GDB 调试 Mysql 实战（二）GDB 调试打印

背景在 https://mengkang.net/1328.html 实验中，我们通过optimizer_trace发现group by会使用intermediate_tmp_table，而且里面的的row_length是20，抱着"打破砂锅问到底"的求学精神，所以想通过 gdb 调试源码的方式看这个row_length为什么是20.通过row_length关键字，我定位到了mysql 5.7 源码里面的sql/sql_tmp_table.cc文件实际操作查找 mysql pid[root@localhost ~]# ps -ef|grep mysqlroot 3739 1 0 09:36 ? 00:00:00 /bin/sh /usr/local/mysql/bin/mysqld_safe –datadir=/var/lib/mysql –pid-file=/var/lib/mysql/localhost.localdomain.pidmysql 3894 3739 0 09:36 ? 00:00:01 /usr/local/mysql/bin/mysqld –basedir=/usr/local/mysql –datadir=/var/lib/mysql –plugin-dir=/usr/local/mysql/lib/plugin –user=mysql –log-error=/var/log/mariadb/mariadb.log –pid-file=/var/lib/mysql/localhost.localdomain.pid –socket=/var/lib/mysql/mysql.sockroot 3956 3940 0 09:48 pts/1 00:00:00 mysql -uroot -px xxxxroot 4002 3985 0 10:11 pts/2 00:00:00 grep –color=auto mysql启动 gdb[root@localhost ~]# gdbGNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-114.el7Copyright (C) 2013 Free Software Foundation, Inc.License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>This is free software: you are free to change and redistribute it.There is NO WARRANTY, to the extent permitted by law. Type “show copying"and “show warranty” for details.This GDB was configured as “x86_64-redhat-linux-gnu”.For bug reporting instructions, please see:<http://www.gnu.org/software/gdb/bugs/>.attach pid(gdb) attach 3894设置断点(gdb) b trace_tmp_tableBreakpoint 1 at 0x15e1eeb: file /root/newdb/mysql-server/sql/sql_tmp_table.cc, line 2300.或者(gdb) b /root/newdb/mysql-server/sql/sql_tmp_table.cc:2306Breakpoint 1 at 0x15e1f8e: file /root/newdb/mysql-server/sql/sql_tmp_table.cc, line 2306.客户端连接[root@localhost ~]# mysql -uroot -p123456 testmysql: [Warning] Using a password on the command line interface can be insecure.阻塞中打印 backtrace(gdb) bt#0 0x00007effcf4ec20d in poll () from /lib64/libc.so.6#1 0x0000000001667759 in Mysqld_socket_listener::listen_for_connection_event (this=0x42e6360) at /root/newdb/mysql-server/sql/conn_handler/socket_connection.cc:852#2 0x0000000000eb14fc in Connection_acceptor<Mysqld_socket_listener>::connection_event_loop (this=0x42ea0a0) at /root/newdb/mysql-server/sql/conn_handler/connection_acceptor.h:66#3 0x0000000000ea8f7a in mysqld_main (argc=12, argv=0x41a60e8) at /root/newdb/mysql-server/sql/mysqld.cc:5149#4 0x0000000000ea00ed in main (argc=8, argv=0x7ffece17c7e8) at /root/newdb/mysql-server/sql/main.cc:25因为主进程在 poll 一直在等待客户端连接请求。执行 continue(gdb) cContinuing.此时，mysql 客户端则进入 mysql 命令行界面[root@localhost ~]# mysql -uroot -p123456 testmysql: [Warning] Using a password on the command line interface can be insecure.Reading table information for completion of table and column namesYou can turn off this feature to get a quicker startup with -AWelcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 3Server version: 5.7.25-debug Source distributionCopyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type ‘help;’ or ‘\h’ for help. Type ‘\c’ to clear the current input statement.mysql>客户端执行查询SET optimizer_trace=‘enabled=on’;select aid,sum(pv) as num from article_rank force index(idx_day_aid_pv) where day>20181223 group by aid order by num desc LIMIT 10;查看断点处信息[Switching to Thread 0x7f7f20145700 (LWP 4392)]Breakpoint 1, trace_tmp_table (trace=0x7eff94003088, table=0x7eff94937200) at /root/newdb/mysql-server/sql/sql_tmp_table.cc:2306warning: Source file is more recent than executable.2306 trace_tmp.add(“row_length”,table->s->reclength).(gdb) p table->s->reclength$1 = 20(gdb) p table->s->fields$2 = 2(gdb) p ((table->field+0))->field_name$3 = 0x7eff94010b0c “aid”(gdb) p ((table->field+1))->field_name$4 = 0x7eff94007518 “num”(gdb) p ((table->field+0))->row_pack_length()$5 = 4(gdb) p ((table->field+1))->row_pack_length()$6 = 15(gdb) p ((table->field+0))->type()$7 = MYSQL_TYPE_LONG(gdb) p ((table->field+1))->type()$8 = MYSQL_TYPE_NEWDECIMAL(gdb)总结原来和 group by没关系，只因为我 sql 种使用了sum行数，使得num字段类型是MYSQL_TYPE_NEWDECIMALThe SUM() and AVG() functions return a DECIMAL value for exact-value arguments (integer or DECIMAL), and a DOUBLE value for approximate-value arguments (FLOAT or DOUBLE). (Before MySQL 5.0.3, SUM() and AVG() return DOUBLE for all numeric arguments.)但是通过我们上面打印信息可以看到两个字段的长度加起来是19，而reclength是20。通过其他实验也发现table->s->reclength的长度就是table->field数组里面所有字段的字段长度和再加1。附录完整的 gdb 调试信息如下[root@localhost ~]# gdbGNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-114.el7Copyright (C) 2013 Free Software Foundation, Inc.License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>This is free software: you are free to change and redistribute it.There is NO WARRANTY, to the extent permitted by law. Type “show copying"and “show warranty” for details.This GDB was configured as “x86_64-redhat-linux-gnu”.For bug reporting instructions, please see:<http://www.gnu.org/software/gdb/bugs/>.(gdb) attach 3894Attaching to process 3894Reading symbols from /usr/local/mysql/bin/mysqld…done.Reading symbols from /lib64/libpthread.so.0…(no debugging symbols found)…done.[New LWP 4392][New LWP 4368][New LWP 4367][New LWP 4366][New LWP 4365][New LWP 4364][New LWP 4363][New LWP 4362][New LWP 4361][New LWP 4360][New LWP 4359][New LWP 4358][New LWP 4357][New LWP 4356][New LWP 4355][New LWP 4353][New LWP 4352][New LWP 4351][New LWP 4350][New LWP 4349][New LWP 4348][New LWP 4347][New LWP 4346][New LWP 4345][New LWP 4344][New LWP 4343][New LWP 4342][Thread debugging using libthread_db enabled]Using host libthread_db library “/lib64/libthread_db.so.1”.Loaded symbols for /lib64/libpthread.so.0Reading symbols from /lib64/libcrypt.so.1…(no debugging symbols found)…done.Loaded symbols for /lib64/libcrypt.so.1Reading symbols from /lib64/libdl.so.2…(no debugging symbols found)…done.Loaded symbols for /lib64/libdl.so.2Reading symbols from /lib64/librt.so.1…(no debugging symbols found)…done.Loaded symbols for /lib64/librt.so.1Reading symbols from /lib64/libstdc++.so.6…(no debugging symbols found)…done.Loaded symbols for /lib64/libstdc++.so.6Reading symbols from /lib64/libm.so.6…(no debugging symbols found)…done.Loaded symbols for /lib64/libm.so.6Reading symbols from /lib64/libgcc_s.so.1…(no debugging symbols found)…done.Loaded symbols for /lib64/libgcc_s.so.1Reading symbols from /lib64/libc.so.6…(no debugging symbols found)…done.Loaded symbols for /lib64/libc.so.6Reading symbols from /lib64/ld-linux-x86-64.so.2…(no debugging symbols found)…done.Loaded symbols for /lib64/ld-linux-x86-64.so.2Reading symbols from /lib64/libfreebl3.so…Reading symbols from /lib64/libfreebl3.so…(no debugging symbols found)…done.(no debugging symbols found)…done.Loaded symbols for /lib64/libfreebl3.soReading symbols from /lib64/libnss_files.so.2…(no debugging symbols found)…done.Loaded symbols for /lib64/libnss_files.so.20x00007f7f2c32620d in poll () from /lib64/libc.so.6Missing separate debuginfos, use: debuginfo-install glibc-2.17-260.el7.x86_64 libgcc-4.8.5-36.el7.x86_64 libstdc++-4.8.5-36.el7.x86_64 nss-softokn-freebl-3.36.0-5.el7_5.x86_64(gdb) b /root/newdb/mysql-server/sql/sql_tmp_table.cc:2306Breakpoint 1 at 0x15e1f8e: file /root/newdb/mysql-server/sql/sql_tmp_table.cc, line 2306.(gdb) bt#0 0x00007f7f2c32620d in poll () from /lib64/libc.so.6#1 0x0000000001667759 in Mysqld_socket_listener::listen_for_connection_event (this=0x3841360) at /root/newdb/mysql-server/sql/conn_handler/socket_connection.cc:852#2 0x0000000000eb14fc in Connection_acceptor<Mysqld_socket_listener>::connection_event_loop (this=0x38450a0) at /root/newdb/mysql-server/sql/conn_handler/connection_acceptor.h:66#3 0x0000000000ea8f7a in mysqld_main (argc=12, argv=0x37010e8) at /root/newdb/mysql-server/sql/mysqld.cc:5149#4 0x0000000000ea00ed in main (argc=8, argv=0x7ffdaf5bc2e8) at /root/newdb/mysql-server/sql/main.cc:25(gdb) cContinuing.[Switching to Thread 0x7f7f20145700 (LWP 4392)]Breakpoint 1, trace_tmp_table (trace=0x7f7ef8016b18, table=0x7f7ef8937380) at /root/newdb/mysql-server/sql/sql_tmp_table.cc:2306warning: Source file is more recent than executable.2306 trace_tmp.add(“row_length”,table->s->reclength).(gdb) p table->s->reclength$1 = 20(gdb) p table->s->fields$2 = 2(gdb) p ((table->field+0))->field_name$3 = 0x7eff94010b0c “aid”(gdb) p ((table->field+1))->field_name$4 = 0x7eff94007518 “num”(gdb) p ((table->field+0))->row_pack_length()$5 = 4(gdb) p ((table->field+1))->row_pack_length()$6 = 15(gdb) p ((table->field+0))->type()$7 = MYSQL_TYPE_LONG(gdb) p ((table->field+1))->type()$8 = MYSQL_TYPE_NEWDECIMAL(gdb)