使用Postgresql中hook，可以不重新编译中断或者改变PG行为。

1.经常使用的Hook汇总

2.Hook工作原理分析

• PG中hook如何工作

Hooks存在一个全局函数指针；并初始为NULL；当PG想使用一个hook时，检查这个全局函数指针，假如被设置，则执行函数。

• 如何设置函数指针

hook函数定义在共享库中；在加载的时候，PG调用共享库中的_PG_init()函数；_PG_init函数设置函数指针（一般会把原来的指针保存）。

• 如何清除函数指针

在卸载时，PG调用共享库中的_PG_fini()函数；_PG_fini函数需要重新设置函数指针（通常将原来的指针恢复）。

3 实例

3.1 ClientAuthentication_hook

• 定义

/* src/include/libpq/auth.h */ /* Hook for plugins to get control in ClientAuthentication() */
typedef void (*ClientAuthentication_hook_type) (Port *,int);
extern PGDLLIMPORT ClientAuthentication_hook_type ClientAuthentication_hook;

• 设置

/* src/backend/libpq/auth.c */
/*
 * This hook allows plugins to get control following client authentication,* but before the user has been informed about the results.  It could be used
 * to record login events,insert a delay after Failed authentication,etc.
 */
ClientAuthentication_hook_type ClientAuthentication_hook = NULL;

• 检查和执行

/* src/backend/libpq/auth.c */

/*
 * Client authentication starts here.  If there is an error,this
 * function does not return and the backend process is terminated.
 */
void
ClientAuthentication(Port *port)
{
    //...
    if (ClientAuthentication_hook)
        (*ClientAuthentication_hook) (port,status);
    //...
}

• 说明

控制：客户端认证之后，通知用户之前

用处：记录登录事件；认证失败后插入一段延迟

• 用到此hook的模块

auth_delay；sepgsql；connection_limits

• ClientAuthentication_hook函数介绍

两个参数 f(Port *port,int status)

Port结构位于src/include/libpq/libpq-be.h，部分字段如下

typedef struct Port
{
    pgsocket    sock;           /* File descriptor */
    bool        noblock;        /* is the socket in non-blocking mode? */
    ProtocolVersion proto;      /* FE/BE protocol version */
    SockAddr    laddr;          /* local addr (postmaster) */
    SockAddr    raddr;          /* remote addr (client) */
    char       *remote_host;    /* name (or ip addr) of remote host */
    char       *remote_hostname;/* name (not ip addr) of remote host,if
                                 * available */
    char       *remote_port;    /* text rep of remote port */
    char       *database_name;
    char       *user_name;
    char       *cmdline_options;
    List       *guc_options;
} Port;

Status为状态代码--STATUS_ERROR，STATUS_OK等

• 实例

写一个具体使用案例：假如某个文件存在则禁止连接。

需要两个函数：

1. 安装hook
2. 检查文件可用性，决定允许或拒绝连接

添加contrib/my_client_auth目录

添加文件：Makefile my_client_auth.c

/* -------------------------------------------------------------------------
 *
 * my_client_auth.c           
 *
 * Copyright (C) 2010-2011,Postgresql Global Development Group
 *
 * IDENTIFICATION
 *      contrib/my_client_auth/my_client_auth.c
 *
 * -------------------------------------------------------------------------
 */
#include <fcntl.h>
#include <sys/stat.h>
#include "postgres.h"
#include "fmgr.h"

#include "libpq/auth.h"

PG_MODULE_MAGIC;

void        _PG_init(void);

/* Original Hook */
static ClientAuthentication_hook_type next_client_auth_hook = NULL;

/*
 * Check authentication
 */
static void
my_client_auth(Port *port,int status)
{
    struct stat buf;
    if (next_client_auth_hook)
        (*next_client_auth_hook) (port,status);
    if (status != STATUS_OK)
            return;
    if(!stat("/tmp/connection.stopped",&buf))
        ereport(FATAL,(errcode(ERRCODE_INTERNAL_ERROR),errmsg("Connection not authorized!!")));
}

/*
 * Module Load Callback
 */
void
_PG_init(void)
{
    /* Install Hooks */
    next_client_auth_hook = ClientAuthentication_hook;
    ClientAuthentication_hook = my_client_auth;
}

#contrib/my_client_auth/Makefile

MODULES = my_client_auth
PGFILEDESC = "my_client_auth - Writing My ClientAuthentication_hook"

ifdef USE_PGXS
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
else
subdir = contrib/my_client_auth
top_builddir = ../..
include $(top_builddir)/src/Makefile.global
include $(top_srcdir)/contrib/contrib-global.mk
endif

data/postgres.conf配置文件汇总添加：

shared_preload_libraries = 'my_client_auth'

重启数据库

/tmp下创建connection.stopped文件

psql连接报错：

psql: FATAL: Connection not authorized!!