发现，发现代码当中有很多这样的代码：

DBUG_ENTER("mysql_real_connect");

说明只要以调试模式启动MYSQL，就可以跟踪代码的执行。

经过查询文档和测试，只要在 cmake 的时候，增加参数 cmake -WITH_DEBUG=1 就可以了。

然后启动一个MYSQL客户端程序之前，更改一个环境变量：

export MYSQL_DEBUG=d:t:O,/tmp/client.trace

使用编辑器打开 /tmp/client.trace 就会得到这样的Debug信息：

| info: Connect socket
| >vio_socket_connect
| | >vio_set_blocking
| | <vio_set_blocking 245
| | >vio_io_wait
| | <vio_io_wait 790
| | >vio_set_blocking
| | <vio_set_blocking 227
| <vio_socket_connect 957
| info: No success, close socket, try next address.
| info: End of connect attempts, sock: 4  status: 1  error: 0
| error: Got error 0 on connect to 'localhost'
| >set_mysql_extended_error
| | enter: error :2003 'Can't connect to MySQL server on '%-.100s' (%d)'
| <set_mysql_extended_error 270
| error: message: 2003/HY000 (Can't connect to MySQL server on 'localhost' (0))

后面的数字是行号。从这个文件就可以追踪程序的执行啦！

好牛逼啊！我们不禁感叹。之余，学习了一下这个调试模式的具体实现：

在 CMakeList.txt 中，有这么一段代码：

IF(WITH_DEBUG)
  SET(CMAKE_BUILD_TYPE "Debug" CACHE STRING ${BUILDTYPE_DOCSTRING} FORCE)

IF(NOT CMAKE_BUILD_TYPE
    AND NOT CMAKE_GENERATOR MATCHES "Visual Studio" 
    AND NOT CMAKE_GENERATOR MATCHES "Xcode")
    # This is the case of no CMAKE_BUILD_TYPE choosen, typical for VS and Xcode
    # or if custom C flags are set. In VS and Xcode for non-Debug configurations 
    # DBUG_OFF is already correctly set. Use DBUG_OFF for Makefile based projects 
    # without build type too, unless user specifically requests DBUG. 
    IF(NOT CMAKE_C_FLAGS MATCHES "-DDBUG_ON")
      ADD_DEFINITIONS(-DDBUG_OFF)
    ENDIF()
ENDIF()

如果没有设置 CMAKE_BUILD_TYPE ，那么就会执行下面一段代码。如果这时候也没有设置一个名为-DDBUG_ON的CFLAGS的环境变量的话，就会增加一个CFLAGS：-DDBUG_OFF。

之后，当编译器编译的时候，根据编译器的参数增加的宏来决定 怎么定义 DBUG_ENTER 之类的函数是空代码，还是实际的报错代码。

/*
 *        These macros provide a user interface into functions in the
 *        dbug runtime support library.  They isolate users from changes
 *        in the MACROS and/or runtime support.
 *
 *        The symbols "__LINE__" and "__FILE__" are expanded by the
 *        preprocessor to the current source file line number and file
 *        name respectively.
 *
 *        WARNING ---  Because the DBUG_ENTER macro allocates space on
 *        the user function's stack, it must precede any executable
 *        statements in the user function.
 *
 */
# ifdef DBUG_OFF
#    define DBUG_ENTER(a1)
#    define DBUG_RETURN(a1) return(a1)
#    define DBUG_VOID_RETURN return
#    define DBUG_EXECUTE(keyword,a1)
#    define DBUG_PRINT(keyword,arglist)
#    define DBUG_2(keyword,format)      /* Obsolete */
#    define DBUG_3(keyword,format,a1)       /* Obsolete */
#    define DBUG_4(keyword,format,a1,a2)    /* Obsolete */
#    define DBUG_5(keyword,format,a1,a2,a3) /* Obsolete */
#    define DBUG_PUSH(a1)
#    define DBUG_POP()
#    define DBUG_PROCESS(a1)
#    define DBUG_FILE (stderr)
#    define DBUG_SETJMP setjmp
#    define DBUG_LONGJMP longjmp
#    define DBUG_DUMP(keyword,a1)
# else
#    define DBUG_ENTER(a) \
    auto char *_db_func_; auto char *_db_file_; auto int _db_level_; \
    auto char **_db_framep_; \
    _db_enter_ (a,__FILE__,__LINE__,&_db_func_,&_db_file_,&_db_level_, \
            &_db_framep_)
#    define DBUG_LEAVE \
          (_db_return_ (__LINE__, &_db_func_, &_db_file_, &_db_level_))
#    define DBUG_RETURN(a1) return (DBUG_LEAVE, (a1))
#    define DBUG_VOID_RETURN {DBUG_LEAVE