PostgreSQL插件hook机制

internal_load_library postgresql->
	PG_init = (PG_init_t) pg_dlsym(file_scanner->handle, "_PG_init");
	if (PG_init)
			(*PG_init) ();
internal_unload_library(const char *libname)->
	PG_fini = (PG_fini_t) pg_dlsym(file_scanner->handle, "_PG_fini");
			if (PG_fini)
				(*PG_fini) ();
		
		
以ClientAuthentication_hook_type为例
auth.h:
//声明插件使用的函数
extern void ClientAuthentication(Port *port);
/* Hook for plugins to get control in ClientAuthentication() */
typedef void (*ClientAuthentication_hook_type) (Port *, int);
extern PGDLLIMPORT ClientAuthentication_hook_type ClientAuthentication_hook;
auth.c:
//全局变量初始化为NULL，在_PG_init函数中进行初始化赋值，如果该插件加载，则ClientAuthentication_hook为
ClientAuthentication_hook_type ClientAuthentication_hook = NULL;
//如果ClientAuthentication_hook被赋值则执行植入的代码
InitPostgres->PerformAuthentication->ClientAuthentication->
	if (ClientAuthentication_hook)
		(*ClientAuthentication_hook) (port, status);
auth_delay.c:
static ClientAuthentication_hook_type original_client_auth_hook = NULL;
/*
 * Module Load Callback
 */
void _PG_init(void)
{
	/* Define custom GUC variables */
	DefineCustomIntVariable("auth_delay.milliseconds",
							"Milliseconds to delay before reporting authentication failure",
							NULL,
							&auth_delay_milliseconds,
							0,
							0, INT_MAX / 1000,
							PGC_SIGHUP,
							GUC_UNIT_MS,
							NULL,
							NULL,
							NULL);
	/* Install Hooks */
	original_client_auth_hook = ClientAuthentication_hook;
	ClientAuthentication_hook = auth_delay_checks;
}
/*
如果卸载则调用该函数，实际上是将ClientAuthentication_hook赋回原值
*/
void
_PG_fini(void)
{
	ClientAuthentication_hook=original_client_auth_hook；
}
/*
*/
static void auth_delay_checks(Port *port, int status)
{
	if (original_client_auth_hook)
		original_client_auth_hook(port, status);
	if (status != STATUS_OK){
		pg_usleep(1000L * auth_delay_milliseconds);
	}
}