postgres/src/backend/access/nbtree/nbtpage.c

/*-------------------------------------------------------------------------
 *
 * nbtpage.c
 *	  BTree-specific page management code for the Postgres btree access
 *	  method.
 *
 * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtpage.c,v 1.60 2003/02/22 00:45:04 tgl Exp $
 *
 *	NOTES
 *	   Postgres btree pages look like ordinary relation pages.	The opaque
 *	   data at high addresses includes pointers to left and right siblings
 *	   and flag data describing page state.  The first page in a btree, page
 *	   zero, is special -- it stores meta-information describing the tree.
 *	   Pages one and higher store the actual tree data.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/nbtree.h"
#include "miscadmin.h"
#include "storage/lmgr.h"


/*
 *	_bt_metapinit() -- Initialize the metadata page of a new btree.
 *
 * Note: there's no real need for any locking here.  Since the transaction
 * creating the index hasn't committed yet, no one else can even see the index
 * much less be trying to use it.
 */
void
_bt_metapinit(Relation rel)
{
	Buffer		buf;
	Page		pg;
	BTMetaPageData *metad;
	BTPageOpaque op;

	if (RelationGetNumberOfBlocks(rel) != 0)
		elog(ERROR, "Cannot initialize non-empty btree %s",
			 RelationGetRelationName(rel));

	buf = ReadBuffer(rel, P_NEW);
	Assert(BufferGetBlockNumber(buf) == BTREE_METAPAGE);
	pg = BufferGetPage(buf);

	/* NO ELOG(ERROR) from here till newmeta op is logged */
	START_CRIT_SECTION();

	_bt_pageinit(pg, BufferGetPageSize(buf));

	metad = BTPageGetMeta(pg);
	metad->btm_magic = BTREE_MAGIC;
	metad->btm_version = BTREE_VERSION;
	metad->btm_root = P_NONE;
	metad->btm_level = 0;
	metad->btm_fastroot = P_NONE;
	metad->btm_fastlevel = 0;

	op = (BTPageOpaque) PageGetSpecialPointer(pg);
	op->btpo_flags = BTP_META;

	/* XLOG stuff */
	if (!rel->rd_istemp)
	{
		xl_btree_newmeta xlrec;
		XLogRecPtr	recptr;
		XLogRecData rdata[1];

		xlrec.node = rel->rd_node;
		xlrec.meta.root = metad->btm_root;
		xlrec.meta.level = metad->btm_level;
		xlrec.meta.fastroot = metad->btm_fastroot;
		xlrec.meta.fastlevel = metad->btm_fastlevel;

		rdata[0].buffer = InvalidBuffer;
		rdata[0].data = (char *) &xlrec;
		rdata[0].len = SizeOfBtreeNewmeta;
		rdata[0].next = NULL;

		recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_NEWMETA, rdata);

		PageSetLSN(pg, recptr);
		PageSetSUI(pg, ThisStartUpID);
	}

	END_CRIT_SECTION();

	WriteBuffer(buf);
}

/*
 *	_bt_getroot() -- Get the root page of the btree.
 *
 *		Since the root page can move around the btree file, we have to read
 *		its location from the metadata page, and then read the root page
 *		itself.  If no root page exists yet, we have to create one.  The
 *		standard class of race conditions exists here; I think I covered
 *		them all in the Hopi Indian rain dance of lock requests below.
 *
 *		The access type parameter (BT_READ or BT_WRITE) controls whether
 *		a new root page will be created or not.  If access = BT_READ,
 *		and no root page exists, we just return InvalidBuffer.	For
 *		BT_WRITE, we try to create the root page if it doesn't exist.
 *		NOTE that the returned root page will have only a read lock set
 *		on it even if access = BT_WRITE!
 *
 *		The returned page is not necessarily the true root --- it could be
 *		a "fast root" (a page that is alone in its level due to deletions).
 *		Also, if the root page is split while we are "in flight" to it,
 *		what we will return is the old root, which is now just the leftmost
 *		page on a probably-not-very-wide level.  For most purposes this is
 *		as good as or better than the true root, so we do not bother to
 *		insist on finding the true root.  We do, however, guarantee to
 *		return a live (not deleted or half-dead) page.
 *
 *		On successful return, the root page is pinned and read-locked.
 *		The metadata page is not locked or pinned on exit.
 */
Buffer
_bt_getroot(Relation rel, int access)
{
	Buffer		metabuf;
	Page		metapg;
	BTPageOpaque metaopaque;
	Buffer		rootbuf;
	Page		rootpage;
	BTPageOpaque rootopaque;
	BlockNumber rootblkno;
	uint32		rootlevel;
	BTMetaPageData *metad;

	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
	metapg = BufferGetPage(metabuf);
	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
	metad = BTPageGetMeta(metapg);

	/* sanity-check the metapage */
	if (!(metaopaque->btpo_flags & BTP_META) ||
		metad->btm_magic != BTREE_MAGIC)
		elog(ERROR, "Index %s is not a btree",
			 RelationGetRelationName(rel));

	if (metad->btm_version != BTREE_VERSION)
		elog(ERROR, "Version mismatch on %s: version %d file, version %d code",
			 RelationGetRelationName(rel),
			 metad->btm_version, BTREE_VERSION);

	/* if no root page initialized yet, do it */
	if (metad->btm_root == P_NONE)
	{
		/* If access = BT_READ, caller doesn't want us to create root yet */
		if (access == BT_READ)
		{
			_bt_relbuf(rel, metabuf);
			return InvalidBuffer;
		}

		/* trade in our read lock for a write lock */
		LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
		LockBuffer(metabuf, BT_WRITE);

		/*
		 * Race condition:	if someone else initialized the metadata
		 * between the time we released the read lock and acquired the
		 * write lock, we must avoid doing it again.
		 */
		if (metad->btm_root != P_NONE)
		{
			/*
			 * Metadata initialized by someone else.  In order to
			 * guarantee no deadlocks, we have to release the metadata
			 * page and start all over again.  (Is that really true?
			 * But it's hardly worth trying to optimize this case.)
			 */
			_bt_relbuf(rel, metabuf);
			return _bt_getroot(rel, access);
		}

		/*
		 * Get, initialize, write, and leave a lock of the appropriate
		 * type on the new root page.  Since this is the first page in
		 * the tree, it's a leaf as well as the root.
		 */
		rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
		rootblkno = BufferGetBlockNumber(rootbuf);
		rootpage = BufferGetPage(rootbuf);

		_bt_pageinit(rootpage, BufferGetPageSize(rootbuf));
		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
		rootopaque->btpo_prev = rootopaque->btpo_next = P_NONE;
		rootopaque->btpo_flags = (BTP_LEAF | BTP_ROOT);
		rootopaque->btpo.level = 0;

		/* NO ELOG(ERROR) till meta is updated */
		START_CRIT_SECTION();

		metad->btm_root = rootblkno;
		metad->btm_level = 0;
		metad->btm_fastroot = rootblkno;
		metad->btm_fastlevel = 0;

		/* XLOG stuff */
		if (!rel->rd_istemp)
		{
			xl_btree_newroot xlrec;
			XLogRecPtr	recptr;
			XLogRecData rdata;

			xlrec.node = rel->rd_node;
			xlrec.rootblk = rootblkno;
			xlrec.level = 0;

			rdata.buffer = InvalidBuffer;
			rdata.data = (char *) &xlrec;
			rdata.len = SizeOfBtreeNewroot;
			rdata.next = NULL;

			recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_NEWROOT, &rdata);

			PageSetLSN(rootpage, recptr);
			PageSetSUI(rootpage, ThisStartUpID);
			PageSetLSN(metapg, recptr);
			PageSetSUI(metapg, ThisStartUpID);
		}

		END_CRIT_SECTION();

		_bt_wrtnorelbuf(rel, rootbuf);

		/*
		 * swap root write lock for read lock.  There is no danger of
		 * anyone else accessing the new root page while it's unlocked,
		 * since no one else knows where it is yet.
		 */
		LockBuffer(rootbuf, BUFFER_LOCK_UNLOCK);
		LockBuffer(rootbuf, BT_READ);

		/* okay, metadata is correct, write and release it */
		_bt_wrtbuf(rel, metabuf);
	}
	else
	{
		rootblkno = metad->btm_fastroot;
		Assert(rootblkno != P_NONE);
		rootlevel = metad->btm_fastlevel;

		_bt_relbuf(rel, metabuf);		/* done with the meta page */

		for (;;)
		{
			rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
			rootpage = BufferGetPage(rootbuf);
			rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);

			if (!P_IGNORE(rootopaque))
				break;

			/* it's dead, Jim.  step right one page */
			if (P_RIGHTMOST(rootopaque))
				elog(ERROR, "No live root page found in %s",
					 RelationGetRelationName(rel));
			rootblkno = rootopaque->btpo_next;

			_bt_relbuf(rel, rootbuf);
		}

		/* Note: can't check btpo.level on deleted pages */
		if (rootopaque->btpo.level != rootlevel)
			elog(ERROR, "Root page %u of %s has level %u, expected %u",
				 rootblkno, RelationGetRelationName(rel),
				 rootopaque->btpo.level, rootlevel);
	}

	/*
	 * By here, we have a pin and read lock on the root page, and no
	 * lock set on the metadata page.  Return the root page's buffer.
	 */
	return rootbuf;
}

/*
 *	_bt_gettrueroot() -- Get the true root page of the btree.
 *
 *		This is the same as the BT_READ case of _bt_getroot(), except
 *		we follow the true-root link not the fast-root link.
 *
 * By the time we acquire lock on the root page, it might have been split and
 * not be the true root anymore.  This is okay for the present uses of this
 * routine; we only really need to be able to move up at least one tree level
 * from whatever non-root page we were at.  If we ever do need to lock the
 * one true root page, we could loop here, re-reading the metapage on each
 * failure.  (Note that it wouldn't do to hold the lock on the metapage while
 * moving to the root --- that'd deadlock against any concurrent root split.)
 */
Buffer
_bt_gettrueroot(Relation rel)
{
	Buffer		metabuf;
	Page		metapg;
	BTPageOpaque metaopaque;
	Buffer		rootbuf;
	Page		rootpage;
	BTPageOpaque rootopaque;
	BlockNumber rootblkno;
	uint32		rootlevel;
	BTMetaPageData *metad;

	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
	metapg = BufferGetPage(metabuf);
	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
	metad = BTPageGetMeta(metapg);

	if (!(metaopaque->btpo_flags & BTP_META) ||
		metad->btm_magic != BTREE_MAGIC)
		elog(ERROR, "Index %s is not a btree",
			 RelationGetRelationName(rel));

	if (metad->btm_version != BTREE_VERSION)
		elog(ERROR, "Version mismatch on %s: version %d file, version %d code",
			 RelationGetRelationName(rel),
			 metad->btm_version, BTREE_VERSION);

	/* if no root page initialized yet, fail */
	if (metad->btm_root == P_NONE)
	{
		_bt_relbuf(rel, metabuf);
		return InvalidBuffer;
	}

	rootblkno = metad->btm_root;
	rootlevel = metad->btm_level;

	_bt_relbuf(rel, metabuf);	/* done with the meta page */

	for (;;)
	{
		rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
		rootpage = BufferGetPage(rootbuf);
		rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);

		if (!P_IGNORE(rootopaque))
			break;

		/* it's dead, Jim.  step right one page */
		if (P_RIGHTMOST(rootopaque))
			elog(ERROR, "No live root page found in %s",
				 RelationGetRelationName(rel));
		rootblkno = rootopaque->btpo_next;

		_bt_relbuf(rel, rootbuf);
	}

	/* Note: can't check btpo.level on deleted pages */
	if (rootopaque->btpo.level != rootlevel)
		elog(ERROR, "Root page %u of %s has level %u, expected %u",
			 rootblkno, RelationGetRelationName(rel),
			 rootopaque->btpo.level, rootlevel);

	return rootbuf;
}

/*
 *	_bt_getbuf() -- Get a buffer by block number for read or write.
 *
 *		blkno == P_NEW means to get an unallocated index page.
 *
 *		When this routine returns, the appropriate lock is set on the
 *		requested buffer and its reference count has been incremented
 *		(ie, the buffer is "locked and pinned").
 */
Buffer
_bt_getbuf(Relation rel, BlockNumber blkno, int access)
{
	Buffer		buf;

	if (blkno != P_NEW)
	{
		/* Read an existing block of the relation */
		buf = ReadBuffer(rel, blkno);
		LockBuffer(buf, access);
	}
	else
	{
		bool		needLock;
		Page		page;

		/* XXX soon: ask FSM about free space */

		/*
		 * Extend the relation by one page.
		 *
		 * We have to use a lock to ensure no one else is extending the rel at
		 * the same time, else we will both try to initialize the same new
		 * page.  We can skip locking for new or temp relations, however,
		 * since no one else could be accessing them.
		 */
		needLock = !(rel->rd_isnew || rel->rd_istemp);

		if (needLock)
			LockPage(rel, 0, ExclusiveLock);

		buf = ReadBuffer(rel, P_NEW);

		/*
		 * Release the file-extension lock; it's now OK for someone else to
		 * extend the relation some more.
		 */
		if (needLock)
			UnlockPage(rel, 0, ExclusiveLock);

		/* Acquire appropriate buffer lock on new page */
		LockBuffer(buf, access);

		/* Initialize the new page before returning it */
		page = BufferGetPage(buf);
		_bt_pageinit(page, BufferGetPageSize(buf));
	}

	/* ref count and lock type are correct */
	return buf;
}

/*
 *	_bt_relbuf() -- release a locked buffer.
 *
 * Lock and pin (refcount) are both dropped.  Note that either read or
 * write lock can be dropped this way, but if we modified the buffer,
 * this is NOT the right way to release a write lock.
 */
void
_bt_relbuf(Relation rel, Buffer buf)
{
	LockBuffer(buf, BUFFER_LOCK_UNLOCK);
	ReleaseBuffer(buf);
}

/*
 *	_bt_wrtbuf() -- write a btree page to disk.
 *
 *		This routine releases the lock held on the buffer and our refcount
 *		for it.  It is an error to call _bt_wrtbuf() without a write lock
 *		and a pin on the buffer.
 *
 * NOTE: actually, the buffer manager just marks the shared buffer page
 * dirty here; the real I/O happens later.  This is okay since we are not
 * relying on write ordering anyway.  The WAL mechanism is responsible for
 * guaranteeing correctness after a crash.
 */
void
_bt_wrtbuf(Relation rel, Buffer buf)
{
	LockBuffer(buf, BUFFER_LOCK_UNLOCK);
	WriteBuffer(buf);
}

/*
 *	_bt_wrtnorelbuf() -- write a btree page to disk, but do not release
 *						 our reference or lock.
 *
 *		It is an error to call _bt_wrtnorelbuf() without a write lock
 *		and a pin on the buffer.
 *
 * See above NOTE.
 */
void
_bt_wrtnorelbuf(Relation rel, Buffer buf)
{
	WriteNoReleaseBuffer(buf);
}

/*
 *	_bt_pageinit() -- Initialize a new page.
 *
 * On return, the page header is initialized; data space is empty;
 * special space is zeroed out.
 */
void
_bt_pageinit(Page page, Size size)
{
	PageInit(page, size, sizeof(BTPageOpaqueData));
}

/*
 *	_bt_metaproot() -- Change the root page of the btree.
 *
 *		Lehman and Yao require that the root page move around in order to
 *		guarantee deadlock-free short-term, fine-granularity locking.  When
 *		we split the root page, we record the new parent in the metadata page
 *		for the relation.  This routine does the work.
 *
 *		No direct preconditions, but if you don't have the write lock on
 *		at least the old root page when you call this, you're making a big
 *		mistake.  On exit, metapage data is correct and we no longer have
 *		a pin or lock on the metapage.
 *
 * Actually this is not used for splitting on-the-fly anymore.  It's only used
 * in nbtsort.c at the completion of btree building, where we know we have
 * sole access to the index anyway.
 */
void
_bt_metaproot(Relation rel, BlockNumber rootbknum, uint32 level)
{
	Buffer		metabuf;
	Page		metap;
	BTPageOpaque metaopaque;
	BTMetaPageData *metad;

	metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
	metap = BufferGetPage(metabuf);
	metaopaque = (BTPageOpaque) PageGetSpecialPointer(metap);
	Assert(metaopaque->btpo_flags & BTP_META);

	/* NO ELOG(ERROR) from here till newmeta op is logged */
	START_CRIT_SECTION();

	metad = BTPageGetMeta(metap);
	metad->btm_root = rootbknum;
	metad->btm_level = level;
	metad->btm_fastroot = rootbknum;
	metad->btm_fastlevel = level;

	/* XLOG stuff */
	if (!rel->rd_istemp)
	{
		xl_btree_newmeta xlrec;
		XLogRecPtr	recptr;
		XLogRecData rdata[1];

		xlrec.node = rel->rd_node;
		xlrec.meta.root = metad->btm_root;
		xlrec.meta.level = metad->btm_level;
		xlrec.meta.fastroot = metad->btm_fastroot;
		xlrec.meta.fastlevel = metad->btm_fastlevel;

		rdata[0].buffer = InvalidBuffer;
		rdata[0].data = (char *) &xlrec;
		rdata[0].len = SizeOfBtreeNewmeta;
		rdata[0].next = NULL;

		recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_NEWMETA, rdata);

		PageSetLSN(metap, recptr);
		PageSetSUI(metap, ThisStartUpID);
	}

	END_CRIT_SECTION();

	_bt_wrtbuf(rel, metabuf);
}

/*
 * Delete an item from a btree page.
 *
 * This must only be used for deleting leaf items.  Deleting an item on a
 * non-leaf page has to be done as part of an atomic action that includes
 * deleting the page it points to.
 *
 * This routine assumes that the caller has pinned and locked the buffer,
 * and will write the buffer afterwards.
 */
void
_bt_itemdel(Relation rel, Buffer buf, ItemPointer tid)
{
	Page		page = BufferGetPage(buf);
	OffsetNumber offno;

	offno = ItemPointerGetOffsetNumber(tid);

	START_CRIT_SECTION();

	PageIndexTupleDelete(page, offno);

	/* XLOG stuff */
	if (!rel->rd_istemp)
	{
		xl_btree_delete xlrec;
		XLogRecPtr	recptr;
		XLogRecData rdata[2];

		xlrec.target.node = rel->rd_node;
		xlrec.target.tid = *tid;

		rdata[0].buffer = InvalidBuffer;
		rdata[0].data = (char *) &xlrec;
		rdata[0].len = SizeOfBtreeDelete;
		rdata[0].next = &(rdata[1]);

		rdata[1].buffer = buf;
		rdata[1].data = NULL;
		rdata[1].len = 0;
		rdata[1].next = NULL;

		recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_DELETE, rdata);

		PageSetLSN(page, recptr);
		PageSetSUI(page, ThisStartUpID);
	}

	END_CRIT_SECTION();
}