Properly access a buffer's LSN using existing access macros instead of abusing

[PostgreSQL.git] / src / backend / storage / buffer / bufmgr.c

/*-------------------------------------------------------------------------
 *
 * bufmgr.c
 *        buffer manager interface routines
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL$
 *
 *-------------------------------------------------------------------------
 */
/*
 * Principal entry points:
 *
 * ReadBuffer() -- find or create a buffer holding the requested page,
 *              and pin it so that no one can destroy it while this process
 *              is using it.
 *
 * ReleaseBuffer() -- unpin a buffer
 *
 * MarkBufferDirty() -- mark a pinned buffer's contents as "dirty".
 *              The disk write is delayed until buffer replacement or checkpoint.
 *
 * See also these files:
 *              freelist.c -- chooses victim for buffer replacement
 *              buf_table.c -- manages the buffer lookup table
 */
#include "postgres.h"

#include <sys/file.h>
#include <unistd.h>

#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
#include "storage/ipc.h"
#include "storage/proc.h"
#include "storage/smgr.h"
#include "utils/rel.h"
#include "utils/resowner.h"


/* Note: these two macros only work on shared buffers, not local ones! */
#define BufHdrGetBlock(bufHdr)  ((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
#define BufferGetLSN(bufHdr)    (PageGetLSN(BufHdrGetBlock(bufHdr)))

/* Note: this macro only works on local buffers, not shared ones! */
#define LocalBufHdrGetBlock(bufHdr) \
        LocalBufferBlockPointers[-((bufHdr)->buf_id + 2)]

/* Bits in SyncOneBuffer's return value */
#define BUF_WRITTEN                             0x01
#define BUF_REUSABLE                    0x02


/* GUC variables */
bool            zero_damaged_pages = false;
int                     bgwriter_lru_maxpages = 100;
double          bgwriter_lru_multiplier = 2.0;

/* local state for StartBufferIO and related functions */
static volatile BufferDesc *InProgressBuf = NULL;
static bool IsForInput;

/* local state for LockBufferForCleanup */
static volatile BufferDesc *PinCountWaitBuf = NULL;


static Buffer ReadBuffer_relcache(Relation reln, ForkNumber forkNum,
                BlockNumber blockNum, bool zeroPage, BufferAccessStrategy strategy);
static Buffer ReadBuffer_common(SMgrRelation reln, bool isLocalBuf,
                                  ForkNumber forkNum, BlockNumber blockNum,
                                  bool zeroPage, BufferAccessStrategy strategy, bool *hit);
static bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
static void PinBuffer_Locked(volatile BufferDesc *buf);
static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
static void BufferSync(int flags);
static int      SyncOneBuffer(int buf_id, bool skip_recently_used);
static void WaitIO(volatile BufferDesc *buf);
static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
static void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
                                  int set_flag_bits);
static void buffer_write_error_callback(void *arg);
static volatile BufferDesc *BufferAlloc(SMgrRelation smgr, ForkNumber forkNum,
                        BlockNumber blockNum,
                        BufferAccessStrategy strategy,
                        bool *foundPtr);
static void FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln);
static void AtProcExit_Buffers(int code, Datum arg);


/*
 * ReadBuffer -- returns a buffer containing the requested
 *              block of the requested relation.  If the blknum
 *              requested is P_NEW, extend the relation file and
 *              allocate a new block.  (Caller is responsible for
 *              ensuring that only one backend tries to extend a
 *              relation at the same time!)
 *
 * Returns: the buffer number for the buffer containing
 *              the block read.  The returned buffer has been pinned.
 *              Does not return on error --- elog's instead.
 *
 * Assume when this function is called, that reln has been
 *              opened already.
 */
Buffer
ReadBuffer(Relation reln, BlockNumber blockNum)
{
        return ReadBuffer_relcache(reln, MAIN_FORKNUM, blockNum, false, NULL);
}

/*
 * ReadBufferWithFork -- same as ReadBuffer, but for accessing relation
 *              forks other than MAIN_FORKNUM.
 */
Buffer
ReadBufferWithFork(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
{
        return ReadBuffer_relcache(reln, forkNum, blockNum, false, NULL);
}

/*
 * ReadBufferWithStrategy -- same as ReadBuffer, except caller can specify
 *              a nondefault buffer access strategy.  See buffer/README for details.
 */
Buffer
ReadBufferWithStrategy(Relation reln, BlockNumber blockNum,
                                           BufferAccessStrategy strategy)
{
        return ReadBuffer_relcache(reln, MAIN_FORKNUM, blockNum, false, strategy);
}

/*
 * ReadOrZeroBuffer -- like ReadBuffer, but if the page isn't in buffer
 *              cache already, it's filled with zeros instead of reading it from
 *              disk.  Useful when the caller intends to fill the page from scratch,
 *              since this saves I/O and avoids unnecessary failure if the
 *              page-on-disk has corrupt page headers.
 *
 *              Caution: do not use this to read a page that is beyond the relation's
 *              current physical EOF; that is likely to cause problems in md.c when
 *              the page is modified and written out.  P_NEW is OK, though.
 */
Buffer
ReadOrZeroBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
{
        return ReadBuffer_relcache(reln, forkNum, blockNum, true, NULL);
}

/*
 * ReadBufferWithoutRelcache -- like ReadBuffer, but doesn't require a
 *              relcache entry for the relation. If zeroPage is true, this behaves
 *              like ReadOrZeroBuffer rather than ReadBuffer.
 */
Buffer
ReadBufferWithoutRelcache(RelFileNode rnode, bool isTemp,
                                          ForkNumber forkNum, BlockNumber blockNum, bool zeroPage)
{
        bool hit;

        SMgrRelation smgr = smgropen(rnode);
        return ReadBuffer_common(smgr, isTemp, forkNum, blockNum, zeroPage, NULL, &hit);
}

/*
 * ReadBuffer_relcache -- common logic for ReadBuffer-variants that
 *              operate on a Relation.
 */
static Buffer
ReadBuffer_relcache(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
                                        bool zeroPage, BufferAccessStrategy strategy)
{
        bool hit;
        Buffer buf;

        /* Open it at the smgr level if not already done */
        RelationOpenSmgr(reln);

        /*
         * Read the buffer, and update pgstat counters to reflect a cache
         * hit or miss.
         */
        pgstat_count_buffer_read(reln);
        buf = ReadBuffer_common(reln->rd_smgr, reln->rd_istemp, forkNum, blockNum,
                                                        zeroPage, strategy, &hit);
        if (hit)
                pgstat_count_buffer_hit(reln);
        return buf;
}

/*
 * ReadBuffer_common -- common logic for all ReadBuffer variants
 *
 * *hit is set to true if the request was satisfied from shared buffer cache.
 */
static Buffer
ReadBuffer_common(SMgrRelation smgr, bool isLocalBuf, ForkNumber forkNum,
                                  BlockNumber blockNum, bool zeroPage,
                                  BufferAccessStrategy strategy, bool *hit)
{
        volatile BufferDesc *bufHdr;
        Block           bufBlock;
        bool            found;
        bool            isExtend;

        *hit = false;

        /* Make sure we will have room to remember the buffer pin */
        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

        isExtend = (blockNum == P_NEW);

        /* Substitute proper block number if caller asked for P_NEW */
        if (isExtend)
                blockNum = smgrnblocks(smgr, forkNum);

        TRACE_POSTGRESQL_BUFFER_READ_START(blockNum, smgr->smgr_rnode.spcNode,
                smgr->smgr_rnode.dbNode, smgr->smgr_rnode.relNode, isLocalBuf);

        if (isLocalBuf)
        {
                ReadLocalBufferCount++;
                bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found);
                if (found)
                {
                        LocalBufferHitCount++;
                        TRACE_POSTGRESQL_BUFFER_HIT(true); /* true == local buffer */
                }
                else
                {
                        TRACE_POSTGRESQL_BUFFER_MISS(true); /* ditto */
                }
        }
        else
        {
                ReadBufferCount++;

                /*
                 * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
                 * not currently in memory.
                 */
                bufHdr = BufferAlloc(smgr, forkNum, blockNum, strategy, &found);
                if (found)
                {
                        BufferHitCount++;
                        TRACE_POSTGRESQL_BUFFER_HIT(false); /* false != local buffer */
                }
                else
                {
                        TRACE_POSTGRESQL_BUFFER_MISS(false); /* ditto */
                }
        }

        /* At this point we do NOT hold any locks. */

        /* if it was already in the buffer pool, we're done */
        if (found)
        {
                if (!isExtend)
                {
                        /* Just need to update stats before we exit */
                        *hit = true;

                        if (VacuumCostActive)
                                VacuumCostBalance += VacuumCostPageHit;

                        TRACE_POSTGRESQL_BUFFER_READ_DONE(blockNum,
                                smgr->smgr_rnode.spcNode,
                                smgr->smgr_rnode.dbNode,
                                smgr->smgr_rnode.relNode, isLocalBuf, found);

                        return BufferDescriptorGetBuffer(bufHdr);
                }

                /*
                 * We get here only in the corner case where we are trying to extend
                 * the relation but we found a pre-existing buffer marked BM_VALID.
                 * This can happen because mdread doesn't complain about reads beyond
                 * EOF (when zero_damaged_pages is ON) and so a previous attempt to
                 * read a block beyond EOF could have left a "valid" zero-filled
                 * buffer.      Unfortunately, we have also seen this case occurring
                 * because of buggy Linux kernels that sometimes return an
                 * lseek(SEEK_END) result that doesn't account for a recent write. In
                 * that situation, the pre-existing buffer would contain valid data
                 * that we don't want to overwrite.  Since the legitimate case should
                 * always have left a zero-filled buffer, complain if not PageIsNew.
                 */
                bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
                if (!PageIsNew((Page) bufBlock))
                        ereport(ERROR,
                                        (errmsg("unexpected data beyond EOF in block %u of relation %u/%u/%u",
                                                        blockNum, smgr->smgr_rnode.spcNode, smgr->smgr_rnode.dbNode, smgr->smgr_rnode.relNode),
                                         errhint("This has been seen to occur with buggy kernels; consider updating your system.")));

                /*
                 * We *must* do smgrextend before succeeding, else the page will not
                 * be reserved by the kernel, and the next P_NEW call will decide to
                 * return the same page.  Clear the BM_VALID bit, do the StartBufferIO
                 * call that BufferAlloc didn't, and proceed.
                 */
                if (isLocalBuf)
                {
                        /* Only need to adjust flags */
                        Assert(bufHdr->flags & BM_VALID);
                        bufHdr->flags &= ~BM_VALID;
                }
                else
                {
                        /*
                         * Loop to handle the very small possibility that someone re-sets
                         * BM_VALID between our clearing it and StartBufferIO inspecting
                         * it.
                         */
                        do
                        {
                                LockBufHdr(bufHdr);
                                Assert(bufHdr->flags & BM_VALID);
                                bufHdr->flags &= ~BM_VALID;
                                UnlockBufHdr(bufHdr);
                        } while (!StartBufferIO(bufHdr, true));
                }
        }

        /*
         * if we have gotten to this point, we have allocated a buffer for the
         * page but its contents are not yet valid.  IO_IN_PROGRESS is set for it,
         * if it's a shared buffer.
         *
         * Note: if smgrextend fails, we will end up with a buffer that is
         * allocated but not marked BM_VALID.  P_NEW will still select the same
         * block number (because the relation didn't get any longer on disk) and
         * so future attempts to extend the relation will find the same buffer (if
         * it's not been recycled) but come right back here to try smgrextend
         * again.
         */
        Assert(!(bufHdr->flags & BM_VALID));            /* spinlock not needed */

        bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);

        if (isExtend)
        {
                /* new buffers are zero-filled */
                MemSet((char *) bufBlock, 0, BLCKSZ);
                smgrextend(smgr, forkNum, blockNum, (char *) bufBlock, isLocalBuf);
        }
        else
        {
                /*
                 * Read in the page, unless the caller intends to overwrite it and
                 * just wants us to allocate a buffer.
                 */
                if (zeroPage)
                        MemSet((char *) bufBlock, 0, BLCKSZ);
                else
                {
                        smgrread(smgr, forkNum, blockNum, (char *) bufBlock);

                        /* check for garbage data */
                        if (!PageHeaderIsValid((PageHeader) bufBlock))
                        {
                                if (zero_damaged_pages)
                                {
                                        ereport(WARNING,
                                                        (errcode(ERRCODE_DATA_CORRUPTED),
                                                         errmsg("invalid page header in block %u of relation %u/%u/%u; zeroing out page",
                                                                        blockNum,
                                                                        smgr->smgr_rnode.spcNode,
                                                                        smgr->smgr_rnode.dbNode,
                                                                        smgr->smgr_rnode.relNode)));
                                        MemSet((char *) bufBlock, 0, BLCKSZ);
                                }
                                else
                                        ereport(ERROR,
                                                        (errcode(ERRCODE_DATA_CORRUPTED),
                                                         errmsg("invalid page header in block %u of relation %u/%u/%u",
                                                                        blockNum, smgr->smgr_rnode.spcNode,
                                                                        smgr->smgr_rnode.dbNode,
                                                                        smgr->smgr_rnode.relNode)));
                        }
                }
        }

        if (isLocalBuf)
        {
                /* Only need to adjust flags */
                bufHdr->flags |= BM_VALID;
        }
        else
        {
                /* Set BM_VALID, terminate IO, and wake up any waiters */
                TerminateBufferIO(bufHdr, false, BM_VALID);
        }

        if (VacuumCostActive)
                VacuumCostBalance += VacuumCostPageMiss;

        TRACE_POSTGRESQL_BUFFER_READ_DONE(blockNum, smgr->smgr_rnode.spcNode,
                        smgr->smgr_rnode.dbNode, smgr->smgr_rnode.relNode,
                        isLocalBuf, found);

        return BufferDescriptorGetBuffer(bufHdr);
}

/*
 * BufferAlloc -- subroutine for ReadBuffer.  Handles lookup of a shared
 *              buffer.  If no buffer exists already, selects a replacement
 *              victim and evicts the old page, but does NOT read in new page.
 *
 * "strategy" can be a buffer replacement strategy object, or NULL for
 * the default strategy.  The selected buffer's usage_count is advanced when
 * using the default strategy, but otherwise possibly not (see PinBuffer).
 *
 * The returned buffer is pinned and is already marked as holding the
 * desired page.  If it already did have the desired page, *foundPtr is
 * set TRUE.  Otherwise, *foundPtr is set FALSE and the buffer is marked
 * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
 *
 * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
 * we keep it for simplicity in ReadBuffer.
 *
 * No locks are held either at entry or exit.
 */
static volatile BufferDesc *
BufferAlloc(SMgrRelation smgr, ForkNumber forkNum,
                        BlockNumber blockNum,
                        BufferAccessStrategy strategy,
                        bool *foundPtr)
{
        BufferTag       newTag;                 /* identity of requested block */
        uint32          newHash;                /* hash value for newTag */
        LWLockId        newPartitionLock;               /* buffer partition lock for it */
        BufferTag       oldTag;                 /* previous identity of selected buffer */
        uint32          oldHash;                /* hash value for oldTag */
        LWLockId        oldPartitionLock;               /* buffer partition lock for it */
        BufFlags        oldFlags;
        int                     buf_id;
        volatile BufferDesc *buf;
        bool            valid;

        /* create a tag so we can lookup the buffer */
        INIT_BUFFERTAG(newTag, smgr->smgr_rnode, forkNum, blockNum);

        /* determine its hash code and partition lock ID */
        newHash = BufTableHashCode(&newTag);
        newPartitionLock = BufMappingPartitionLock(newHash);

        /* see if the block is in the buffer pool already */
        LWLockAcquire(newPartitionLock, LW_SHARED);
        buf_id = BufTableLookup(&newTag, newHash);
        if (buf_id >= 0)
        {
                /*
                 * Found it.  Now, pin the buffer so no one can steal it from the
                 * buffer pool, and check to see if the correct data has been loaded
                 * into the buffer.
                 */
                buf = &BufferDescriptors[buf_id];

                valid = PinBuffer(buf, strategy);

                /* Can release the mapping lock as soon as we've pinned it */
                LWLockRelease(newPartitionLock);

                *foundPtr = TRUE;

                if (!valid)
                {
                        /*
                         * We can only get here if (a) someone else is still reading in
                         * the page, or (b) a previous read attempt failed.  We have to
                         * wait for any active read attempt to finish, and then set up our
                         * own read attempt if the page is still not BM_VALID.
                         * StartBufferIO does it all.
                         */
                        if (StartBufferIO(buf, true))
                        {
                                /*
                                 * If we get here, previous attempts to read the buffer must
                                 * have failed ... but we shall bravely try again.
                                 */
                                *foundPtr = FALSE;
                        }
                }

                return buf;
        }

        /*
         * Didn't find it in the buffer pool.  We'll have to initialize a new
         * buffer.      Remember to unlock the mapping lock while doing the work.
         */
        LWLockRelease(newPartitionLock);

        /* Loop here in case we have to try another victim buffer */
        for (;;)
        {
                bool            lock_held;

                /*
                 * Select a victim buffer.      The buffer is returned with its header
                 * spinlock still held!  Also (in most cases) the BufFreelistLock is
                 * still held, since it would be bad to hold the spinlock while
                 * possibly waking up other processes.
                 */
                buf = StrategyGetBuffer(strategy, &lock_held);

                Assert(buf->refcount == 0);

                /* Must copy buffer flags while we still hold the spinlock */
                oldFlags = buf->flags;

                /* Pin the buffer and then release the buffer spinlock */
                PinBuffer_Locked(buf);

                /* Now it's safe to release the freelist lock */
                if (lock_held)
                        LWLockRelease(BufFreelistLock);

                /*
                 * If the buffer was dirty, try to write it out.  There is a race
                 * condition here, in that someone might dirty it after we released it
                 * above, or even while we are writing it out (since our share-lock
                 * won't prevent hint-bit updates).  We will recheck the dirty bit
                 * after re-locking the buffer header.
                 */
                if (oldFlags & BM_DIRTY)
                {
                        /*
                         * We need a share-lock on the buffer contents to write it out
                         * (else we might write invalid data, eg because someone else is
                         * compacting the page contents while we write).  We must use a
                         * conditional lock acquisition here to avoid deadlock.  Even
                         * though the buffer was not pinned (and therefore surely not
                         * locked) when StrategyGetBuffer returned it, someone else could
                         * have pinned and exclusive-locked it by the time we get here. If
                         * we try to get the lock unconditionally, we'd block waiting for
                         * them; if they later block waiting for us, deadlock ensues.
                         * (This has been observed to happen when two backends are both
                         * trying to split btree index pages, and the second one just
                         * happens to be trying to split the page the first one got from
                         * StrategyGetBuffer.)
                         */
                        if (LWLockConditionalAcquire(buf->content_lock, LW_SHARED))
                        {
                                /*
                                 * If using a nondefault strategy, and writing the buffer
                                 * would require a WAL flush, let the strategy decide whether
                                 * to go ahead and write/reuse the buffer or to choose another
                                 * victim.      We need lock to inspect the page LSN, so this
                                 * can't be done inside StrategyGetBuffer.
                                 */
                                if (strategy != NULL &&
                                        XLogNeedsFlush(BufferGetLSN(buf)) &&
                                        StrategyRejectBuffer(strategy, buf))
                                {
                                        /* Drop lock/pin and loop around for another buffer */
                                        LWLockRelease(buf->content_lock);
                                        UnpinBuffer(buf, true);
                                        continue;
                                }

                                /* OK, do the I/O */
                                FlushBuffer(buf, NULL);
                                LWLockRelease(buf->content_lock);
                        }
                        else
                        {
                                /*
                                 * Someone else has locked the buffer, so give it up and loop
                                 * back to get another one.
                                 */
                                UnpinBuffer(buf, true);
                                continue;
                        }
                }

                /*
                 * To change the association of a valid buffer, we'll need to have
                 * exclusive lock on both the old and new mapping partitions.
                 */
                if (oldFlags & BM_TAG_VALID)
                {
                        /*
                         * Need to compute the old tag's hashcode and partition lock ID.
                         * XXX is it worth storing the hashcode in BufferDesc so we need
                         * not recompute it here?  Probably not.
                         */
                        oldTag = buf->tag;
                        oldHash = BufTableHashCode(&oldTag);
                        oldPartitionLock = BufMappingPartitionLock(oldHash);

                        /*
                         * Must lock the lower-numbered partition first to avoid
                         * deadlocks.
                         */
                        if (oldPartitionLock < newPartitionLock)
                        {
                                LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
                                LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
                        }
                        else if (oldPartitionLock > newPartitionLock)
                        {
                                LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
                                LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
                        }
                        else
                        {
                                /* only one partition, only one lock */
                                LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
                        }
                }
                else
                {
                        /* if it wasn't valid, we need only the new partition */
                        LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
                        /* these just keep the compiler quiet about uninit variables */
                        oldHash = 0;
                        oldPartitionLock = 0;
                }

                /*
                 * Try to make a hashtable entry for the buffer under its new tag.
                 * This could fail because while we were writing someone else
                 * allocated another buffer for the same block we want to read in.
                 * Note that we have not yet removed the hashtable entry for the old
                 * tag.
                 */
                buf_id = BufTableInsert(&newTag, newHash, buf->buf_id);

                if (buf_id >= 0)
                {
                        /*
                         * Got a collision. Someone has already done what we were about to
                         * do. We'll just handle this as if it were found in the buffer
                         * pool in the first place.  First, give up the buffer we were
                         * planning to use.
                         */
                        UnpinBuffer(buf, true);

                        /* Can give up that buffer's mapping partition lock now */
                        if ((oldFlags & BM_TAG_VALID) &&
                                oldPartitionLock != newPartitionLock)
                                LWLockRelease(oldPartitionLock);

                        /* remaining code should match code at top of routine */

                        buf = &BufferDescriptors[buf_id];

                        valid = PinBuffer(buf, strategy);

                        /* Can release the mapping lock as soon as we've pinned it */
                        LWLockRelease(newPartitionLock);

                        *foundPtr = TRUE;

                        if (!valid)
                        {
                                /*
                                 * We can only get here if (a) someone else is still reading
                                 * in the page, or (b) a previous read attempt failed.  We
                                 * have to wait for any active read attempt to finish, and
                                 * then set up our own read attempt if the page is still not
                                 * BM_VALID.  StartBufferIO does it all.
                                 */
                                if (StartBufferIO(buf, true))
                                {
                                        /*
                                         * If we get here, previous attempts to read the buffer
                                         * must have failed ... but we shall bravely try again.
                                         */
                                        *foundPtr = FALSE;
                                }
                        }

                        return buf;
                }

                /*
                 * Need to lock the buffer header too in order to change its tag.
                 */
                LockBufHdr(buf);

                /*
                 * Somebody could have pinned or re-dirtied the buffer while we were
                 * doing the I/O and making the new hashtable entry.  If so, we can't
                 * recycle this buffer; we must undo everything we've done and start
                 * over with a new victim buffer.
                 */
                oldFlags = buf->flags;
                if (buf->refcount == 1 && !(oldFlags & BM_DIRTY))
                        break;

                UnlockBufHdr(buf);
                BufTableDelete(&newTag, newHash);
                if ((oldFlags & BM_TAG_VALID) &&
                        oldPartitionLock != newPartitionLock)
                        LWLockRelease(oldPartitionLock);
                LWLockRelease(newPartitionLock);
                UnpinBuffer(buf, true);
        }

        /*
         * Okay, it's finally safe to rename the buffer.
         *
         * Clearing BM_VALID here is necessary, clearing the dirtybits is just
         * paranoia.  We also reset the usage_count since any recency of use of
         * the old content is no longer relevant.  (The usage_count starts out at
         * 1 so that the buffer can survive one clock-sweep pass.)
         */
        buf->tag = newTag;
        buf->flags &= ~(BM_VALID | BM_DIRTY | BM_JUST_DIRTIED | BM_CHECKPOINT_NEEDED | BM_IO_ERROR);
        buf->flags |= BM_TAG_VALID;
        buf->usage_count = 1;

        UnlockBufHdr(buf);

        if (oldFlags & BM_TAG_VALID)
        {
                BufTableDelete(&oldTag, oldHash);
                if (oldPartitionLock != newPartitionLock)
                        LWLockRelease(oldPartitionLock);
        }

        LWLockRelease(newPartitionLock);

        /*
         * Buffer contents are currently invalid.  Try to get the io_in_progress
         * lock.  If StartBufferIO returns false, then someone else managed to
         * read it before we did, so there's nothing left for BufferAlloc() to do.
         */
        if (StartBufferIO(buf, true))
                *foundPtr = FALSE;
        else
                *foundPtr = TRUE;

        return buf;
}

/*
 * InvalidateBuffer -- mark a shared buffer invalid and return it to the
 * freelist.
 *
 * The buffer header spinlock must be held at entry.  We drop it before
 * returning.  (This is sane because the caller must have locked the
 * buffer in order to be sure it should be dropped.)
 *
 * This is used only in contexts such as dropping a relation.  We assume
 * that no other backend could possibly be interested in using the page,
 * so the only reason the buffer might be pinned is if someone else is
 * trying to write it out.      We have to let them finish before we can
 * reclaim the buffer.
 *
 * The buffer could get reclaimed by someone else while we are waiting
 * to acquire the necessary locks; if so, don't mess it up.
 */
static void
InvalidateBuffer(volatile BufferDesc *buf)
{
        BufferTag       oldTag;
        uint32          oldHash;                /* hash value for oldTag */
        LWLockId        oldPartitionLock;               /* buffer partition lock for it */
        BufFlags        oldFlags;

        /* Save the original buffer tag before dropping the spinlock */
        oldTag = buf->tag;

        UnlockBufHdr(buf);

        /*
         * Need to compute the old tag's hashcode and partition lock ID. XXX is it
         * worth storing the hashcode in BufferDesc so we need not recompute it
         * here?  Probably not.
         */
        oldHash = BufTableHashCode(&oldTag);
        oldPartitionLock = BufMappingPartitionLock(oldHash);

retry:

        /*
         * Acquire exclusive mapping lock in preparation for changing the buffer's
         * association.
         */
        LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);

        /* Re-lock the buffer header */
        LockBufHdr(buf);

        /* If it's changed while we were waiting for lock, do nothing */
        if (!BUFFERTAGS_EQUAL(buf->tag, oldTag))
        {
                UnlockBufHdr(buf);
                LWLockRelease(oldPartitionLock);
                return;
        }

        /*
         * We assume the only reason for it to be pinned is that someone else is
         * flushing the page out.  Wait for them to finish.  (This could be an
         * infinite loop if the refcount is messed up... it would be nice to time
         * out after awhile, but there seems no way to be sure how many loops may
         * be needed.  Note that if the other guy has pinned the buffer but not
         * yet done StartBufferIO, WaitIO will fall through and we'll effectively
         * be busy-looping here.)
         */
        if (buf->refcount != 0)
        {
                UnlockBufHdr(buf);
                LWLockRelease(oldPartitionLock);
                /* safety check: should definitely not be our *own* pin */
                if (PrivateRefCount[buf->buf_id] != 0)
                        elog(ERROR, "buffer is pinned in InvalidateBuffer");
                WaitIO(buf);
                goto retry;
        }

        /*
         * Clear out the buffer's tag and flags.  We must do this to ensure that
         * linear scans of the buffer array don't think the buffer is valid.
         */
        oldFlags = buf->flags;
        CLEAR_BUFFERTAG(buf->tag);
        buf->flags = 0;
        buf->usage_count = 0;

        UnlockBufHdr(buf);

        /*
         * Remove the buffer from the lookup hashtable, if it was in there.
         */
        if (oldFlags & BM_TAG_VALID)
                BufTableDelete(&oldTag, oldHash);

        /*
         * Done with mapping lock.
         */
        LWLockRelease(oldPartitionLock);

        /*
         * Insert the buffer at the head of the list of free buffers.
         */
        StrategyFreeBuffer(buf);
}

/*
 * MarkBufferDirty
 *
 *              Marks buffer contents as dirty (actual write happens later).
 *
 * Buffer must be pinned and exclusive-locked.  (If caller does not hold
 * exclusive lock, then somebody could be in process of writing the buffer,
 * leading to risk of bad data written to disk.)
 */
void
MarkBufferDirty(Buffer buffer)
{
        volatile BufferDesc *bufHdr;

        if (!BufferIsValid(buffer))
                elog(ERROR, "bad buffer id: %d", buffer);

        if (BufferIsLocal(buffer))
        {
                MarkLocalBufferDirty(buffer);
                return;
        }

        bufHdr = &BufferDescriptors[buffer - 1];

        Assert(PrivateRefCount[buffer - 1] > 0);
        /* unfortunately we can't check if the lock is held exclusively */
        Assert(LWLockHeldByMe(bufHdr->content_lock));

        LockBufHdr(bufHdr);

        Assert(bufHdr->refcount > 0);

        /*
         * If the buffer was not dirty already, do vacuum cost accounting.
         */
        if (!(bufHdr->flags & BM_DIRTY) && VacuumCostActive)
                VacuumCostBalance += VacuumCostPageDirty;

        bufHdr->flags |= (BM_DIRTY | BM_JUST_DIRTIED);

        UnlockBufHdr(bufHdr);
}

/*
 * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
 *
 * Formerly, this saved one cycle of acquiring/releasing the BufMgrLock
 * compared to calling the two routines separately.  Now it's mainly just
 * a convenience function.      However, if the passed buffer is valid and
 * already contains the desired block, we just return it as-is; and that
 * does save considerable work compared to a full release and reacquire.
 *
 * Note: it is OK to pass buffer == InvalidBuffer, indicating that no old
 * buffer actually needs to be released.  This case is the same as ReadBuffer,
 * but can save some tests in the caller.
 */
Buffer
ReleaseAndReadBuffer(Buffer buffer,
                                         Relation relation,
                                         BlockNumber blockNum)
{
        ForkNumber forkNum = MAIN_FORKNUM;
        volatile BufferDesc *bufHdr;

        if (BufferIsValid(buffer))
        {
                if (BufferIsLocal(buffer))
                {
                        Assert(LocalRefCount[-buffer - 1] > 0);
                        bufHdr = &LocalBufferDescriptors[-buffer - 1];
                        if (bufHdr->tag.blockNum == blockNum &&
                                RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
                                bufHdr->tag.forkNum == forkNum)
                                return buffer;
                        ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
                        LocalRefCount[-buffer - 1]--;
                }
                else
                {
                        Assert(PrivateRefCount[buffer - 1] > 0);
                        bufHdr = &BufferDescriptors[buffer - 1];
                        /* we have pin, so it's ok to examine tag without spinlock */
                        if (bufHdr->tag.blockNum == blockNum &&
                                RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
                                bufHdr->tag.forkNum == forkNum)
                                return buffer;
                        UnpinBuffer(bufHdr, true);
                }
        }

        return ReadBuffer(relation, blockNum);
}

/*
 * PinBuffer -- make buffer unavailable for replacement.
 *
 * For the default access strategy, the buffer's usage_count is incremented
 * when we first pin it; for other strategies we just make sure the usage_count
 * isn't zero.  (The idea of the latter is that we don't want synchronized
 * heap scans to inflate the count, but we need it to not be zero to discourage
 * other backends from stealing buffers from our ring.  As long as we cycle
 * through the ring faster than the global clock-sweep cycles, buffers in
 * our ring won't be chosen as victims for replacement by other backends.)
 *
 * This should be applied only to shared buffers, never local ones.
 *
 * Note that ResourceOwnerEnlargeBuffers must have been done already.
 *
 * Returns TRUE if buffer is BM_VALID, else FALSE.      This provision allows
 * some callers to avoid an extra spinlock cycle.
 */
static bool
PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
{
        int                     b = buf->buf_id;
        bool            result;

        if (PrivateRefCount[b] == 0)
        {
                LockBufHdr(buf);
                buf->refcount++;
                if (strategy == NULL)
                {
                        if (buf->usage_count < BM_MAX_USAGE_COUNT)
                                buf->usage_count++;
                }
                else
                {
                        if (buf->usage_count == 0)
                                buf->usage_count = 1;
                }
                result = (buf->flags & BM_VALID) != 0;
                UnlockBufHdr(buf);
        }
        else
        {
                /* If we previously pinned the buffer, it must surely be valid */
                result = true;
        }
        PrivateRefCount[b]++;
        Assert(PrivateRefCount[b] > 0);
        ResourceOwnerRememberBuffer(CurrentResourceOwner,
                                                                BufferDescriptorGetBuffer(buf));
        return result;
}

/*
 * PinBuffer_Locked -- as above, but caller already locked the buffer header.
 * The spinlock is released before return.
 *
 * Currently, no callers of this function want to modify the buffer's
 * usage_count at all, so there's no need for a strategy parameter.
 * Also we don't bother with a BM_VALID test (the caller could check that for
 * itself).
 *
 * Note: use of this routine is frequently mandatory, not just an optimization
 * to save a spin lock/unlock cycle, because we need to pin a buffer before
 * its state can change under us.
 */
static void
PinBuffer_Locked(volatile BufferDesc *buf)
{
        int                     b = buf->buf_id;

        if (PrivateRefCount[b] == 0)
                buf->refcount++;
        UnlockBufHdr(buf);
        PrivateRefCount[b]++;
        Assert(PrivateRefCount[b] > 0);
        ResourceOwnerRememberBuffer(CurrentResourceOwner,
                                                                BufferDescriptorGetBuffer(buf));
}

/*
 * UnpinBuffer -- make buffer available for replacement.
 *
 * This should be applied only to shared buffers, never local ones.
 *
 * Most but not all callers want CurrentResourceOwner to be adjusted.
 * Those that don't should pass fixOwner = FALSE.
 */
static void
UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
{
        int                     b = buf->buf_id;

        if (fixOwner)
                ResourceOwnerForgetBuffer(CurrentResourceOwner,
                                                                  BufferDescriptorGetBuffer(buf));

        Assert(PrivateRefCount[b] > 0);
        PrivateRefCount[b]--;
        if (PrivateRefCount[b] == 0)
        {
                /* I'd better not still hold any locks on the buffer */
                Assert(!LWLockHeldByMe(buf->content_lock));
                Assert(!LWLockHeldByMe(buf->io_in_progress_lock));

                LockBufHdr(buf);

                /* Decrement the shared reference count */
                Assert(buf->refcount > 0);
                buf->refcount--;

                /* Support LockBufferForCleanup() */
                if ((buf->flags & BM_PIN_COUNT_WAITER) &&
                        buf->refcount == 1)
                {
                        /* we just released the last pin other than the waiter's */
                        int                     wait_backend_pid = buf->wait_backend_pid;

                        buf->flags &= ~BM_PIN_COUNT_WAITER;
                        UnlockBufHdr(buf);
                        ProcSendSignal(wait_backend_pid);
                }
                else
                        UnlockBufHdr(buf);
        }
}

/*
 * BufferSync -- Write out all dirty buffers in the pool.
 *
 * This is called at checkpoint time to write out all dirty shared buffers.
 * The checkpoint request flags should be passed in; currently the only one
 * examined is CHECKPOINT_IMMEDIATE, which disables delays between writes.
 */
static void
BufferSync(int flags)
{
        int                     buf_id;
        int                     num_to_scan;
        int                     num_to_write;
        int                     num_written;

        /* Make sure we can handle the pin inside SyncOneBuffer */
        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

        /*
         * Loop over all buffers, and mark the ones that need to be written with
         * BM_CHECKPOINT_NEEDED.  Count them as we go (num_to_write), so that we
         * can estimate how much work needs to be done.
         *
         * This allows us to write only those pages that were dirty when the
         * checkpoint began, and not those that get dirtied while it proceeds.
         * Whenever a page with BM_CHECKPOINT_NEEDED is written out, either by us
         * later in this function, or by normal backends or the bgwriter cleaning
         * scan, the flag is cleared.  Any buffer dirtied after this point won't
         * have the flag set.
         *
         * Note that if we fail to write some buffer, we may leave buffers with
         * BM_CHECKPOINT_NEEDED still set.      This is OK since any such buffer would
         * certainly need to be written for the next checkpoint attempt, too.
         */
        num_to_write = 0;
        for (buf_id = 0; buf_id < NBuffers; buf_id++)
        {
                volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];

                /*
                 * Header spinlock is enough to examine BM_DIRTY, see comment in
                 * SyncOneBuffer.
                 */
                LockBufHdr(bufHdr);

                if (bufHdr->flags & BM_DIRTY)
                {
                        bufHdr->flags |= BM_CHECKPOINT_NEEDED;
                        num_to_write++;
                }

                UnlockBufHdr(bufHdr);
        }

        if (num_to_write == 0)
                return;                                 /* nothing to do */

        TRACE_POSTGRESQL_BUFFER_SYNC_START(NBuffers, num_to_write);

        /*
         * Loop over all buffers again, and write the ones (still) marked with
         * BM_CHECKPOINT_NEEDED.  In this loop, we start at the clock sweep point
         * since we might as well dump soon-to-be-recycled buffers first.
         *
         * Note that we don't read the buffer alloc count here --- that should be
         * left untouched till the next BgBufferSync() call.
         */
        buf_id = StrategySyncStart(NULL, NULL);
        num_to_scan = NBuffers;
        num_written = 0;
        while (num_to_scan-- > 0)
        {
                volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];

                /*
                 * We don't need to acquire the lock here, because we're only looking
                 * at a single bit. It's possible that someone else writes the buffer
                 * and clears the flag right after we check, but that doesn't matter
                 * since SyncOneBuffer will then do nothing.  However, there is a
                 * further race condition: it's conceivable that between the time we
                 * examine the bit here and the time SyncOneBuffer acquires lock,
                 * someone else not only wrote the buffer but replaced it with another
                 * page and dirtied it.  In that improbable case, SyncOneBuffer will
                 * write the buffer though we didn't need to.  It doesn't seem worth
                 * guarding against this, though.
                 */
                if (bufHdr->flags & BM_CHECKPOINT_NEEDED)
                {
                        if (SyncOneBuffer(buf_id, false) & BUF_WRITTEN)
                        {
                                TRACE_POSTGRESQL_BUFFER_SYNC_WRITTEN(buf_id);
                                BgWriterStats.m_buf_written_checkpoints++;
                                num_written++;

                                /*
                                 * We know there are at most num_to_write buffers with
                                 * BM_CHECKPOINT_NEEDED set; so we can stop scanning if
                                 * num_written reaches num_to_write.
                                 *
                                 * Note that num_written doesn't include buffers written by
                                 * other backends, or by the bgwriter cleaning scan. That
                                 * means that the estimate of how much progress we've made is
                                 * conservative, and also that this test will often fail to
                                 * trigger.  But it seems worth making anyway.
                                 */
                                if (num_written >= num_to_write)
                                        break;

                                /*
                                 * Perform normal bgwriter duties and sleep to throttle our
                                 * I/O rate.
                                 */
                                CheckpointWriteDelay(flags,
                                                                         (double) num_written / num_to_write);
                        }
                }

                if (++buf_id >= NBuffers)
                        buf_id = 0;
        }

        TRACE_POSTGRESQL_BUFFER_SYNC_DONE(NBuffers, num_written, num_to_write);

        /*
         * Update checkpoint statistics. As noted above, this doesn't include
         * buffers written by other backends or bgwriter scan.
         */
        CheckpointStats.ckpt_bufs_written += num_written;
}

/*
 * BgBufferSync -- Write out some dirty buffers in the pool.
 *
 * This is called periodically by the background writer process.
 */
void
BgBufferSync(void)
{
        /* info obtained from freelist.c */
        int                     strategy_buf_id;
        uint32          strategy_passes;
        uint32          recent_alloc;

        /*
         * Information saved between calls so we can determine the strategy
         * point's advance rate and avoid scanning already-cleaned buffers.
         */
        static bool saved_info_valid = false;
        static int      prev_strategy_buf_id;
        static uint32 prev_strategy_passes;
        static int      next_to_clean;
        static uint32 next_passes;

        /* Moving averages of allocation rate and clean-buffer density */
        static float smoothed_alloc = 0;
        static float smoothed_density = 10.0;

        /* Potentially these could be tunables, but for now, not */
        float           smoothing_samples = 16;
        float           scan_whole_pool_milliseconds = 120000.0;

        /* Used to compute how far we scan ahead */
        long            strategy_delta;
        int                     bufs_to_lap;
        int                     bufs_ahead;
        float           scans_per_alloc;
        int                     reusable_buffers_est;
        int                     upcoming_alloc_est;
        int                     min_scan_buffers;

        /* Variables for the scanning loop proper */
        int                     num_to_scan;
        int                     num_written;
        int                     reusable_buffers;

        /*
         * Find out where the freelist clock sweep currently is, and how many
         * buffer allocations have happened since our last call.
         */
        strategy_buf_id = StrategySyncStart(&strategy_passes, &recent_alloc);

        /* Report buffer alloc counts to pgstat */
        BgWriterStats.m_buf_alloc += recent_alloc;

        /*
         * If we're not running the LRU scan, just stop after doing the stats
         * stuff.  We mark the saved state invalid so that we can recover sanely
         * if LRU scan is turned back on later.
         */
        if (bgwriter_lru_maxpages <= 0)
        {
                saved_info_valid = false;
                return;
        }

        /*
         * Compute strategy_delta = how many buffers have been scanned by the
         * clock sweep since last time.  If first time through, assume none. Then
         * see if we are still ahead of the clock sweep, and if so, how many
         * buffers we could scan before we'd catch up with it and "lap" it. Note:
         * weird-looking coding of xxx_passes comparisons are to avoid bogus
         * behavior when the passes counts wrap around.
         */
        if (saved_info_valid)
        {
                int32           passes_delta = strategy_passes - prev_strategy_passes;

                strategy_delta = strategy_buf_id - prev_strategy_buf_id;
                strategy_delta += (long) passes_delta *NBuffers;

                Assert(strategy_delta >= 0);

                if ((int32) (next_passes - strategy_passes) > 0)
                {
                        /* we're one pass ahead of the strategy point */
                        bufs_to_lap = strategy_buf_id - next_to_clean;
#ifdef BGW_DEBUG
                        elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
                                 next_passes, next_to_clean,
                                 strategy_passes, strategy_buf_id,
                                 strategy_delta, bufs_to_lap);
#endif
                }
                else if (next_passes == strategy_passes &&
                                 next_to_clean >= strategy_buf_id)
                {
                        /* on same pass, but ahead or at least not behind */
                        bufs_to_lap = NBuffers - (next_to_clean - strategy_buf_id);
#ifdef BGW_DEBUG
                        elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
                                 next_passes, next_to_clean,
                                 strategy_passes, strategy_buf_id,
                                 strategy_delta, bufs_to_lap);
#endif
                }
                else
                {
                        /*
                         * We're behind, so skip forward to the strategy point and start
                         * cleaning from there.
                         */
#ifdef BGW_DEBUG
                        elog(DEBUG2, "bgwriter behind: bgw %u-%u strategy %u-%u delta=%ld",
                                 next_passes, next_to_clean,
                                 strategy_passes, strategy_buf_id,
                                 strategy_delta);
#endif
                        next_to_clean = strategy_buf_id;
                        next_passes = strategy_passes;
                        bufs_to_lap = NBuffers;
                }
        }
        else
        {
                /*
                 * Initializing at startup or after LRU scanning had been off. Always
                 * start at the strategy point.
                 */
#ifdef BGW_DEBUG
                elog(DEBUG2, "bgwriter initializing: strategy %u-%u",
                         strategy_passes, strategy_buf_id);
#endif
                strategy_delta = 0;
                next_to_clean = strategy_buf_id;
                next_passes = strategy_passes;
                bufs_to_lap = NBuffers;
        }

        /* Update saved info for next time */
        prev_strategy_buf_id = strategy_buf_id;
        prev_strategy_passes = strategy_passes;
        saved_info_valid = true;

        /*
         * Compute how many buffers had to be scanned for each new allocation, ie,
         * 1/density of reusable buffers, and track a moving average of that.
         *
         * If the strategy point didn't move, we don't update the density estimate
         */
        if (strategy_delta > 0 && recent_alloc > 0)
        {
                scans_per_alloc = (float) strategy_delta / (float) recent_alloc;
                smoothed_density += (scans_per_alloc - smoothed_density) /
                        smoothing_samples;
        }

        /*
         * Estimate how many reusable buffers there are between the current
         * strategy point and where we've scanned ahead to, based on the smoothed
         * density estimate.
         */
        bufs_ahead = NBuffers - bufs_to_lap;
        reusable_buffers_est = (float) bufs_ahead / smoothed_density;

        /*
         * Track a moving average of recent buffer allocations.  Here, rather than
         * a true average we want a fast-attack, slow-decline behavior: we
         * immediately follow any increase.
         */
        if (smoothed_alloc <= (float) recent_alloc)
                smoothed_alloc = recent_alloc;
        else
                smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
                        smoothing_samples;

        /* Scale the estimate by a GUC to allow more aggressive tuning. */
        upcoming_alloc_est = smoothed_alloc * bgwriter_lru_multiplier;

        /*
         * Even in cases where there's been little or no buffer allocation
         * activity, we want to make a small amount of progress through the buffer
         * cache so that as many reusable buffers as possible are clean after an
         * idle period.
         *
         * (scan_whole_pool_milliseconds / BgWriterDelay) computes how many times
         * the BGW will be called during the scan_whole_pool time; slice the
         * buffer pool into that many sections.
         */
        min_scan_buffers = (int) (NBuffers / (scan_whole_pool_milliseconds / BgWriterDelay));

        if (upcoming_alloc_est < (min_scan_buffers + reusable_buffers_est))
        {
#ifdef BGW_DEBUG
                elog(DEBUG2, "bgwriter: alloc_est=%d too small, using min=%d + reusable_est=%d",
                         upcoming_alloc_est, min_scan_buffers, reusable_buffers_est);
#endif
                upcoming_alloc_est = min_scan_buffers + reusable_buffers_est;
        }

        /*
         * Now write out dirty reusable buffers, working forward from the
         * next_to_clean point, until we have lapped the strategy scan, or cleaned
         * enough buffers to match our estimate of the next cycle's allocation
         * requirements, or hit the bgwriter_lru_maxpages limit.
         */

        /* Make sure we can handle the pin inside SyncOneBuffer */
        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

        num_to_scan = bufs_to_lap;
        num_written = 0;
        reusable_buffers = reusable_buffers_est;

        /* Execute the LRU scan */
        while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
        {
                int                     buffer_state = SyncOneBuffer(next_to_clean, true);

                if (++next_to_clean >= NBuffers)
                {
                        next_to_clean = 0;
                        next_passes++;
                }
                num_to_scan--;

                if (buffer_state & BUF_WRITTEN)
                {
                        reusable_buffers++;
                        if (++num_written >= bgwriter_lru_maxpages)
                        {
                                BgWriterStats.m_maxwritten_clean++;
                                break;
                        }
                }
                else if (buffer_state & BUF_REUSABLE)
                        reusable_buffers++;
        }

        BgWriterStats.m_buf_written_clean += num_written;

#ifdef BGW_DEBUG
        elog(DEBUG1, "bgwriter: recent_alloc=%u smoothed=%.2f delta=%ld ahead=%d density=%.2f reusable_est=%d upcoming_est=%d scanned=%d wrote=%d reusable=%d",
                 recent_alloc, smoothed_alloc, strategy_delta, bufs_ahead,
                 smoothed_density, reusable_buffers_est, upcoming_alloc_est,
                 bufs_to_lap - num_to_scan,
                 num_written,
                 reusable_buffers - reusable_buffers_est);
#endif

        /*
         * Consider the above scan as being like a new allocation scan.
         * Characterize its density and update the smoothed one based on it. This
         * effectively halves the moving average period in cases where both the
         * strategy and the background writer are doing some useful scanning,
         * which is helpful because a long memory isn't as desirable on the
         * density estimates.
         */
        strategy_delta = bufs_to_lap - num_to_scan;
        recent_alloc = reusable_buffers - reusable_buffers_est;
        if (strategy_delta > 0 && recent_alloc > 0)
        {
                scans_per_alloc = (float) strategy_delta / (float) recent_alloc;
                smoothed_density += (scans_per_alloc - smoothed_density) /
                        smoothing_samples;

#ifdef BGW_DEBUG
                elog(DEBUG2, "bgwriter: cleaner density alloc=%u scan=%ld density=%.2f new smoothed=%.2f",
                         recent_alloc, strategy_delta, scans_per_alloc, smoothed_density);
#endif
        }
}

/*
 * SyncOneBuffer -- process a single buffer during syncing.
 *
 * If skip_recently_used is true, we don't write currently-pinned buffers, nor
 * buffers marked recently used, as these are not replacement candidates.
 *
 * Returns a bitmask containing the following flag bits:
 *      BUF_WRITTEN: we wrote the buffer.
 *      BUF_REUSABLE: buffer is available for replacement, ie, it has
 *              pin count 0 and usage count 0.
 *
 * (BUF_WRITTEN could be set in error if FlushBuffers finds the buffer clean
 * after locking it, but we don't care all that much.)
 *
 * Note: caller must have done ResourceOwnerEnlargeBuffers.
 */
static int
SyncOneBuffer(int buf_id, bool skip_recently_used)
{
        volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
        int                     result = 0;

        /*
         * Check whether buffer needs writing.
         *
         * We can make this check without taking the buffer content lock so long
         * as we mark pages dirty in access methods *before* logging changes with
         * XLogInsert(): if someone marks the buffer dirty just after our check we
         * don't worry because our checkpoint.redo points before log record for
         * upcoming changes and so we are not required to write such dirty buffer.
         */
        LockBufHdr(bufHdr);

        if (bufHdr->refcount == 0 && bufHdr->usage_count == 0)
                result |= BUF_REUSABLE;
        else if (skip_recently_used)
        {
                /* Caller told us not to write recently-used buffers */
                UnlockBufHdr(bufHdr);
                return result;
        }

        if (!(bufHdr->flags & BM_VALID) || !(bufHdr->flags & BM_DIRTY))
        {
                /* It's clean, so nothing to do */
                UnlockBufHdr(bufHdr);
                return result;
        }

        /*
         * Pin it, share-lock it, write it.  (FlushBuffer will do nothing if the
         * buffer is clean by the time we've locked it.)
         */
        PinBuffer_Locked(bufHdr);
        LWLockAcquire(bufHdr->content_lock, LW_SHARED);

        FlushBuffer(bufHdr, NULL);

        LWLockRelease(bufHdr->content_lock);
        UnpinBuffer(bufHdr, true);

        return result | BUF_WRITTEN;
}


/*
 * Return a palloc'd string containing buffer usage statistics.
 */
char *
ShowBufferUsage(void)
{
        StringInfoData str;
        float           hitrate;
        float           localhitrate;

        initStringInfo(&str);

        if (ReadBufferCount == 0)
                hitrate = 0.0;
        else
                hitrate = (float) BufferHitCount *100.0 / ReadBufferCount;

        if (ReadLocalBufferCount == 0)
                localhitrate = 0.0;
        else
                localhitrate = (float) LocalBufferHitCount *100.0 / ReadLocalBufferCount;

        appendStringInfo(&str,
        "!\tShared blocks: %10ld read, %10ld written, buffer hit rate = %.2f%%\n",
                                ReadBufferCount - BufferHitCount, BufferFlushCount, hitrate);
        appendStringInfo(&str,
        "!\tLocal  blocks: %10ld read, %10ld written, buffer hit rate = %.2f%%\n",
                                         ReadLocalBufferCount - LocalBufferHitCount, LocalBufferFlushCount, localhitrate);
        appendStringInfo(&str,
                                         "!\tDirect blocks: %10ld read, %10ld written\n",
                                         BufFileReadCount, BufFileWriteCount);

        return str.data;
}

void
ResetBufferUsage(void)
{
        BufferHitCount = 0;
        ReadBufferCount = 0;
        BufferFlushCount = 0;
        LocalBufferHitCount = 0;
        ReadLocalBufferCount = 0;
        LocalBufferFlushCount = 0;
        BufFileReadCount = 0;
        BufFileWriteCount = 0;
}

/*
 *              AtEOXact_Buffers - clean up at end of transaction.
 *
 *              As of PostgreSQL 8.0, buffer pins should get released by the
 *              ResourceOwner mechanism.  This routine is just a debugging
 *              cross-check that no pins remain.
 */
void
AtEOXact_Buffers(bool isCommit)
{
#ifdef USE_ASSERT_CHECKING
        if (assert_enabled)
        {
                int                     i;

                for (i = 0; i < NBuffers; i++)
                {
                        Assert(PrivateRefCount[i] == 0);
                }
        }
#endif

        AtEOXact_LocalBuffers(isCommit);
}

/*
 * InitBufferPoolBackend --- second-stage initialization of a new backend
 *
 * This is called after we have acquired a PGPROC and so can safely get
 * LWLocks.  We don't currently need to do anything at this stage ...
 * except register a shmem-exit callback.  AtProcExit_Buffers needs LWLock
 * access, and thereby has to be called at the corresponding phase of
 * backend shutdown.
 */
void
InitBufferPoolBackend(void)
{
        on_shmem_exit(AtProcExit_Buffers, 0);
}

/*
 * Ensure we have released all shared-buffer locks and pins during backend exit
 */
static void
AtProcExit_Buffers(int code, Datum arg)
{
        int                     i;

        AbortBufferIO();
        UnlockBuffers();

        for (i = 0; i < NBuffers; i++)
        {
                if (PrivateRefCount[i] != 0)
                {
                        volatile BufferDesc *buf = &(BufferDescriptors[i]);

                        /*
                         * We don't worry about updating ResourceOwner; if we even got
                         * here, it suggests that ResourceOwners are messed up.
                         */
                        PrivateRefCount[i] = 1;         /* make sure we release shared pin */
                        UnpinBuffer(buf, false);
                        Assert(PrivateRefCount[i] == 0);
                }
        }

        /* localbuf.c needs a chance too */
        AtProcExit_LocalBuffers();
}

/*
 * Helper routine to issue warnings when a buffer is unexpectedly pinned
 */
void
PrintBufferLeakWarning(Buffer buffer)
{
        volatile BufferDesc *buf;
        int32           loccount;

        Assert(BufferIsValid(buffer));
        if (BufferIsLocal(buffer))
        {
                buf = &LocalBufferDescriptors[-buffer - 1];
                loccount = LocalRefCount[-buffer - 1];
        }
        else
        {
                buf = &BufferDescriptors[buffer - 1];
                loccount = PrivateRefCount[buffer - 1];
        }

        /* theoretically we should lock the bufhdr here */
        elog(WARNING,
                 "buffer refcount leak: [%03d] "
                 "(rel=%u/%u/%u, blockNum=%u, flags=0x%x, refcount=%u %d)",
                 buffer,
                 buf->tag.rnode.spcNode, buf->tag.rnode.dbNode,
                 buf->tag.rnode.relNode,
                 buf->tag.blockNum, buf->flags,
                 buf->refcount, loccount);
}

/*
 * CheckPointBuffers
 *
 * Flush all dirty blocks in buffer pool to disk at checkpoint time.
 *
 * Note: temporary relations do not participate in checkpoints, so they don't
 * need to be flushed.
 */
void
CheckPointBuffers(int flags)
{
        TRACE_POSTGRESQL_BUFFER_CHECKPOINT_START(flags);
        CheckpointStats.ckpt_write_t = GetCurrentTimestamp();
        BufferSync(flags);
        CheckpointStats.ckpt_sync_t = GetCurrentTimestamp();
        smgrsync();
        CheckpointStats.ckpt_sync_end_t = GetCurrentTimestamp();
        TRACE_POSTGRESQL_BUFFER_CHECKPOINT_DONE();
}


/*
 * Do whatever is needed to prepare for commit at the bufmgr and smgr levels
 */
void
BufmgrCommit(void)
{
        /* Nothing to do in bufmgr anymore... */

        smgrcommit();
}

/*
 * BufferGetBlockNumber
 *              Returns the block number associated with a buffer.
 *
 * Note:
 *              Assumes that the buffer is valid and pinned, else the
 *              value may be obsolete immediately...
 */
BlockNumber
BufferGetBlockNumber(Buffer buffer)
{
        volatile BufferDesc *bufHdr;

        Assert(BufferIsPinned(buffer));

        if (BufferIsLocal(buffer))
                bufHdr = &(LocalBufferDescriptors[-buffer - 1]);
        else
                bufHdr = &BufferDescriptors[buffer - 1];

        /* pinned, so OK to read tag without spinlock */
        return bufHdr->tag.blockNum;
}

/*
 * BufferGetTag
 *              Returns the relfilenode, fork number and block number associated with
 *              a buffer.
 */
void
BufferGetTag(Buffer buffer, RelFileNode *rnode, ForkNumber *forknum,
                         BlockNumber *blknum)
{
        volatile BufferDesc *bufHdr;

        /* Do the same checks as BufferGetBlockNumber. */
        Assert(BufferIsPinned(buffer));

        if (BufferIsLocal(buffer))
                bufHdr = &(LocalBufferDescriptors[-buffer - 1]);
        else
                bufHdr = &BufferDescriptors[buffer - 1];

        /* pinned, so OK to read tag without spinlock */
        *rnode = bufHdr->tag.rnode;
        *forknum = bufHdr->tag.forkNum;
        *blknum = bufHdr->tag.blockNum;
}

/*
 * FlushBuffer
 *              Physically write out a shared buffer.
 *
 * NOTE: this actually just passes the buffer contents to the kernel; the
 * real write to disk won't happen until the kernel feels like it.  This
 * is okay from our point of view since we can redo the changes from WAL.
 * However, we will need to force the changes to disk via fsync before
 * we can checkpoint WAL.
 *
 * The caller must hold a pin on the buffer and have share-locked the
 * buffer contents.  (Note: a share-lock does not prevent updates of
 * hint bits in the buffer, so the page could change while the write
 * is in progress, but we assume that that will not invalidate the data
 * written.)
 *
 * If the caller has an smgr reference for the buffer's relation, pass it
 * as the second parameter.  If not, pass NULL.
 */
static void
FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln)
{
        XLogRecPtr      recptr;
        ErrorContextCallback errcontext;

        /*
         * Acquire the buffer's io_in_progress lock.  If StartBufferIO returns
         * false, then someone else flushed the buffer before we could, so we need
         * not do anything.
         */
        if (!StartBufferIO(buf, false))
                return;

        /* Setup error traceback support for ereport() */
        errcontext.callback = buffer_write_error_callback;
        errcontext.arg = (void *) buf;
        errcontext.previous = error_context_stack;
        error_context_stack = &errcontext;

        /* Find smgr relation for buffer */
        if (reln == NULL)
                reln = smgropen(buf->tag.rnode);

        TRACE_POSTGRESQL_BUFFER_FLUSH_START(reln->smgr_rnode.spcNode,
                 reln->smgr_rnode.dbNode,
                 reln->smgr_rnode.relNode);

        /*
         * Force XLOG flush up to buffer's LSN.  This implements the basic WAL
         * rule that log updates must hit disk before any of the data-file changes
         * they describe do.
         */
        recptr = BufferGetLSN(buf);
        XLogFlush(recptr);

        /*
         * Now it's safe to write buffer to disk. Note that no one else should
         * have been able to write it while we were busy with log flushing because
         * we have the io_in_progress lock.
         */

        /* To check if block content changes while flushing. - vadim 01/17/97 */
        LockBufHdr(buf);
        buf->flags &= ~BM_JUST_DIRTIED;
        UnlockBufHdr(buf);

        smgrwrite(reln,
                          buf->tag.forkNum,
                          buf->tag.blockNum,
                          (char *) BufHdrGetBlock(buf),
                          false);

        BufferFlushCount++;

        TRACE_POSTGRESQL_BUFFER_FLUSH_DONE(reln->smgr_rnode.spcNode,
                 reln->smgr_rnode.dbNode, reln->smgr_rnode.relNode);

        /*
         * Mark the buffer as clean (unless BM_JUST_DIRTIED has become set) and
         * end the io_in_progress state.
         */
        TerminateBufferIO(buf, true, 0);

        /* Pop the error context stack */
        error_context_stack = errcontext.previous;
}

/*
 * RelationGetNumberOfBlocks
 *              Determines the current number of pages in the relation.
 */
BlockNumber
RelationGetNumberOfBlocks(Relation relation)
{
        /* Open it at the smgr level if not already done */
        RelationOpenSmgr(relation);

        return smgrnblocks(relation->rd_smgr, MAIN_FORKNUM);
}

/*
 * RelationTruncate
 *              Physically truncate a relation to the specified number of blocks.
 *
 * As of Postgres 8.1, this includes getting rid of any buffers for the
 * blocks that are to be dropped; previously, callers had to do that.
 */
void
RelationTruncate(Relation rel, BlockNumber nblocks)
{
        /* Open it at the smgr level if not already done */
        RelationOpenSmgr(rel);

        /* Make sure rd_targblock isn't pointing somewhere past end */
        rel->rd_targblock = InvalidBlockNumber;

        /* Do the real work */
        smgrtruncate(rel->rd_smgr, MAIN_FORKNUM, nblocks, rel->rd_istemp);
}

/* ---------------------------------------------------------------------
 *              DropRelFileNodeBuffers
 *
 *              This function removes from the buffer pool all the pages of the
 *              specified relation that have block numbers >= firstDelBlock.
 *              (In particular, with firstDelBlock = 0, all pages are removed.)
 *              Dirty pages are simply dropped, without bothering to write them
 *              out first.      Therefore, this is NOT rollback-able, and so should be
 *              used only with extreme caution!
 *
 *              Currently, this is called only from smgr.c when the underlying file
 *              is about to be deleted or truncated (firstDelBlock is needed for
 *              the truncation case).  The data in the affected pages would therefore
 *              be deleted momentarily anyway, and there is no point in writing it.
 *              It is the responsibility of higher-level code to ensure that the
 *              deletion or truncation does not lose any data that could be needed
 *              later.  It is also the responsibility of higher-level code to ensure
 *              that no other process could be trying to load more pages of the
 *              relation into buffers.
 *
 *              XXX currently it sequentially searches the buffer pool, should be
 *              changed to more clever ways of searching.  However, this routine
 *              is used only in code paths that aren't very performance-critical,
 *              and we shouldn't slow down the hot paths to make it faster ...
 * --------------------------------------------------------------------
 */
void
DropRelFileNodeBuffers(RelFileNode rnode, ForkNumber forkNum, bool istemp,
                                           BlockNumber firstDelBlock)
{
        int                     i;

        if (istemp)
        {
                DropRelFileNodeLocalBuffers(rnode, forkNum, firstDelBlock);
                return;
        }

        for (i = 0; i < NBuffers; i++)
        {
                volatile BufferDesc *bufHdr = &BufferDescriptors[i];

                LockBufHdr(bufHdr);
                if (RelFileNodeEquals(bufHdr->tag.rnode, rnode) &&
                        bufHdr->tag.forkNum == forkNum &&
                        bufHdr->tag.blockNum >= firstDelBlock)
                        InvalidateBuffer(bufHdr);       /* releases spinlock */
                else
                        UnlockBufHdr(bufHdr);
        }
}

/* ---------------------------------------------------------------------
 *              DropDatabaseBuffers
 *
 *              This function removes all the buffers in the buffer cache for a
 *              particular database.  Dirty pages are simply dropped, without
 *              bothering to write them out first.      This is used when we destroy a
 *              database, to avoid trying to flush data to disk when the directory
 *              tree no longer exists.  Implementation is pretty similar to
 *              DropRelFileNodeBuffers() which is for destroying just one relation.
 * --------------------------------------------------------------------
 */
void
DropDatabaseBuffers(Oid dbid)
{
        int                     i;
        volatile BufferDesc *bufHdr;

        /*
         * We needn't consider local buffers, since by assumption the target
         * database isn't our own.
         */

        for (i = 0; i < NBuffers; i++)
        {
                bufHdr = &BufferDescriptors[i];
                LockBufHdr(bufHdr);
                if (bufHdr->tag.rnode.dbNode == dbid)
                        InvalidateBuffer(bufHdr);       /* releases spinlock */
                else
                        UnlockBufHdr(bufHdr);
        }
}

/* -----------------------------------------------------------------
 *              PrintBufferDescs
 *
 *              this function prints all the buffer descriptors, for debugging
 *              use only.
 * -----------------------------------------------------------------
 */
#ifdef NOT_USED
void
PrintBufferDescs(void)
{
        int                     i;
        volatile BufferDesc *buf = BufferDescriptors;

        for (i = 0; i < NBuffers; ++i, ++buf)
        {
                /* theoretically we should lock the bufhdr here */
                elog(LOG,
                         "[%02d] (freeNext=%d, rel=%u/%u/%u, "
                         "blockNum=%u, flags=0x%x, refcount=%u %d)",
                         i, buf->freeNext,
                         buf->tag.rnode.spcNode, buf->tag.rnode.dbNode,
                         buf->tag.rnode.relNode,
                         buf->tag.blockNum, buf->flags,
                         buf->refcount, PrivateRefCount[i]);
        }
}
#endif

#ifdef NOT_USED
void
PrintPinnedBufs(void)
{
        int                     i;
        volatile BufferDesc *buf = BufferDescriptors;

        for (i = 0; i < NBuffers; ++i, ++buf)
        {
                if (PrivateRefCount[i] > 0)
                {
                        /* theoretically we should lock the bufhdr here */
                        elog(LOG,
                                 "[%02d] (freeNext=%d, rel=%u/%u/%u, "
                                 "blockNum=%u, flags=0x%x, refcount=%u %d)",
                                 i, buf->freeNext,
                                 buf->tag.rnode.spcNode, buf->tag.rnode.dbNode,
                                 buf->tag.rnode.relNode,
                                 buf->tag.blockNum, buf->flags,
                                 buf->refcount, PrivateRefCount[i]);
                }
        }
}
#endif

/* ---------------------------------------------------------------------
 *              FlushRelationBuffers
 *
 *              This function writes all dirty pages of a relation out to disk
 *              (or more accurately, out to kernel disk buffers), ensuring that the
 *              kernel has an up-to-date view of the relation.
 *
 *              Generally, the caller should be holding AccessExclusiveLock on the
 *              target relation to ensure that no other backend is busy dirtying
 *              more blocks of the relation; the effects can't be expected to last
 *              after the lock is released.
 *
 *              XXX currently it sequentially searches the buffer pool, should be
 *              changed to more clever ways of searching.  This routine is not
 *              used in any performance-critical code paths, so it's not worth
 *              adding additional overhead to normal paths to make it go faster;
 *              but see also DropRelFileNodeBuffers.
 * --------------------------------------------------------------------
 */
void
FlushRelationBuffers(Relation rel)
{
        int                     i;
        volatile BufferDesc *bufHdr;

        /* Open rel at the smgr level if not already done */
        RelationOpenSmgr(rel);

        if (rel->rd_istemp)
        {
                for (i = 0; i < NLocBuffer; i++)
                {
                        bufHdr = &LocalBufferDescriptors[i];
                        if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
                                (bufHdr->flags & BM_VALID) && (bufHdr->flags & BM_DIRTY))
                        {
                                ErrorContextCallback errcontext;

                                /* Setup error traceback support for ereport() */
                                errcontext.callback = buffer_write_error_callback;
                                errcontext.arg = (void *) bufHdr;
                                errcontext.previous = error_context_stack;
                                error_context_stack = &errcontext;

                                smgrwrite(rel->rd_smgr,
                                                  bufHdr->tag.forkNum,
                                                  bufHdr->tag.blockNum,
                                                  (char *) LocalBufHdrGetBlock(bufHdr),
                                                  true);

                                bufHdr->flags &= ~(BM_DIRTY | BM_JUST_DIRTIED);

                                /* Pop the error context stack */
                                error_context_stack = errcontext.previous;
                        }
                }

                return;
        }

        /* Make sure we can handle the pin inside the loop */
        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

        for (i = 0; i < NBuffers; i++)
        {
                bufHdr = &BufferDescriptors[i];
                LockBufHdr(bufHdr);
                if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
                        (bufHdr->flags & BM_VALID) && (bufHdr->flags & BM_DIRTY))
                {
                        PinBuffer_Locked(bufHdr);
                        LWLockAcquire(bufHdr->content_lock, LW_SHARED);
                        FlushBuffer(bufHdr, rel->rd_smgr);
                        LWLockRelease(bufHdr->content_lock);
                        UnpinBuffer(bufHdr, true);
                }
                else
                        UnlockBufHdr(bufHdr);
        }
}

/* ---------------------------------------------------------------------
 *              FlushDatabaseBuffers
 *
 *              This function writes all dirty pages of a database out to disk
 *              (or more accurately, out to kernel disk buffers), ensuring that the
 *              kernel has an up-to-date view of the database.
 *
 *              Generally, the caller should be holding an appropriate lock to ensure
 *              no other backend is active in the target database; otherwise more
 *              pages could get dirtied.
 *
 *              Note we don't worry about flushing any pages of temporary relations.
 *              It's assumed these wouldn't be interesting.
 * --------------------------------------------------------------------
 */
void
FlushDatabaseBuffers(Oid dbid)
{
        int                     i;
        volatile BufferDesc *bufHdr;

        /* Make sure we can handle the pin inside the loop */
        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

        for (i = 0; i < NBuffers; i++)
        {
                bufHdr = &BufferDescriptors[i];
                LockBufHdr(bufHdr);
                if (bufHdr->tag.rnode.dbNode == dbid &&
                        (bufHdr->flags & BM_VALID) && (bufHdr->flags & BM_DIRTY))
                {
                        PinBuffer_Locked(bufHdr);
                        LWLockAcquire(bufHdr->content_lock, LW_SHARED);
                        FlushBuffer(bufHdr, NULL);
                        LWLockRelease(bufHdr->content_lock);
                        UnpinBuffer(bufHdr, true);
                }
                else
                        UnlockBufHdr(bufHdr);
        }
}

/*
 * ReleaseBuffer -- release the pin on a buffer
 */
void
ReleaseBuffer(Buffer buffer)
{
        volatile BufferDesc *bufHdr;

        if (!BufferIsValid(buffer))
                elog(ERROR, "bad buffer id: %d", buffer);

        ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);

        if (BufferIsLocal(buffer))
        {
                Assert(LocalRefCount[-buffer - 1] > 0);
                LocalRefCount[-buffer - 1]--;
                return;
        }

        bufHdr = &BufferDescriptors[buffer - 1];

        Assert(PrivateRefCount[buffer - 1] > 0);

        if (PrivateRefCount[buffer - 1] > 1)
                PrivateRefCount[buffer - 1]--;
        else
                UnpinBuffer(bufHdr, false);
}

/*
 * UnlockReleaseBuffer -- release the content lock and pin on a buffer
 *
 * This is just a shorthand for a common combination.
 */
void
UnlockReleaseBuffer(Buffer buffer)
{
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        ReleaseBuffer(buffer);
}

/*
 * IncrBufferRefCount
 *              Increment the pin count on a buffer that we have *already* pinned
 *              at least once.
 *
 *              This function cannot be used on a buffer we do not have pinned,
 *              because it doesn't change the shared buffer state.
 */
void
IncrBufferRefCount(Buffer buffer)
{
        Assert(BufferIsPinned(buffer));
        ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
        ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
        if (BufferIsLocal(buffer))
                LocalRefCount[-buffer - 1]++;
        else
                PrivateRefCount[buffer - 1]++;
}

/*
 * SetBufferCommitInfoNeedsSave
 *
 *      Mark a buffer dirty when we have updated tuple commit-status bits in it.
 *
 * This is essentially the same as MarkBufferDirty, except that the caller
 * might have only share-lock instead of exclusive-lock on the buffer's
 * content lock.  We preserve the distinction mainly as a way of documenting
 * that the caller has not made a critical data change --- the status-bit
 * update could be redone by someone else just as easily.  Therefore, no WAL
 * log record need be generated, whereas calls to MarkBufferDirty really ought
 * to be associated with a WAL-entry-creating action.
 */
void
SetBufferCommitInfoNeedsSave(Buffer buffer)
{
        volatile BufferDesc *bufHdr;

        if (!BufferIsValid(buffer))
                elog(ERROR, "bad buffer id: %d", buffer);

        if (BufferIsLocal(buffer))
        {
                MarkLocalBufferDirty(buffer);
                return;
        }

        bufHdr = &BufferDescriptors[buffer - 1];

        Assert(PrivateRefCount[buffer - 1] > 0);
        /* here, either share or exclusive lock is OK */
        Assert(LWLockHeldByMe(bufHdr->content_lock));

        /*
         * This routine might get called many times on the same page, if we are
         * making the first scan after commit of an xact that added/deleted many
         * tuples.      So, be as quick as we can if the buffer is already dirty.  We
         * do this by not acquiring spinlock if it looks like the status bits are
         * already OK.  (Note it is okay if someone else clears BM_JUST_DIRTIED
         * immediately after we look, because the buffer content update is already
         * done and will be reflected in the I/O.)
         */
        if ((bufHdr->flags & (BM_DIRTY | BM_JUST_DIRTIED)) !=
                (BM_DIRTY | BM_JUST_DIRTIED))
        {
                LockBufHdr(bufHdr);
                Assert(bufHdr->refcount > 0);
                if (!(bufHdr->flags & BM_DIRTY) && VacuumCostActive)
                        VacuumCostBalance += VacuumCostPageDirty;
                bufHdr->flags |= (BM_DIRTY | BM_JUST_DIRTIED);
                UnlockBufHdr(bufHdr);
        }
}

/*
 * Release buffer content locks for shared buffers.
 *
 * Used to clean up after errors.
 *
 * Currently, we can expect that lwlock.c's LWLockReleaseAll() took care
 * of releasing buffer content locks per se; the only thing we need to deal
 * with here is clearing any PIN_COUNT request that was in progress.
 */
void
UnlockBuffers(void)
{
        volatile BufferDesc *buf = PinCountWaitBuf;

        if (buf)
        {
                LockBufHdr(buf);

                /*
                 * Don't complain if flag bit not set; it could have been reset but we
                 * got a cancel/die interrupt before getting the signal.
                 */
                if ((buf->flags & BM_PIN_COUNT_WAITER) != 0 &&
                        buf->wait_backend_pid == MyProcPid)
                        buf->flags &= ~BM_PIN_COUNT_WAITER;

                UnlockBufHdr(buf);

                PinCountWaitBuf = NULL;
        }
}

/*
 * Acquire or release the content_lock for the buffer.
 */
void
LockBuffer(Buffer buffer, int mode)
{
        volatile BufferDesc *buf;

        Assert(BufferIsValid(buffer));
        if (BufferIsLocal(buffer))
                return;                                 /* local buffers need no lock */

        buf = &(BufferDescriptors[buffer - 1]);

        if (mode == BUFFER_LOCK_UNLOCK)
                LWLockRelease(buf->content_lock);
        else if (mode == BUFFER_LOCK_SHARE)
                LWLockAcquire(buf->content_lock, LW_SHARED);
        else if (mode == BUFFER_LOCK_EXCLUSIVE)
                LWLockAcquire(buf->content_lock, LW_EXCLUSIVE);
        else
                elog(ERROR, "unrecognized buffer lock mode: %d", mode);
}

/*
 * Acquire the content_lock for the buffer, but only if we don't have to wait.
 *
 * This assumes the caller wants BUFFER_LOCK_EXCLUSIVE mode.
 */
bool
ConditionalLockBuffer(Buffer buffer)
{
        volatile BufferDesc *buf;

        Assert(BufferIsValid(buffer));
        if (BufferIsLocal(buffer))
                return true;                    /* act as though we got it */

        buf = &(BufferDescriptors[buffer - 1]);

        return LWLockConditionalAcquire(buf->content_lock, LW_EXCLUSIVE);
}

/*
 * LockBufferForCleanup - lock a buffer in preparation for deleting items
 *
 * Items may be deleted from a disk page only when the caller (a) holds an
 * exclusive lock on the buffer and (b) has observed that no other backend
 * holds a pin on the buffer.  If there is a pin, then the other backend
 * might have a pointer into the buffer (for example, a heapscan reference
 * to an item --- see README for more details).  It's OK if a pin is added
 * after the cleanup starts, however; the newly-arrived backend will be
 * unable to look at the page until we release the exclusive lock.
 *
 * To implement this protocol, a would-be deleter must pin the buffer and
 * then call LockBufferForCleanup().  LockBufferForCleanup() is similar to
 * LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE), except that it loops until
 * it has successfully observed pin count = 1.
 */
void
LockBufferForCleanup(Buffer buffer)
{
        volatile BufferDesc *bufHdr;

        Assert(BufferIsValid(buffer));
        Assert(PinCountWaitBuf == NULL);

        if (BufferIsLocal(buffer))
        {
                /* There should be exactly one pin */
                if (LocalRefCount[-buffer - 1] != 1)
                        elog(ERROR, "incorrect local pin count: %d",
                                 LocalRefCount[-buffer - 1]);
                /* Nobody else to wait for */
                return;
        }

        /* There should be exactly one local pin */
        if (PrivateRefCount[buffer - 1] != 1)
                elog(ERROR, "incorrect local pin count: %d",
                         PrivateRefCount[buffer - 1]);

        bufHdr = &BufferDescriptors[buffer - 1];

        for (;;)
        {
                /* Try to acquire lock */
                LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
                LockBufHdr(bufHdr);
                Assert(bufHdr->refcount > 0);
                if (bufHdr->refcount == 1)
                {
                        /* Successfully acquired exclusive lock with pincount 1 */
                        UnlockBufHdr(bufHdr);
                        return;
                }
                /* Failed, so mark myself as waiting for pincount 1 */
                if (bufHdr->flags & BM_PIN_COUNT_WAITER)
                {
                        UnlockBufHdr(bufHdr);
                        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
                        elog(ERROR, "multiple backends attempting to wait for pincount 1");
                }
                bufHdr->wait_backend_pid = MyProcPid;
                bufHdr->flags |= BM_PIN_COUNT_WAITER;
                PinCountWaitBuf = bufHdr;
                UnlockBufHdr(bufHdr);
                LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
                /* Wait to be signaled by UnpinBuffer() */
                ProcWaitForSignal();
                PinCountWaitBuf = NULL;
                /* Loop back and try again */
        }
}

/*
 * ConditionalLockBufferForCleanup - as above, but don't wait to get the lock
 *
 * We won't loop, but just check once to see if the pin count is OK.  If
 * not, return FALSE with no lock held.
 */
bool
ConditionalLockBufferForCleanup(Buffer buffer)
{
        volatile BufferDesc *bufHdr;

        Assert(BufferIsValid(buffer));

        if (BufferIsLocal(buffer))
        {
                /* There should be exactly one pin */
                Assert(LocalRefCount[-buffer - 1] > 0);
                if (LocalRefCount[-buffer - 1] != 1)
                        return false;
                /* Nobody else to wait for */
                return true;
        }

        /* There should be exactly one local pin */
        Assert(PrivateRefCount[buffer - 1] > 0);
        if (PrivateRefCount[buffer - 1] != 1)
                return false;

        /* Try to acquire lock */
        if (!ConditionalLockBuffer(buffer))
                return false;

        bufHdr = &BufferDescriptors[buffer - 1];
        LockBufHdr(bufHdr);
        Assert(bufHdr->refcount > 0);
        if (bufHdr->refcount == 1)
        {
                /* Successfully acquired exclusive lock with pincount 1 */
                UnlockBufHdr(bufHdr);
                return true;
        }

        /* Failed, so release the lock */
        UnlockBufHdr(bufHdr);
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        return false;
}


/*
 *      Functions for buffer I/O handling
 *
 *      Note: We assume that nested buffer I/O never occurs.
 *      i.e at most one io_in_progress lock is held per proc.
 *
 *      Also note that these are used only for shared buffers, not local ones.
 */

/*
 * WaitIO -- Block until the IO_IN_PROGRESS flag on 'buf' is cleared.
 */
static void
WaitIO(volatile BufferDesc *buf)
{
        /*
         * Changed to wait until there's no IO - Inoue 01/13/2000
         *
         * Note this is *necessary* because an error abort in the process doing
         * I/O could release the io_in_progress_lock prematurely. See
         * AbortBufferIO.
         */
        for (;;)
        {
                BufFlags        sv_flags;

                /*
                 * It may not be necessary to acquire the spinlock to check the flag
                 * here, but since this test is essential for correctness, we'd better
                 * play it safe.
                 */
                LockBufHdr(buf);
                sv_flags = buf->flags;
                UnlockBufHdr(buf);
                if (!(sv_flags & BM_IO_IN_PROGRESS))
                        break;
                LWLockAcquire(buf->io_in_progress_lock, LW_SHARED);
                LWLockRelease(buf->io_in_progress_lock);
        }
}

/*
 * StartBufferIO: begin I/O on this buffer
 *      (Assumptions)
 *      My process is executing no IO
 *      The buffer is Pinned
 *
 * In some scenarios there are race conditions in which multiple backends
 * could attempt the same I/O operation concurrently.  If someone else
 * has already started I/O on this buffer then we will block on the
 * io_in_progress lock until he's done.
 *
 * Input operations are only attempted on buffers that are not BM_VALID,
 * and output operations only on buffers that are BM_VALID and BM_DIRTY,
 * so we can always tell if the work is already done.
 *
 * Returns TRUE if we successfully marked the buffer as I/O busy,
 * FALSE if someone else already did the work.
 */
static bool
StartBufferIO(volatile BufferDesc *buf, bool forInput)
{
        Assert(!InProgressBuf);

        for (;;)
        {
                /*
                 * Grab the io_in_progress lock so that other processes can wait for
                 * me to finish the I/O.
                 */
                LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);

                LockBufHdr(buf);

                if (!(buf->flags & BM_IO_IN_PROGRESS))
                        break;

                /*
                 * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
                 * lock isn't held is if the process doing the I/O is recovering from
                 * an error (see AbortBufferIO).  If that's the case, we must wait for
                 * him to get unwedged.
                 */
                UnlockBufHdr(buf);
                LWLockRelease(buf->io_in_progress_lock);
                WaitIO(buf);
        }

        /* Once we get here, there is definitely no I/O active on this buffer */

        if (forInput ? (buf->flags & BM_VALID) : !(buf->flags & BM_DIRTY))
        {
                /* someone else already did the I/O */
                UnlockBufHdr(buf);
                LWLockRelease(buf->io_in_progress_lock);
                return false;
        }

        buf->flags |= BM_IO_IN_PROGRESS;

        UnlockBufHdr(buf);

        InProgressBuf = buf;
        IsForInput = forInput;

        return true;
}

/*
 * TerminateBufferIO: release a buffer we were doing I/O on
 *      (Assumptions)
 *      My process is executing IO for the buffer
 *      BM_IO_IN_PROGRESS bit is set for the buffer
 *      We hold the buffer's io_in_progress lock
 *      The buffer is Pinned
 *
 * If clear_dirty is TRUE and BM_JUST_DIRTIED is not set, we clear the
 * buffer's BM_DIRTY flag.  This is appropriate when terminating a
 * successful write.  The check on BM_JUST_DIRTIED is necessary to avoid
 * marking the buffer clean if it was re-dirtied while we were writing.
 *
 * set_flag_bits gets ORed into the buffer's flags.  It must include
 * BM_IO_ERROR in a failure case.  For successful completion it could
 * be 0, or BM_VALID if we just finished reading in the page.
 */
static void
TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
                                  int set_flag_bits)
{
        Assert(buf == InProgressBuf);

        LockBufHdr(buf);

        Assert(buf->flags & BM_IO_IN_PROGRESS);
        buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
        if (clear_dirty && !(buf->flags & BM_JUST_DIRTIED))
                buf->flags &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
        buf->flags |= set_flag_bits;

        UnlockBufHdr(buf);

        InProgressBuf = NULL;

        LWLockRelease(buf->io_in_progress_lock);
}

/*
 * AbortBufferIO: Clean up any active buffer I/O after an error.
 *
 *      All LWLocks we might have held have been released,
 *      but we haven't yet released buffer pins, so the buffer is still pinned.
 *
 *      If I/O was in progress, we always set BM_IO_ERROR, even though it's
 *      possible the error condition wasn't related to the I/O.
 */
void
AbortBufferIO(void)
{
        volatile BufferDesc *buf = InProgressBuf;

        if (buf)
        {
                /*
                 * Since LWLockReleaseAll has already been called, we're not holding
                 * the buffer's io_in_progress_lock. We have to re-acquire it so that
                 * we can use TerminateBufferIO. Anyone who's executing WaitIO on the
                 * buffer will be in a busy spin until we succeed in doing this.
                 */
                LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);

                LockBufHdr(buf);
                Assert(buf->flags & BM_IO_IN_PROGRESS);
                if (IsForInput)
                {
                        Assert(!(buf->flags & BM_DIRTY));
                        /* We'd better not think buffer is valid yet */
                        Assert(!(buf->flags & BM_VALID));
                        UnlockBufHdr(buf);
                }
                else
                {
                        BufFlags        sv_flags;

                        sv_flags = buf->flags;
                        Assert(sv_flags & BM_DIRTY);
                        UnlockBufHdr(buf);
                        /* Issue notice if this is not the first failure... */
                        if (sv_flags & BM_IO_ERROR)
                        {
                                /* Buffer is pinned, so we can read tag without spinlock */
                                ereport(WARNING,
                                                (errcode(ERRCODE_IO_ERROR),
                                                 errmsg("could not write block %u of %u/%u/%u",
                                                                buf->tag.blockNum,
                                                                buf->tag.rnode.spcNode,
                                                                buf->tag.rnode.dbNode,
                                                                buf->tag.rnode.relNode),
                                                 errdetail("Multiple failures --- write error might be permanent.")));
                        }
                }
                TerminateBufferIO(buf, false, BM_IO_ERROR);
        }
}

/*
 * Error context callback for errors occurring during buffer writes.
 */
static void
buffer_write_error_callback(void *arg)
{
        volatile BufferDesc *bufHdr = (volatile BufferDesc *) arg;

        /* Buffer is pinned, so we can read the tag without locking the spinlock */
        if (bufHdr != NULL)
                errcontext("writing block %u of relation %u/%u/%u",
                                   bufHdr->tag.blockNum,
                                   bufHdr->tag.rnode.spcNode,
                                   bufHdr->tag.rnode.dbNode,
                                   bufHdr->tag.rnode.relNode);
}