MySQL 源码中的 ut_a ut_ad

Posted 2021-03-07 菁芜·Firmamentfell

在 ut0dbg.h 中,可以看到如下宏

#ifdef UNIV_DEBUG
/** Debug assertion. Does nothing unless UNIV_DEBUG is defined. */
#define ut_ad(EXPR) ut_a(EXPR)
/** Debug statement. Does nothing unless UNIV_DEBUG is defined. */
#define ut_d(EXPR) EXPR
#else
/** Debug assertion. Does nothing unless UNIV_DEBUG is defined. */
#define ut_ad(EXPR)
/** Debug statement. Does nothing unless UNIV_DEBUG is defined. */
#define ut_d(EXPR)
#endif

可见， ut_ad 是在开启 UNIV_DEBUG 下的 ut_a，ut_d 是开启 UNIV_DEBUG 下才执行 EXPR；当 UNIV_DEBUG 关闭时， EXPR 均不执行。

/** Abort execution if EXPR does not evaluate to nonzero.
@param EXPR assertion expression that should hold */
#define ut_a(EXPR)                                                 do {                                                               if (UNIV_UNLIKELY(!(ulint)(EXPR))) {                               ut_dbg_assertion_failed(#EXPR, __FILE__, (ulint)__LINE__);     }                                                              } while (0)

/* Tell the compiler that cond is unlikely to hold */
#define UNIV_UNLIKELY(cond) UNIV_EXPECT(cond, FALSE)

例子如下，在开启 UNIV_DEBUG 时，会执行 lock_mutex_own() 和 trx_mutex_own(),并且会在其返回值为 0 时报错。

const trx_t *DeadlockChecker::check_and_resolve(const lock_t *lock,
                                                trx_t *trx) {
  ut_ad(lock_mutex_own());
  ut_ad(trx_mutex_own(trx));
  check_trx_state(trx);
  ut_ad(!srv_read_only_mode);