Function get_oldest_buffer

src/jrd/cch.cpp:3667–3775

static BufferDesc* get_oldest_buffer(thread_db* tdbb, BufferControl* bcb)
{
/**************************************
 * Function description:
 *       Get candidate for preemption
 *       Found page buffer must have SYNC_EXCLUSIVE lock.
 **************************************/

	int walk = bcb->bcb_free_minimum;
	BufferDesc* bdb = nullptr;

	Sync lruSync(&bcb->bcb_syncLRU, FB_FUNCTION);
	if (bcb->bcb_lru_chain.load() != NULL)
	{
		lruSync.lock(SYNC_EXCLUSIVE);
		requeueRecentlyUsed(bcb);
		lruSync.downgrade(SYNC_SHARED);
	}
	else
		lruSync.lock(SYNC_SHARED);

	for (QUE que_inst = bcb->bcb_in_use.que_backward;
		 que_inst != &bcb->bcb_in_use;
		 que_inst = que_inst->que_backward)
	{
		bdb = nullptr;

		// get the oldest buffer as the least recently used -- note
		// that since there are no empty buffers this queue cannot be empty

		if (bcb->bcb_in_use.que_forward == &bcb->bcb_in_use)
			BUGCHECK(213);	// msg 213 insufficient cache size

		BufferDesc* oldest = BLOCK(que_inst, BufferDesc, bdb_in_use);

		if (oldest->bdb_flags & BDB_lru_chained)
			continue;

		if (oldest->bdb_use_count || !oldest->addRefConditional(tdbb, SYNC_EXCLUSIVE))
			continue;

		/*if (!writeable(oldest))
		{
			oldest->release(tdbb, true);
			continue;
		}*/

		bdb = oldest;
		if (!(bdb->bdb_flags & (BDB_dirty | BDB_db_dirty)) || !walk)
			break;

		if (!(bcb->bcb_flags & BCB_cache_writer))
			break;

		bcb->bcb_flags |= BCB_free_pending;
		if (!(bcb->bcb_flags & BCB_writer_active))
			bcb->bcb_writer_sem.release();