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

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; return;}
#    define DBUG_EXECUTE(keyword,a1) 
          {if (_db_on_) {if (_db_keyword_ (keyword)) { a1 }}}
#    define DBUG_PRINT(keyword,arglist) 
          {if (_db_on_) {_db_pargs_(__LINE__,keyword); _db_doprnt_ arglist;}}
#    define DBUG_2(keyword,format) 
          DBUG_PRINT(keyword,(format))        /* Obsolete */
#    define DBUG_3(keyword,format,a1) 
          DBUG_PRINT(keyword,(format,a1))         /* Obsolete */
#    define DBUG_4(keyword,format,a1,a2) 
          DBUG_PRINT(keyword,(format,a1,a2))      /* Obsolete */
#    define DBUG_5(keyword,format,a1,a2,a3) 
          DBUG_PRINT(keyword,(format,a1,a2,a3))   /* Obsolete */
#    define DBUG_PUSH(a1) _db_push_ (a1)
#    define DBUG_POP() _db_pop_ ()
#    define DBUG_PROCESS(a1) (_db_process_ = a1)
#    define DBUG_FILE (_db_fp_)
#    define DBUG_SETJMP(a1) (_db_setjmp_ (), setjmp (a1))
#    define DBUG_LONGJMP(a1,a2) (_db_longjmp_ (), longjmp (a1, a2))
#    define DBUG_DUMP(keyword,a1,a2) _db_dump_(__LINE__,keyword,a1,a2)
# endif