miniob调试篇

-- by caizj

调试c/c++程序，常用的有两种方式，一是打印日志调试，二是gdb调试，调试不仅可以定位问题，也可以用来熟悉代码。

miniob关键代码

首先，拿到一份陌生的代码，要先确定代码的大致结构，一些关键数据结构和方法，这里的技巧和经验不展开了

miniob的关键数据结构

部分关键数据结构：

parse_def.h:
	struct Selects;//查询相关
	struct CreateTable;//建表相关
	struct DropTable;//删表相关
	enum SqlCommandFlag;//sql语句对应的command枚举
	union Queries;//各类dml和ddl操作的联合
table.h
	class Table;
db.h
	class Db;

miniob的关键接口

部分关键接口

RC parse(const char *st, Query *sqln);//sql parse入口
ExecuteStage::handle_request
ExecuteStage::do_select
DefaultStorageStage::handle_event
DefaultHandler::create_index
DefaultHandler::insert_record
DefaultHandler::delete_record
DefaultHandler::update_record
Db::create_table
Db::find_table
Table::create
Table::scan_record
Table::insert_record
Table::update_record
Table::delete_record
Table::scan_record
Table::create_index

打印日志调试

miniob提供的日志接口

deps/common/log/log.h:
#define LOG_PANIC(fmt, ...)
#define LOG_ERROR(fmt, ...)
#define LOG_WARN(fmt, ...)
#define LOG_INFO(fmt, ...)
#define LOG_DEBUG(fmt, ...)
#define LOG_TRACE(fmt, ...)

日志相关配置项observer.ini

LOG_FILE_NAME = observer.log
#  LOG_LEVEL_PANIC = 0,
#  LOG_LEVEL_ERR = 1,
#  LOG_LEVEL_WARN = 2,
#  LOG_LEVEL_INFO = 3,
#  LOG_LEVEL_DEBUG = 4,
#  LOG_LEVEL_TRACE = 5,
#  LOG_LEVEL_LAST
LOG_FILE_LEVEL=5
LOG_CONSOLE_LEVEL=1

gdb调试

调试工具有很多种，但是它们的关键点都是类似的，比如关联到进程、运行时查看变量值、单步运行、跟踪变量等。GDB是在Linux环境中常用的调试工具。其它环境上也有类似的工具，比如LLDB，或者Windows可能使用Visual Studio直接启动调试。Java的调试工具是jdb。

另外，很多同学喜欢使用Visual Studio Code(vscode)开发项目，vscode提供了很多插件，包括调试的插件，这些调试插件支持gdb、lldb等，可以按照自己的平台环境，设置不同的调试工具。

这里介绍了gdb的基本使用，其它工具的使用方法类似。

Attach进程

[caizj@localhost run]$ gdb -p `pidof observer` 

GNU gdb (GDB) Red Hat Enterprise Linux 8.2-15.el8 Copyright (C) 2018 Free Software Foundation, Inc.

(gdb)

设置断点

(gdb) break do_select
Breakpoint 1 at 0x44b636: file /home/caizj/source/stunning-engine/src/observer/sql/executor/execute_stage.cpp, line 526.
(gdb) info b
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x000000000044b636 in ExecuteStage::do_select(char const*, Query*, SessionEvent*)
                                                   at /home/caizj/source/stunning-engine/src/observer/sql/executor/execute_stage.cpp:526

(gdb) break Table::scan_record
Breakpoint 2 at 0x50b82b: Table::scan_record. (2 locations)
(gdb) inf b
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x000000000044b636 in ExecuteStage::do_select(char const*, Query*, SessionEvent*)
                                                   at /home/caizj/source/stunning-engine/src/observer/sql/executor/execute_stage.cpp:526
2       breakpoint     keep y   <MULTIPLE>
2.1                         y     0x000000000050b82b in Table::scan_record(Trx*, ConditionFilter*, int, void*, void (*)(char const*, void*))
                                                   at /home/caizj/source/stunning-engine/src/observer/storage/common/table.cpp:421
2.2                         y     0x000000000050ba00 in Table::scan_record(Trx*, ConditionFilter*, int, void*, RC (*)(Record*, void*))
                                                   at /home/caizj/source/stunning-engine/src/observer/storage/common/table.cpp:426
(gdb)

继续执行
```
(gdb) c
Continuing.
```

触发断点

执行：miniob > select * from t1;

[Switching to Thread 0x7f51345f9700 (LWP 54706)]

Thread 8 "observer" hit Breakpoint 1, ExecuteStage::do_select (this=0x611000000540,
    db=0x6040000005e0 "sys", sql=0x620000023080, session_event=0x608000003d20)
    at /home/caizj/source/stunning-engine/src/observer/sql/executor/execute_stage.cpp:526
526	  RC rc = RC::SUCCESS;
(gdb)

单步调式

575	  std::vector<TupleSet> tuple_sets;
(gdb) next
576	  for (SelectExeNode *&node: select_nodes) {
(gdb) n
577	    TupleSet tuple_set;
(gdb)
578	    rc = node->execute(tuple_set);
(gdb)

跳入跟踪到函数内部

(gdb) s
SelectExeNode::execute (this=0x60700002ce80, tuple_set=...)
    at /home/caizj/source/stunning-engine/src/observer/sql/executor/execution_node.cpp:43
43	  CompositeConditionFilter condition_filter;
(gdb)

打印变量

(gdb) p tuple_set
$3 = (TupleSet &) @0x7f51345f1760: {tuples_ = std::vector of length 0, capacity 0, schema_ = {
    fields_ = std::vector of length 0, capacity 0}}
(gdb)

watch变量

(gdb) n
443	  RC rc = RC::SUCCESS;
(gdb) n
444	  RecordFileScanner scanner;
(gdb) n
445	  rc = scanner.open_scan(*data_buffer_pool_, file_id_, filter);
(gdb) watch -l rc
Hardware watchpoint 3: -location rc
(gdb) c
Continuing.

Thread 8 "observer" hit Hardware watchpoint 3: -location rc

Old value = SUCCESS
New value = RECORD_EOF
0x000000000050c2de in Table::scan_record (this=0x60f000007840, trx=0x606000009920,
    filter=0x7f51345f12a0, limit=2147483647, context=0x7f51345f11c0,
    record_reader=0x50b74a <scan_record_reader_adapter(Record*, void*)>)
    at /home/caizj/source/stunning-engine/src/observer/storage/common/table.cpp:454
454	  for ( ; RC::SUCCESS == rc && record_count < limit; rc = scanner.get_next_record(&record)) {
(gdb)

结束函数调用

(gdb) finish
Run till exit from #0  0x000000000050c2de in Table::scan_record (this=0x60f000007840,
    trx=0x606000009920, filter=0x7f51345f12a0, limit=2147483647, context=0x7f51345f11c0,
    record_reader=0x50b74a <scan_record_reader_adapter(Record*, void*)>)
    at /home/caizj/source/stunning-engine/src/observer/storage/common/table.cpp:454

结束调试

(gdb) quit
A debugging session is active.

	Inferior 1 [process 54699] will be detached.

Quit anyway? (y or n) y
Detaching from program: /home/caizj/local/bin/observer, process 54699
[Inferior 1 (process 54699) detached]

代码中已经为vscode配置了launch.json，可以直接启动调试。 launch.json中有两个调试配置，一个是Debug，一个是LLDB。其中Debug使用cppdbg，会自动探测调试工具gdb或lldb，而LLDB会使用lldb调试工具。通常情况下，大家使用Debug就可以了，但是我在测试过程中发现cppdbg不能在macos上正常工作，因而增加了LLDB的配置，以便在macos上调试，如果使用macos的同学，可以使用LLDB配置启动调试程序。

MiniOB