Function garbage_collect

src/jrd/btr.cpp:5106–5625

static contents garbage_collect(thread_db* tdbb, WIN* window, ULONG parent_number)
{
/**************************************
 *
 *	g a r b a g e _ c o l l e c t
 *
 **************************************
 *
 * Functional description
 *	Garbage collect an index page.  This requires
 * 	care so that we don't step on other processes
 * 	that might be traversing the tree forwards,
 *	backwards, or top to bottom.  We must also
 *	keep in mind that someone might be adding a node
 *	at the same time we are deleting.  Therefore we
 *	must lock all the pages involved to prevent
 *	such operations while we are garbage collecting.
 *
 **************************************/

	SET_TDBB(tdbb);
	const Database* dbb = tdbb->getDatabase();
	CHECK_DBB(dbb);

	const USHORT pageSpaceID = window->win_page.getPageSpaceID();
	btree_page* gc_page = (btree_page*) window->win_buffer;
	contents result = contents_above_threshold;

	// check to see if the page was marked not to be garbage collected
	if ( !BtrPageGCLock::isPageGCAllowed(tdbb, window->win_page) )
	{
		CCH_RELEASE(tdbb, window);
		return contents_above_threshold;
	}

	// record the left sibling now since this is the only way to
	// get to it quickly; don't worry if it's not accurate now or
	// is changed after we release the page, since we will fetch
	// it in a fault-tolerant way anyway.
	const ULONG left_number = gc_page->btr_left_sibling;

	// if the left sibling is blank, that indicates we are the leftmost page,
	// so don't garbage-collect the page; do this for several reasons:
	//   1.  The leftmost page needs a degenerate zero length node as its first node
	//       (for a non-leaf, non-top-level page).
	//   2.  The parent page would need to be fixed up to have a degenerate node
	//       pointing to the right sibling.
	//   3.  If we remove all pages on the level, we would need to re-add it next
	//       time a record is inserted, so why constantly garbage-collect and re-create
	//       this page?

	if (!left_number)
	{
		CCH_RELEASE(tdbb, window);
		return contents_above_threshold;
	}

	// record some facts for later validation