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);
}
}