MySQL embedded server简介
MySQL可以作为一个embedded server执行，这时它就充当了一个与应用紧密耦合的轻量级数据中心，适合资源有限、应用需要连接很少（通常1个）的场景，使用方便，编译后就可以随应用一起启动，具有部署方便，速度更快的优点，当然它有很多限制，如：不支持UDF，不支持主备复制，InnoDB层不支持多个连接等，更多请参考 libmysqld, the Embedded MySQL Server Library.
因为DRC团队在使用embedded server的过程中遇到了执行prepared statment时内存泄露的问题，为了定位并fix问题，分析了这部分逻辑
prepared statement API
| 函数 | 说明 |
|---|---|
| mysql_stmt_init | 初始化statement |
| mysql_stmt_prepare | prepare一条SQL语句 |
| mysql_stmt_bind_param | 绑定传入变量 |
| mysql_stmt_bind_result | 绑定结果变量 |
| mysql_stmt_execute | 设置不同的绑定变量的值，可以多次执行 |
| mysql_stmt_restore_result | 存储执行结果 |
| mysql_stmt_fetch | 读取一行结果到绑定结果变量 |
| mysql_stmt_affected_rows | 对于insert/update/delete返回影响的数据条数 |
| mysql_stmt_num_rows | 对于select返回结果集大小 |
| mysql_stmt_free_result | 清空结果集 |
| mysql_stmt_close | 关闭statement |
embedded server数据交互
相对于C/S架构的MySQL server，embedded server是运行于应用内部的，因此数据传输不需要走网络，通过内存拷贝就可以完成，但是在逻辑上还是维持C/S结构，libmysqld中重新实现了Protocol类的一些方法如：Protocol::send_fields，以及net_send_ok，net_send_eof，net_send_error_packet，所有这些操作都是些内存操作，结果集数据写到THD中为embedded server增加的st_mysql_data类型的链表中：(sql/sql_class.h)
#ifdef EMBEDDED_LIBRARY struct st_mysql *mysql; unsigned long client_stmt_id; unsigned long client_param_count; struct st_mysql_bind *client_params; char *extra_data; ulong extra_length; struct st_mysql_data *cur_data; struct st_mysql_data *first_data; struct st_mysql_data **data_tail; void clear_data_list(); struct st_mysql_data *alloc_new_dataset(); /* In embedded server it points to the statement that is processed in the current query. We store some results directly in statement fields then. */ struct st_mysql_stmt *current_stmt; #endif |
重要数据结构
MYSQL_DATA是一个数据集，其中alloc负责为embedded_info中的fields_list分配内存，data为结果数据集（链表），rows为结果集数据行数
typedef struct st_mysql_data { MYSQL_ROWS *data; struct embedded_query_result *embedded_info; MEM_ROOT alloc; my_ulonglong rows; unsigned int fields; /* extra info for embedded library */ void *extension; } MYSQL_DATA; |
embedded_query_result主要包含结果集的状态信息，结果集的列集合field_list(用于解析MYSQL_ROWS)以及指向下一个结果集的指针
/* This one is used by embedded library to gather returning data */ typedef struct embedded_query_result { MYSQL_ROWS **prev_ptr; unsigned int warning_count, server_status; struct st_mysql_data *next; my_ulonglong affected_rows, insert_id; char info[MYSQL_ERRMSG_SIZE]; MYSQL_FIELD *fields_list; unsigned int last_errno; char sqlstate[SQLSTATE_LENGTH+1]; } EQR; |
MYSQL_ROWS是一个链表节点，保存一行数据和指向下行数据的指针
typedef struct st_mysql_rows { struct st_mysql_rows *next; /* list of rows */ MYSQL_ROW data; unsigned long length; } MYSQL_ROWS; |
另外两个重要数据结构st_mysql_stmt和st_mysql，在此不列举了
函数调用分析
mysql_stmt_init
为stmt分配内存，初始化变量，为stmt->mem_root和stmt->result.alloc分配内存
init_alloc_root(&stmt->mem_root, 2048, 2048); init_alloc_root(&stmt->result.alloc, 4096, 4096); |
mysql_stmt_prepare
1、stmt_command发送COM_STMT_PREPARE命令，最终通过emb_advanced_command调用dispatch_command执行mysqld_stmt_prepare
2、read_prepare_resut调用emb_read_prepare_result读取prepare语句执行结果
3、为stmt->param（绑定变量参数）分配内存（在stmt->mem_root上）
mysql_stmt_bind_param
绑定prepare语句变量，对参数做一些判断，初始化stmt->param，根据数据类型设置param->store_param_func
mysql_stmt_bind_result
绑定结果集变量，对参数做一些判断，初始化stmt->bind，调用setup_one_fetch_function根据数据类型设置bind->fetch_result
mysql_stmt_execute
1、reset_stmt_handle，释放stmt->result.alloc内存（MY_KEEP_PREALLOC方式）
2、emb_stmt_execute，通过emb_advanced_command以COM_STMT_EXECUTE作为参数调用dispatch_command执行mysqld_stmt_execute
3、emb_read_query_result设置结果读取位置（thd->cur_data），主要执行一下代码：
static my_bool emb_read_query_result(MYSQL *mysql) { THD *thd= (THD*) mysql->thd; MYSQL_DATA *res= thd->first_data; thd->first_data= res->embedded_info->next; .. if (res->embedded_info->fields_list) { mysql->status=MYSQL_STATUS_GET_RESULT; thd->cur_data= res; } .. |
mysql_stmt_restore_result
1、emb_read_binary_rows：调用emb_read_rows获得结果集读取位置（thd->cur_data）之后赋值：stmt->result= *data，到此歩，stmt结果集可以通过stmt->result返回了
2、为stmt设置fetch结果集的函数指针和游标：
stmt->read_row_func= stmt_read_row_buffered; stmt->data_cursor= stmt->result->data |
mysql_stmt_fetch
1、stmt_read_row_buffered，代码如下：
static int stmt_read_row_buffered(MYSQL_STMT *stmt, unsigned char **row) { if (stmt->data_cursor) { *row= (uchar *) stmt->data_cursor->data; stmt->data_cursor= stmt->data_cursor->next; return 0; } *row= 0; return MYSQL_NO_DATA; } |
2、stmt_fetch_row，根据stmt_read_row_buffered得到的行数据，将结果解析到绑定结果变量中
mysql_stmt_affected_rows, mysql_stmt_num_rows
返回stmt->affected_rows和stmt->result.rows
mysql_stmt_close
关闭stmt，释放stmt->mem_root和stmt->result.alloc内存
两个Memeory leak的bug
代码调试过程中通过valgrind发现了两处内存泄露（mysql-5.6.11）
==29639== 2,064 bytes in 1 blocks are definitely lost in loss record 3 of 4 ==29639== at 0x4A05FDE: malloc (vg_replace_malloc.c:236) ==29639== by 0x4635E1: my_malloc (my_malloc.c:38) ==29639== by 0x462C72: init_alloc_root (my_alloc.c:63) ==29639== by 0x448935: mysql_stmt_init (libmysql.c:1512) ==29639== by 0x444277: main (in /u01/mysql5611/bug62136) ==29639== ==29639== 4,112 bytes in 1 blocks are definitely lost in loss record 4 of 4 ==29639== at 0x4A05FDE: malloc (vg_replace_malloc.c:236) ==29639== by 0x4635E1: my_malloc (my_malloc.c:38) ==29639== by 0x462C72: init_alloc_root (my_alloc.c:63) ==29639== by 0x44894B: mysql_stmt_init (libmysql.c:1513) ==29639== by 0x444277: main (in /u01/mysql5611/bug62136) ==29639== ==29639== LEAK SUMMARY: ==29639== definitely lost: 6,176 bytes in 2 blocks ==29639== indirectly lost: 0 bytes in 0 blocks ==29639== possibly lost: 0 bytes in 0 blocks ==29639== still reachable: 568 bytes in 2 blocks ==29639== suppressed: 0 bytes in 0 blocks ==29639== ==29639== For counts of detected and suppressed errors, rerun with: -v ==29639== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 6 from 6) |
经过分析，最终定位到问题原因并fix，向官方提交了一个bug report，详情：mysql bug#70238