Function down_grade

src/jrd/cch.cpp:3369–3573

static void down_grade(thread_db* tdbb, BufferDesc* bdb, int high)
{
/**************************************
 *
 *	d o w n _ g r a d e
 *
 **************************************
 *
 * Functional description
 *	A lock on a page is blocking another process.  If possible, downgrade
 *	the lock on the buffer.  This may be called from either AST or
 *	regular level.  Return true if the down grade was successful.  If the
 *	down grade was deferred for any reason, return false.
 *
 **************************************/
	SET_TDBB(tdbb);

	const bool oldBlocking = (bdb->bdb_ast_flags.exchangeBitOr(BDB_blocking) & BDB_blocking);
	Lock* lock = bdb->bdb_lock;
	Database* dbb = tdbb->getDatabase();
	BufferControl* bcb = bdb->bdb_bcb;

	if (dbb->dbb_flags & DBB_bugcheck)
	{
		PAGE_LOCK_RELEASE(tdbb, bcb, lock);
		bdb->bdb_ast_flags &= ~BDB_blocking;

		clear_dirty_flag_and_nbak_state(tdbb, bdb);
		return; // true;
	}

	// If the BufferDesc is in use and, being written or already
	// downgraded to read, mark it as blocking and exit.

	bool justWrite = false;

	if (bdb->isLocked() || !bdb->addRefConditional(tdbb, SYNC_EXCLUSIVE))
	{
		if (!high)
			return; // false;

		// hvlad: buffer is in use and we can't downgrade its lock. But if this
		// buffer blocks some lower precedence buffer, it is enough to just write
		// our (high) buffer to clear precedence and thus allow blocked (low)
		// buffer to be downgraded. IO lock guarantees that there is no buffer
		// modification in progress currently and it is safe to write it right now.
		// No need to mark our buffer as blocking nor to change state of our lock.

		bdb->lockIO(tdbb);
		if (!(bdb->bdb_flags & BDB_dirty))
		{
			bdb->unLockIO(tdbb);
			return; // true
		}

		if (!oldBlocking)
			bdb->bdb_ast_flags &= ~BDB_blocking;