postgres/src/backend/access/transam/slru.c

/*-------------------------------------------------------------------------
 *
 * slru.c
 *		Simple LRU buffering for transaction status logfiles
 *
 * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * $Header: /cvsroot/pgsql/src/backend/access/transam/slru.c,v 1.2 2003/07/19 21:37:37 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <fcntl.h>
#include <dirent.h>
#include <errno.h>
#include <sys/stat.h>
#include <unistd.h>

#include "access/slru.h"
#include "storage/lwlock.h"
#include "miscadmin.h"


/*
 * Define segment size.  A page is the same BLCKSZ as is used everywhere
 * else in Postgres.  The segment size can be chosen somewhat arbitrarily;
 * we make it 32 pages by default, or 256Kb, i.e. 1M transactions for CLOG
 * or 64K transactions for SUBTRANS.
 *
 * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
 * page numbering also wraps around at 0xFFFFFFFF/xxxx_XACTS_PER_PAGE (where
 * xxxx is CLOG or SUBTRANS, respectively), and segment numbering at
 * 0xFFFFFFFF/xxxx_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT.  We need
 * take no explicit notice of that fact in this module, except when comparing
 * segment and page numbers in SimpleLruTruncate (see PagePrecedes()).
 */

#define SLRU_PAGES_PER_SEGMENT	32


/*----------
 * Shared-memory data structures for SLRU control
 *
 * We use a simple least-recently-used scheme to manage a pool of page
 * buffers.  Under ordinary circumstances we expect that write
 * traffic will occur mostly to the latest page (and to the just-prior
 * page, soon after a page transition).  Read traffic will probably touch
 * a larger span of pages, but in any case a fairly small number of page
 * buffers should be sufficient.  So, we just search the buffers using plain
 * linear search; there's no need for a hashtable or anything fancy.
 * The management algorithm is straight LRU except that we will never swap
 * out the latest page (since we know it's going to be hit again eventually).
 *
 * We use a control LWLock to protect the shared data structures, plus
 * per-buffer LWLocks that synchronize I/O for each buffer.  A process
 * that is reading in or writing out a page buffer does not hold the control
 * lock, only the per-buffer lock for the buffer it is working on.
 *
 * To change the page number or state of a buffer, one must normally hold
 * the control lock.  (The sole exception to this rule is that a writer
 * process changes the state from DIRTY to WRITE_IN_PROGRESS while holding
 * only the per-buffer lock.)  If the buffer's state is neither EMPTY nor
 * CLEAN, then there may be processes doing (or waiting to do) I/O on the
 * buffer, so the page number may not be changed, and the only allowed state
 * transition is to change WRITE_IN_PROGRESS to DIRTY after dirtying the page.
 * To do any other state transition involving a buffer with potential I/O
 * processes, one must hold both the per-buffer lock and the control lock.
 * (Note the control lock must be acquired second; do not wait on a buffer
 * lock while holding the control lock.)  A process wishing to read a page
 * marks the buffer state as READ_IN_PROGRESS, then drops the control lock,
 * acquires the per-buffer lock, and rechecks the state before proceeding.
 * This recheck takes care of the possibility that someone else already did
 * the read, while the early marking prevents someone else from trying to
 * read the same page into a different buffer.
 *
 * Note we are assuming that read and write of the state value is atomic,
 * since I/O processes may examine and change the state while not holding
 * the control lock.
 *
 * As with the regular buffer manager, it is possible for another process
 * to re-dirty a page that is currently being written out.	This is handled
 * by setting the page's state from WRITE_IN_PROGRESS to DIRTY.  The writing
 * process must notice this and not mark the page CLEAN when it's done.
 *----------
 */

typedef enum
{
	SLRU_PAGE_EMPTY,			/* buffer is not in use */
	SLRU_PAGE_READ_IN_PROGRESS, /* page is being read in */
	SLRU_PAGE_CLEAN,			/* page is valid and not dirty */
	SLRU_PAGE_DIRTY,			/* page is valid but needs write */
	SLRU_PAGE_WRITE_IN_PROGRESS /* page is being written out */
} SlruPageStatus;

/*
 * Shared-memory state
 */
typedef struct SlruSharedData
{
	/*
	 * Info for each buffer slot.  Page number is undefined when status is
	 * EMPTY.  lru_count is essentially the number of page switches since
	 * last use of this page; the page with highest lru_count is the best
	 * candidate to replace.
	 */
	char	   *page_buffer[NUM_CLOG_BUFFERS];
	SlruPageStatus page_status[NUM_CLOG_BUFFERS];
	int			page_number[NUM_CLOG_BUFFERS];
	unsigned int page_lru_count[NUM_CLOG_BUFFERS];

	/*
	 * latest_page_number is the page number of the current end of the
	 * CLOG; this is not critical data, since we use it only to avoid
	 * swapping out the latest page.
	 */
	int			latest_page_number;
} SlruSharedData;
typedef SlruSharedData *SlruShared;


#define SlruFileName(ctl, path, seg) \
	snprintf(path, MAXPGPATH, "%s/%04X", (ctl)->Dir, seg)

/*
 * Macro to mark a buffer slot "most recently used".
 */
#define SlruRecentlyUsed(shared, slotno)	\
	do { \
		if ((shared)->page_lru_count[slotno] != 0) { \
			int		iilru; \
			for (iilru = 0; iilru < NUM_CLOG_BUFFERS; iilru++) \
				(shared)->page_lru_count[iilru]++; \
			(shared)->page_lru_count[slotno] = 0; \
		} \
	} while (0)

/* Saved info for SlruReportIOError */
typedef enum
{
	SLRU_OPEN_FAILED,
	SLRU_CREATE_FAILED,
	SLRU_SEEK_FAILED,
	SLRU_READ_FAILED,
	SLRU_WRITE_FAILED
} SlruErrorCause;
static SlruErrorCause slru_errcause;
static int	slru_errno;


static bool SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno);
static bool SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno);
static void SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid);
static int	SlruSelectLRUPage(SlruCtl ctl, int pageno);
static bool SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions);


/*
 * Initialization of shared memory
 */

int
SimpleLruShmemSize(void)
{
	return MAXALIGN(sizeof(SlruSharedData)) + BLCKSZ * NUM_CLOG_BUFFERS
#ifdef EXEC_BACKEND
			+ MAXALIGN(sizeof(SlruLockData))
#endif
	;
}

void
SimpleLruInit(SlruCtl ctl, const char *name, const char *subdir)
{
	bool		found;
	char	   *ptr;
	SlruShared	shared;
	SlruLock	locks;

	ptr = ShmemInitStruct(name, SimpleLruShmemSize(), &found);
	shared = (SlruShared) ptr;

#ifdef EXEC_BACKEND
	/*
	 * Locks are in shared memory
	 */
	locks = (SlruLock)(ptr + MAXALIGN(sizeof(SlruSharedData)) +
	                         BLCKSZ * NUM_CLOG_BUFFERS);
#else
	/*
	 * Locks are in private memory
	 */
	Assert(!IsUnderPostmaster);
	locks = malloc(sizeof(SlruLockData));
	Assert(locks);
#endif


	if (!IsUnderPostmaster)
	/* Initialize locks and shared memory area */
	{
		char	   *bufptr;
		int			slotno;

		Assert(!found);

		locks->ControlLock = LWLockAssign();

		memset(shared, 0, sizeof(SlruSharedData));

		bufptr = (char *)shared + MAXALIGN(sizeof(SlruSharedData));
	
		for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
		{
			locks->BufferLocks[slotno] = LWLockAssign();
			shared->page_buffer[slotno] = bufptr;
			shared->page_status[slotno] = SLRU_PAGE_EMPTY;
			shared->page_lru_count[slotno] = 1;
			bufptr += BLCKSZ;
		}

		/* shared->latest_page_number will be set later */
	}
	else
		Assert(found);


	ctl->locks = locks;
	ctl->shared = shared;


	/* Init directory path */
	snprintf(ctl->Dir, MAXPGPATH, "%s/%s", DataDir, subdir);
}

/*
 * Initialize (or reinitialize) a page to zeroes.
 *
 * The page is not actually written, just set up in shared memory.
 * The slot number of the new page is returned.
 *
 * Control lock must be held at entry, and will be held at exit.
 */
int
SimpleLruZeroPage(SlruCtl ctl, int pageno)
{
	int			slotno;
	SlruShared shared = (SlruShared) ctl->shared;

	/* Find a suitable buffer slot for the page */
	slotno = SlruSelectLRUPage(ctl, pageno);
	Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
		   shared->page_status[slotno] == SLRU_PAGE_CLEAN ||
		   shared->page_number[slotno] == pageno);

	/* Mark the slot as containing this page */
	shared->page_number[slotno] = pageno;
	shared->page_status[slotno] = SLRU_PAGE_DIRTY;
	SlruRecentlyUsed(shared, slotno);

	/* Set the buffer to zeroes */
	MemSet(shared->page_buffer[slotno], 0, BLCKSZ);

	/* Assume this page is now the latest active page */
	shared->latest_page_number = pageno;

	return slotno;
}

/*
 * Find a page in a shared buffer, reading it in if necessary.
 * The page number must correspond to an already-initialized page.
 *
 * The passed-in xid is used only for error reporting, and may be
 * InvalidTransactionId if no specific xid is associated with the action.
 *
 * Return value is the shared-buffer address of the page.
 * The buffer's LRU access info is updated.
 * If forwrite is true, the buffer is marked as dirty.
 *
 * Control lock must be held at entry, and will be held at exit.
 */
char *
SimpleLruReadPage(SlruCtl ctl, int pageno, TransactionId xid, bool forwrite)
{
	SlruShared shared = (SlruShared) ctl->shared;

	/* Outer loop handles restart if we lose the buffer to someone else */
	for (;;)
	{
		int			slotno;
		bool		ok;

		/* See if page already is in memory; if not, pick victim slot */
		slotno = SlruSelectLRUPage(ctl, pageno);

		/* Did we find the page in memory? */
		if (shared->page_number[slotno] == pageno &&
			shared->page_status[slotno] != SLRU_PAGE_EMPTY)
		{
			/* If page is still being read in, we cannot use it yet */
			if (shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS)
			{
				/* otherwise, it's ready to use */
				SlruRecentlyUsed(shared, slotno);
				if (forwrite)
					shared->page_status[slotno] = SLRU_PAGE_DIRTY;
				return shared->page_buffer[slotno];
			}
		}
		else
		{
			/* We found no match; assert we selected a freeable slot */
			Assert(shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
				   shared->page_status[slotno] == SLRU_PAGE_CLEAN);
		}

		/* Mark the slot read-busy (no-op if it already was) */
		shared->page_number[slotno] = pageno;
		shared->page_status[slotno] = SLRU_PAGE_READ_IN_PROGRESS;

		/*
		 * Temporarily mark page as recently-used to discourage
		 * SlruSelectLRUPage from selecting it again for someone else.
		 */
		SlruRecentlyUsed(shared, slotno);

		/* Release shared lock, grab per-buffer lock instead */
		LWLockRelease(ctl->locks->ControlLock);
		LWLockAcquire(ctl->locks->BufferLocks[slotno], LW_EXCLUSIVE);

		/*
		 * Check to see if someone else already did the read, or took the
		 * buffer away from us.  If so, restart from the top.
		 */
		if (shared->page_number[slotno] != pageno ||
			shared->page_status[slotno] != SLRU_PAGE_READ_IN_PROGRESS)
		{
			LWLockRelease(ctl->locks->BufferLocks[slotno]);
			LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE);
			continue;
		}

		/* Okay, do the read */
		ok = SlruPhysicalReadPage(ctl, pageno, slotno);

		/* Re-acquire shared control lock and update page state */
		LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE);

		Assert(shared->page_number[slotno] == pageno &&
			   shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS);

		shared->page_status[slotno] = ok ? SLRU_PAGE_CLEAN : SLRU_PAGE_EMPTY;

		LWLockRelease(ctl->locks->BufferLocks[slotno]);

		/* Now it's okay to ereport if we failed */
		if (!ok)
			SlruReportIOError(ctl, pageno, xid);

		SlruRecentlyUsed(shared, slotno);
		if (forwrite)
			shared->page_status[slotno] = SLRU_PAGE_DIRTY;
		return shared->page_buffer[slotno];
	}
}

/*
 * Write a page from a shared buffer, if necessary.
 * Does nothing if the specified slot is not dirty.
 *
 * NOTE: only one write attempt is made here.  Hence, it is possible that
 * the page is still dirty at exit (if someone else re-dirtied it during
 * the write).	However, we *do* attempt a fresh write even if the page
 * is already being written; this is for checkpoints.
 *
 * Control lock must be held at entry, and will be held at exit.
 */
void
SimpleLruWritePage(SlruCtl ctl, int slotno)
{
	int			pageno;
	bool		ok;
	SlruShared shared = (SlruShared) ctl->shared;

	/* Do nothing if page does not need writing */
	if (shared->page_status[slotno] != SLRU_PAGE_DIRTY &&
		shared->page_status[slotno] != SLRU_PAGE_WRITE_IN_PROGRESS)
		return;

	pageno = shared->page_number[slotno];

	/* Release shared lock, grab per-buffer lock instead */
	LWLockRelease(ctl->locks->ControlLock);
	LWLockAcquire(ctl->locks->BufferLocks[slotno], LW_EXCLUSIVE);

	/*
	 * Check to see if someone else already did the write, or took the
	 * buffer away from us.  If so, do nothing.  NOTE: we really should
	 * never see WRITE_IN_PROGRESS here, since that state should only
	 * occur while the writer is holding the buffer lock.  But accept it
	 * so that we have a recovery path if a writer aborts.
	 */
	if (shared->page_number[slotno] != pageno ||
		(shared->page_status[slotno] != SLRU_PAGE_DIRTY &&
		 shared->page_status[slotno] != SLRU_PAGE_WRITE_IN_PROGRESS))
	{
		LWLockRelease(ctl->locks->BufferLocks[slotno]);
		LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE);
		return;
	}

	/*
	 * Mark the slot write-busy.  After this point, a transaction status
	 * update on this page will mark it dirty again.  NB: we are assuming
	 * that read/write of the page status field is atomic, since we change
	 * the state while not holding control lock.  However, we cannot set
	 * this state any sooner, or we'd possibly fool a previous writer into
	 * thinking he's successfully dumped the page when he hasn't.
	 * (Scenario: other writer starts, page is redirtied, we come along
	 * and set WRITE_IN_PROGRESS again, other writer completes and sets
	 * CLEAN because redirty info has been lost, then we think it's clean
	 * too.)
	 */
	shared->page_status[slotno] = SLRU_PAGE_WRITE_IN_PROGRESS;

	/* Okay, do the write */
	ok = SlruPhysicalWritePage(ctl, pageno, slotno);

	/* Re-acquire shared control lock and update page state */
	LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE);

	Assert(shared->page_number[slotno] == pageno &&
		   (shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS ||
			shared->page_status[slotno] == SLRU_PAGE_DIRTY));

	/* Cannot set CLEAN if someone re-dirtied page since write started */
	if (shared->page_status[slotno] == SLRU_PAGE_WRITE_IN_PROGRESS)
		shared->page_status[slotno] = ok ? SLRU_PAGE_CLEAN : SLRU_PAGE_DIRTY;

	LWLockRelease(ctl->locks->BufferLocks[slotno]);

	/* Now it's okay to ereport if we failed */
	if (!ok)
		SlruReportIOError(ctl, pageno, InvalidTransactionId);
}

/*
 * Physical read of a (previously existing) page into a buffer slot
 *
 * On failure, we cannot just ereport(ERROR) since caller has put state in
 * shared memory that must be undone.  So, we return FALSE and save enough
 * info in static variables to let SlruReportIOError make the report.
 *
 * For now, assume it's not worth keeping a file pointer open across
 * read/write operations.  We could cache one virtual file pointer ...
 */
static bool
SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno)
{
	SlruShared	shared = (SlruShared) ctl->shared;
	int			segno = pageno / SLRU_PAGES_PER_SEGMENT;
	int			rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
	int			offset = rpageno * BLCKSZ;
	char		path[MAXPGPATH];
	int			fd;

	SlruFileName(ctl, path, segno);

	/*
	 * In a crash-and-restart situation, it's possible for us to receive
	 * commands to set the commit status of transactions whose bits are in
	 * already-truncated segments of the commit log (see notes in
	 * SlruPhysicalWritePage).	Hence, if we are InRecovery, allow the
	 * case where the file doesn't exist, and return zeroes instead.
	 */
	fd = BasicOpenFile(path, O_RDWR | PG_BINARY, S_IRUSR | S_IWUSR);
	if (fd < 0)
	{
		if (errno != ENOENT || !InRecovery)
		{
			slru_errcause = SLRU_OPEN_FAILED;
			slru_errno = errno;
			return false;
		}

		ereport(LOG,
				(errmsg("file \"%s\" doesn't exist, reading as zeroes",
						path)));
		MemSet(shared->page_buffer[slotno], 0, BLCKSZ);
		return true;
	}

	if (lseek(fd, (off_t) offset, SEEK_SET) < 0)
	{
		slru_errcause = SLRU_SEEK_FAILED;
		slru_errno = errno;
		return false;
	}

	errno = 0;
	if (read(fd, shared->page_buffer[slotno], BLCKSZ) != BLCKSZ)
	{
		slru_errcause = SLRU_READ_FAILED;
		slru_errno = errno;
		return false;
	}

	close(fd);
	return true;
}

/*
 * Physical write of a page from a buffer slot
 *
 * On failure, we cannot just ereport(ERROR) since caller has put state in
 * shared memory that must be undone.  So, we return FALSE and save enough
 * info in static variables to let SlruReportIOError make the report.
 *
 * For now, assume it's not worth keeping a file pointer open across
 * read/write operations.  We could cache one virtual file pointer ...
 */
static bool
SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno)
{
	SlruShared	shared = (SlruShared) ctl->shared;
	int			segno = pageno / SLRU_PAGES_PER_SEGMENT;
	int			rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
	int			offset = rpageno * BLCKSZ;
	char		path[MAXPGPATH];
	int			fd;

	SlruFileName(ctl, path, segno);

	/*
	 * If the file doesn't already exist, we should create it.  It is
	 * possible for this to need to happen when writing a page that's not
	 * first in its segment; we assume the OS can cope with that.  (Note:
	 * it might seem that it'd be okay to create files only when
	 * SimpleLruZeroPage is called for the first page of a segment.	However,
	 * if after a crash and restart the REDO logic elects to replay the
	 * log from a checkpoint before the latest one, then it's possible
	 * that we will get commands to set transaction status of transactions
	 * that have already been truncated from the commit log.  Easiest way
	 * to deal with that is to accept references to nonexistent files here
	 * and in SlruPhysicalReadPage.)
	 */
	fd = BasicOpenFile(path, O_RDWR | PG_BINARY, S_IRUSR | S_IWUSR);
	if (fd < 0)
	{
		if (errno != ENOENT)
		{
			slru_errcause = SLRU_OPEN_FAILED;
			slru_errno = errno;
			return false;
		}

		fd = BasicOpenFile(path, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
						   S_IRUSR | S_IWUSR);
		if (fd < 0)
		{
			slru_errcause = SLRU_CREATE_FAILED;
			slru_errno = errno;
			return false;
		}
	}

	if (lseek(fd, (off_t) offset, SEEK_SET) < 0)
	{
		slru_errcause = SLRU_SEEK_FAILED;
		slru_errno = errno;
		return false;
	}

	errno = 0;
	if (write(fd, shared->page_buffer[slotno], BLCKSZ) != BLCKSZ)
	{
		/* if write didn't set errno, assume problem is no disk space */
		if (errno == 0)
			errno = ENOSPC;
		slru_errcause = SLRU_WRITE_FAILED;
		slru_errno = errno;
		return false;
	}

	close(fd);
	return true;
}

/*
 * Issue the error message after failure of SlruPhysicalReadPage or
 * SlruPhysicalWritePage.  Call this after cleaning up shared-memory state.
 */
static void
SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid)
{
	int			segno = pageno / SLRU_PAGES_PER_SEGMENT;
	int			rpageno = pageno % SLRU_PAGES_PER_SEGMENT;
	int			offset = rpageno * BLCKSZ;
	char		path[MAXPGPATH];

	SlruFileName(ctl, path, segno);
	errno = slru_errno;
	switch (slru_errcause)
	{
		case SLRU_OPEN_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
					 errdetail("open of file \"%s\" failed: %m",
							   path)));
			break;
		case SLRU_CREATE_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
					 errdetail("creation of file \"%s\" failed: %m",
							   path)));
			break;
		case SLRU_SEEK_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
					 errdetail("lseek of file \"%s\", offset %u failed: %m",
							   path, offset)));
			break;
		case SLRU_READ_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
					 errdetail("read of file \"%s\", offset %u failed: %m",
							   path, offset)));
			break;
		case SLRU_WRITE_FAILED:
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not access status of transaction %u", xid),
					 errdetail("write of file \"%s\", offset %u failed: %m",
							   path, offset)));
			break;
		default:
			/* can't get here, we trust */
			elog(ERROR, "unrecognized SimpleLru error cause: %d",
				 (int) slru_errcause);
			break;
	}
}

/*
 * Select the slot to re-use when we need a free slot.
 *
 * The target page number is passed because we need to consider the
 * possibility that some other process reads in the target page while
 * we are doing I/O to free a slot.  Hence, check or recheck to see if
 * any slot already holds the target page, and return that slot if so.
 * Thus, the returned slot is *either* a slot already holding the pageno
 * (could be any state except EMPTY), *or* a freeable slot (state EMPTY
 * or CLEAN).
 *
 * Control lock must be held at entry, and will be held at exit.
 */
static int
SlruSelectLRUPage(SlruCtl ctl, int pageno)
{
	SlruShared	shared = (SlruShared) ctl->shared;
	/* Outer loop handles restart after I/O */
	for (;;)
	{
		int			slotno;
		int			bestslot = 0;
		unsigned int bestcount = 0;

		/* See if page already has a buffer assigned */
		for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
		{
			if (shared->page_number[slotno] == pageno &&
				shared->page_status[slotno] != SLRU_PAGE_EMPTY)
				return slotno;
		}

		/*
		 * If we find any EMPTY slot, just select that one. Else locate
		 * the least-recently-used slot that isn't the latest page.
		 */
		for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
		{
			if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
				return slotno;
			if (shared->page_lru_count[slotno] > bestcount &&
			    shared->page_number[slotno] != shared->latest_page_number)
			{
				bestslot = slotno;
				bestcount = shared->page_lru_count[slotno];
			}
		}

		/*
		 * If the selected page is clean, we're set.
		 */
		if (shared->page_status[bestslot] == SLRU_PAGE_CLEAN)
			return bestslot;

		/*
		 * We need to do I/O.  Normal case is that we have to write it
		 * out, but it's possible in the worst case to have selected a
		 * read-busy page.	In that case we use SimpleLruReadPage to wait for
		 * the read to complete.
		 */
		if (shared->page_status[bestslot] == SLRU_PAGE_READ_IN_PROGRESS)
			(void) SimpleLruReadPage(ctl, shared->page_number[bestslot],
			                         InvalidTransactionId, false);
		else
			SimpleLruWritePage(ctl, bestslot);

		/*
		 * Now loop back and try again.  This is the easiest way of
		 * dealing with corner cases such as the victim page being
		 * re-dirtied while we wrote it.
		 */
	}
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
SimpleLruSetLatestPage(SlruCtl ctl, int pageno)
{
	SlruShared	shared = (SlruShared) ctl->shared;

	shared->latest_page_number = pageno;
}

/*
 * This is called during checkpoint and postmaster/standalone-backend shutdown
 */
void
SimpleLruFlush(SlruCtl ctl, bool checkpoint)
{
	SlruShared	shared = (SlruShared) ctl->shared;
	int			slotno;

	LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE);

	for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
	{
		SimpleLruWritePage(ctl, slotno);
		/*
		 * When called during a checkpoint,
		 * we cannot assert that the slot is clean now, since another
		 * process might have re-dirtied it already.  That's okay.
		 */
		Assert(checkpoint ||
			   shared->page_status[slotno] == SLRU_PAGE_EMPTY ||
			   shared->page_status[slotno] == SLRU_PAGE_CLEAN);
	}

	LWLockRelease(ctl->locks->ControlLock);
}

/*
 * Remove all segments before the one holding the passed page number
 *
 * When this is called, we know that the database logically contains no
 * reference to transaction IDs older than oldestXact.	However, we must
 * not remove any segment until we have performed a checkpoint, to ensure
 * that no such references remain on disk either; else a crash just after
 * the truncation might leave us with a problem.  Since CLOG segments hold
 * a large number of transactions, the opportunity to actually remove a
 * segment is fairly rare, and so it seems best not to do the checkpoint
 * unless we have confirmed that there is a removable segment.	Therefore
 * we issue the checkpoint command here, not in higher-level code as might
 * seem cleaner.
 */
void
SimpleLruTruncate(SlruCtl ctl, int cutoffPage)
{
	int			slotno;
	SlruShared	shared = (SlruShared) ctl->shared;

	/*
	 * The cutoff point is the start of the segment containing cutoffPage.
	 */
	cutoffPage -= cutoffPage % SLRU_PAGES_PER_SEGMENT;

	if (!SlruScanDirectory(ctl, cutoffPage, false))
		return;					/* nothing to remove */

	/* Perform a forced CHECKPOINT */
	CreateCheckPoint(false, true);

	/*
	 * Scan shared memory and remove any pages preceding the cutoff
	 * page, to ensure we won't rewrite them later.  (Any dirty pages
	 * should have been flushed already during the checkpoint, we're just
	 * being extra careful here.)
	 */
	LWLockAcquire(ctl->locks->ControlLock, LW_EXCLUSIVE);

restart:;

	/*
	 * While we are holding the lock, make an important safety check: the
	 * planned cutoff point must be <= the current endpoint page.
	 * Otherwise we have already wrapped around, and proceeding with the
	 * truncation would risk removing the current segment.
	 */
	if (ctl->PagePrecedes(shared->latest_page_number, cutoffPage))
	{
		LWLockRelease(ctl->locks->ControlLock);
		ereport(LOG,
				(errmsg("unable to truncate \"%s\": apparent wraparound",
						ctl->Dir)));
		return;
	}

	for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
	{
		if (shared->page_status[slotno] == SLRU_PAGE_EMPTY)
			continue;
		if (!ctl->PagePrecedes(shared->page_number[slotno], cutoffPage))
			continue;

		/*
		 * If page is CLEAN, just change state to EMPTY (expected case).
		 */
		if (shared->page_status[slotno] == SLRU_PAGE_CLEAN)
		{
			shared->page_status[slotno] = SLRU_PAGE_EMPTY;
			continue;
		}

		/*
		 * Hmm, we have (or may have) I/O operations acting on the page,
		 * so we've got to wait for them to finish and then start again.
		 * This is the same logic as in SlruSelectLRUPage.
		 */
		if (shared->page_status[slotno] == SLRU_PAGE_READ_IN_PROGRESS)
			(void) SimpleLruReadPage(ctl, shared->page_number[slotno],
									 InvalidTransactionId, false);
		else
			SimpleLruWritePage(ctl, slotno);
		goto restart;
	}

	LWLockRelease(ctl->locks->ControlLock);

	/* Now we can remove the old segment(s) */
	(void) SlruScanDirectory(ctl, cutoffPage, true);
}

/*
 * SlruTruncate subroutine: scan directory for removable segments.
 * Actually remove them iff doDeletions is true.  Return TRUE iff any
 * removable segments were found.  Note: no locking is needed.
 */
static bool
SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions)
{
	bool		found = false;
	DIR		   *cldir;
	struct dirent *clde;
	int			segno;
	int			segpage;
	char		path[MAXPGPATH];

	cldir = opendir(ctl->Dir);
	if (cldir == NULL)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not open directory \"%s\": %m", ctl->Dir)));

	errno = 0;
	while ((clde = readdir(cldir)) != NULL)
	{
		if (strlen(clde->d_name) == 4 &&
			strspn(clde->d_name, "0123456789ABCDEF") == 4)
		{
			segno = (int) strtol(clde->d_name, NULL, 16);
			segpage = segno * SLRU_PAGES_PER_SEGMENT;
			if (ctl->PagePrecedes(segpage, cutoffPage))
			{
				found = true;
				if (doDeletions)
				{
					ereport(LOG,
							(errmsg("removing file \"%s/%s\"",
									ctl->Dir, clde->d_name)));
					snprintf(path, MAXPGPATH, "%s/%s", ctl->Dir, clde->d_name);
					unlink(path);
				}
			}
		}
		errno = 0;
	}
	if (errno)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not read directory \"%s\": %m", ctl->Dir)));
	closedir(cldir);

	return found;
}