PostgreSQL資料庫插件——pgaudit初始化

_PG_init首先使用DefineCustomTypeVariable定義GUC變量，以pgaudit.log為例，auditLog是pgaudit.c中定義的char *指針變量，check_pgaudit_log和assign_pgaudit_log是pgaudit.c中定義的函數。

/* Define GUC variables and install hooks upon module load. */
void _PG_init(void) {
    ...
    /* Define pgaudit.log */
    DefineCustomStringVariable(
        "pgaudit.log",
        "Specifies which classes of statements will be logged by session audit "
        "logging. Multiple classes can be provided using a comma-separated "
        "list and classes can be subtracted by prefacing the class with a "
        "- sign.",
        NULL,
        &auditLog,
        "none",
        PGC_SUSET,
        GUC_LIST_INPUT | GUC_NOT_IN_SAMPLE,
        check_pgaudit_log,
        assign_pgaudit_log,
        NULL);
    ....      

将我們定義的鈎子加入PG的鈎子鍊。如下圖就是pgaudit.c中定義的臨時鈎子指針變量，用于儲存PG已經引入的鈎子變量，然後将我們定義的鈎子挂在标準的鈎子上，也就是我們熟悉的單連結清單頭插法。

/* Install our hook functions after saving the existing pointers to preserve the chains. */
    next_ExecutorStart_hook = ExecutorStart_hook;
    ExecutorStart_hook = pgaudit_ExecutorStart_hook;

    next_ExecutorCheckPerms_hook = ExecutorCheckPerms_hook;
    ExecutorCheckPerms_hook = pgaudit_ExecutorCheckPerms_hook;

    next_ProcessUtility_hook = ProcessUtility_hook;
    ProcessUtility_hook = pgaudit_ProcessUtility_hook;

    next_object_access_hook = object_access_hook;
    object_access_hook = pgaudit_object_access_hook;

    /* The following hook functions are required to get rows */
    next_ExecutorRun_hook = ExecutorRun_hook;
    ExecutorRun_hook = pgaudit_ExecutorRun_hook;

    next_ExecutorEnd_hook = ExecutorEnd_hook;
    ExecutorEnd_hook = pgaudit_ExecutorEnd_hook;      

以pgaudit_ExecutorStart_hook為例，先執行pgaudit_ExecutorStart_hook鈎子函數，最後判斷後續有沒有鈎子需要繼續執行，沒有就執行執行PG标準應該執行的函數。