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本文的目的一是为了备忘，二是为了抛砖引玉，希望有更多的人来研究如何使用好PostgreSQL的hook机制。

在研究pg_stat_statments的源代码的时候，发现其中使用了hook机制：例如其中提到了如下几种hook：

void _PG_fini(void) { /* Uninstall hooks. */ shmem_startup_hook = prev_shmem_startup_hook; ExecutorStart_hook = prev_ExecutorStart; ExecutorRun_hook = prev_ExecutorRun; ExecutorFinish_hook = prev_ExecutorFinish; ExecutorEnd_hook = prev_ExecutorEnd; ProcessUtility_hook = prev_ProcessUtility; } 

hook机制，是一把双刃剑：

它十分强大，可以让用户有机会切入到PostgreSQL的内部运行机制中，完成用户自定义的控制登录过程、查看系统状态、收集数据库活动的统计信息，甚至控制数据库中特定活动的执行。

当然，一般而言要求hook函数用C语言来编写，如果稍有不慎可能造成数据库系统失常乃至崩溃。

如果按照如下的日文网站的列举，其种类繁多，功能丰富：

http://postgresql.g.hatena.ne.jp/pgsql/20090325

遗憾的是，PostgreSQL的官方文档中没有讲如何使用hook的。只是在release note 中隐约提及。

目前，最权威的资料，来自于：http://wiki.postgresql.org/images/e/e3/Hooks_in_postgresql.pdf

为了进行初步的学习，参考了如下的URL的例子：

http://michael.otacoo.com/postgresql-2/hooks-in-postgres-super-superuser-restrictions/

下面描述建立hook的过程：

此处使用的是 ProcessUtility_hook，作为了例子，当我们从psql等发起 drop database xxx命令的时候。

本hook被激活，然后进行检查，如果被删除的数据库名为 fooddb，那么只有用户foo才有机会删除它。

步骤1：建立contrib目录：

[root@server contrib]# pwd /soft/postgresql-9.1.2/contrib [root@server contrib]# mkdir dbrestrict [root@server contrib]# cd dbrestrict

步骤2：编写程序代码：

需要指出的是，由于我用的是postgresql9.1.2，和上述URL中所用的数据库版本有所不同，故 standard_ProcessUtility 的入口参数有所不同。

[root@server dbrestrict]# pwd /soft/postgresql-9.1.2/contrib/dbrestrict [root@server dbrestrict]# cat dbrestrict.c /* * dbrestrict.c * Restrict drop of a given database to a given super-superuser only. * Michael Paquier, 2013 * Under license "do whatever you want with that" */ #include "postgres.h" #include "miscadmin.h" #include "tcop/utility.h" PG_MODULE_MAGIC; void _PG_init(void); void _PG_fini(void); static char *undroppabledb = "foodb"; static char *supersuperuser = "foo"; static ProcessUtility_hook_type prev_utility_hook = NULL; static void dbrestrict_utility(Node *parsetree, const char *queryString, ParamListInfo params,bool isTopLevel, DestReceiver *dest,char *completionTag); /* * Personal process for DB drop restriction */
static
void dbrestrict_utility(Node *parsetree,const char *queryString,ParamListInfo params,bool isTopLevel,DestReceiver *dest,char *completionTag) { /* Do our custom process on drop database */
        switch (nodeTag(parsetree)) { case T_DropdbStmt: { DropdbStmt *stmt = (DropdbStmt *) parsetree; char *username = GetUserNameFromId(GetUserId()); /* * Check that only the authorized superuser foo can * drop the database undroppable_foodb. */
                        if (strcmp(stmt->dbname, undroppabledb) == 0 && strcmp(username, supersuperuser) != 0) ereport(ERROR, (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE), errmsg("Only super-superuser \"%s\" can drop database \"%s\"", supersuperuser, undroppabledb))); break; } default: break; } /* Fallback to normal process */ standard_ProcessUtility(parsetree, queryString, params, isTopLevel,dest,completionTag); } /* * _PG_init * Install the hook. */
void _PG_init(void) { prev_utility_hook = ProcessUtility_hook; ProcessUtility_hook = dbrestrict_utility; } /* * _PG_fini * Uninstall the hook. */
void _PG_fini(void) { ProcessUtility_hook = prev_utility_hook; } [root@server dbrestrict]# 

步骤3：建立Makefile:

[root@server dbrestrict]# cat Makefile # contrib/dbrestrict/Makefile MODULES = dbrestrict OBJS = dbrestreict.so ifdef USE_PGXS PG_CONFIG = pg_config PGXS := $(shell $(PG_CONFIG) --pgxs) include $(PGXS) else subdir = contrib/dbrestrict top_builddir = ../.. include $(top_builddir)/src/Makefile.global include $(top_srcdir)/contrib/contrib-global.mk endif [root@server dbrestrict]# 

步骤4：编译和安装:

[root@server dbrestrict]# make gcc -O2 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wformat-security -fno-strict-aliasing -fwrapv -fpic -I. -I. -I../../src/include -D_GNU_SOURCE   -c -o dbrestrict.o dbrestrict.c gcc -O2 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wformat-security -fno-strict-aliasing -fwrapv -fpic -L../../src/port  -Wl,-rpath,'/usr/local/pgsql/lib',--enable-new-dtags  -shared -o dbrestrict.so dbrestrict.o rm dbrestrict.o [root@server dbrestrict]# ls Makefile dbrestrict.c dbrestrict.so [root@server dbrestrict]# make install /bin/mkdir -p '/usr/local/pgsql/lib'
/bin/sh ../../config/install-sh -c -m 755  dbrestrict.so '/usr/local/pgsql/lib/' [root@server dbrestrict]# 

步骤5：配置postgresql.conf文件：把编译好的名为dbrestrict的动态库，追加到 shared_preload_libraries中。

[postgres@server data]$ vim postgresql.conf

[postgres@server data]$ cat postgresql.conf | grep 'preload' shared_preload_libraries = 'dbrestrict' # (change requires restart) #local_preload_libraries = '' [postgres@server data]$ 

这中 shared_preload_libraries 机制还是不错的，事实上EnterpriseDB PPAS的postgresql.conf的此行是这样的：

shared_preload_libraries = '$libdir/dbms_pipe,$libdir/edb_gen'

步骤6：运行验证：

重新启动postgresql

[postgres@server pgsql]$ ./bin/pg_ctl -D ./data start server starting [postgres@server pgsql]$ LOG: loaded library "dbrestrict" LOG: database system was shut down at 2013-06-19 21:05:17 CST LOG: autovacuum launcher started LOG: database system is ready to accept connections

可以看到，对用户 foo 和 goo，他们意图drop database foodb时候，得到的信息是不同的；如果foo是super user，就已经可以删除数据库了：

postgres=# create database foodb; CREATE DATABASE postgres=# create role foo; CREATE ROLE postgres=# \c postgres foo; FATAL: role "foo" is not permitted to log in Previous connection kept postgres=# drop role foo; DROP ROLE postgres=# create role foo login; CREATE ROLE postgres=# \c postgres foo; You are now connected to database "postgres" as user "foo". postgres=> drop database foodb; ERROR: must be owner of database foodb postgres=> postgres=# create role goo login; CREATE ROLE postgres=# \c postgres goo; You are now connected to database "postgres" as user "goo". postgres=> drop database foodb; ERROR: Only super-superuser "foo" can drop database "foodb" postgres=> 

结束！如果有可能，希望更进一步地学习PostgreSQL的各种hook，如果有人可以提供更加详细的信息，十分感谢！ 

磨砺技术珠矶，践行数据之道，追求卓越价值

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