freespace.c

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/*-------------------------------------------------------------------------
 *
 * freespace.c
 *    POSTGRES free space map for quickly finding free space in relations
 *
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/storage/freespace/freespace.c
 *
 *
 * NOTES:
 *
 *  Free Space Map keeps track of the amount of free space on pages, and
 *  allows quickly searching for a page with enough free space. The FSM is
 *  stored in a dedicated relation fork of all heap relations, and those
 *  index access methods that need it (see also indexfsm.c). See README for
 *  more information.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/htup_details.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "storage/freespace.h"
#include "storage/fsm_internals.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"
 
 
/*
 * We use just one byte to store the amount of free space on a page, so we
 * divide the amount of free space a page can have into 256 different
 * categories. The highest category, 255, represents a page with at least
 * MaxFSMRequestSize bytes of free space, and the second highest category
 * represents the range from 254 * FSM_CAT_STEP, inclusive, to
 * MaxFSMRequestSize, exclusive.
 *
 * MaxFSMRequestSize depends on the architecture and BLCKSZ, but assuming
 * default 8k BLCKSZ, and that MaxFSMRequestSize is 8164 bytes, the
 * categories look like this:
 *
 *
 * Range     Category
 * 0    - 31   0
 * 32   - 63   1
 * ...    ...  ...
 * 8096 - 8127 253
 * 8128 - 8163 254
 * 8164 - 8192 255
 *
 * The reason that MaxFSMRequestSize is special is that if MaxFSMRequestSize
 * isn't equal to a range boundary, a page with exactly MaxFSMRequestSize
 * bytes of free space wouldn't satisfy a request for MaxFSMRequestSize
 * bytes. If there isn't more than MaxFSMRequestSize bytes of free space on a
 * completely empty page, that would mean that we could never satisfy a
 * request of exactly MaxFSMRequestSize bytes.
 */
#define FSM_CATEGORIES  256
#define FSM_CAT_STEP    (BLCKSZ / FSM_CATEGORIES)
#define MaxFSMRequestSize   MaxHeapTupleSize
 
/*
 * Depth of the on-disk tree. We need to be able to address 2^32-1 blocks,
 * and 1626 is the smallest number that satisfies X^3 >= 2^32-1. Likewise,
 * 256 is the smallest number that satisfies X^4 >= 2^32-1. In practice,
 * this means that 4096 bytes is the smallest BLCKSZ that we can get away
 * with a 3-level tree, and 512 is the smallest we support.
 */
#define FSM_TREE_DEPTH  ((SlotsPerFSMPage >= 1626) ? 3 : 4)
 
#define FSM_ROOT_LEVEL  (FSM_TREE_DEPTH - 1)
#define FSM_BOTTOM_LEVEL 0
 
/*
 * The internal FSM routines work on a logical addressing scheme. Each
 * level of the tree can be thought of as a separately addressable file.
 */
typedef struct
{
    int         level;          /* level */
    int         logpageno;      /* page number within the level */
} FSMAddress;
 
/* Address of the root page. */
static const FSMAddress FSM_ROOT_ADDRESS = {FSM_ROOT_LEVEL, 0};
 
/* functions to navigate the tree */
static FSMAddress fsm_get_child(FSMAddress parent, uint16 slot);
static FSMAddress fsm_get_parent(FSMAddress child, uint16 *slot);
static FSMAddress fsm_get_location(BlockNumber heapblk, uint16 *slot);
static BlockNumber fsm_get_heap_blk(FSMAddress addr, uint16 slot);
static BlockNumber fsm_logical_to_physical(FSMAddress addr);
 
static Buffer fsm_readbuf(Relation rel, FSMAddress addr, bool extend);
static Buffer fsm_extend(Relation rel, BlockNumber fsm_nblocks);
 
/* functions to convert amount of free space to a FSM category */
static uint8 fsm_space_avail_to_cat(Size avail);
static uint8 fsm_space_needed_to_cat(Size needed);
static Size fsm_space_cat_to_avail(uint8 cat);
 
/* workhorse functions for various operations */
static int  fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
                               uint8 newValue, uint8 minValue);
static BlockNumber fsm_search(Relation rel, uint8 min_cat);
static uint8 fsm_vacuum_page(Relation rel, FSMAddress addr,
                             BlockNumber start, BlockNumber end,
                             bool *eof_p);
 
 
/******** Public API ********/
 
/*
 * GetPageWithFreeSpace - try to find a page in the given relation with
 *      at least the specified amount of free space.
 *
 * If successful, return the block number; if not, return InvalidBlockNumber.
 *
 * The caller must be prepared for the possibility that the returned page
 * will turn out to have too little space available by the time the caller
 * gets a lock on it.  In that case, the caller should report the actual
 * amount of free space available on that page and then try again (see
 * RecordAndGetPageWithFreeSpace).  If InvalidBlockNumber is returned,
 * extend the relation.
 */
BlockNumber
GetPageWithFreeSpace(Relation rel, Size spaceNeeded)
{
    uint8       min_cat = fsm_space_needed_to_cat(spaceNeeded);
 
    return fsm_search(rel, min_cat);
}
 
/*
 * RecordAndGetPageWithFreeSpace - update info about a page and try again.
 *
 * We provide this combo form to save some locking overhead, compared to
 * separate RecordPageWithFreeSpace + GetPageWithFreeSpace calls. There's
 * also some effort to return a page close to the old page; if there's a
 * page with enough free space on the same FSM page where the old one page
 * is located, it is preferred.
 */
BlockNumber
RecordAndGetPageWithFreeSpace(Relation rel, BlockNumber oldPage,
                              Size oldSpaceAvail, Size spaceNeeded)
{
    int         old_cat = fsm_space_avail_to_cat(oldSpaceAvail);
    int         search_cat = fsm_space_needed_to_cat(spaceNeeded);
    FSMAddress  addr;
    uint16      slot;
    int         search_slot;
 
    /* Get the location of the FSM byte representing the heap block */
    addr = fsm_get_location(oldPage, &slot);
 
    search_slot = fsm_set_and_search(rel, addr, slot, old_cat, search_cat);
 
    /*
     * If fsm_set_and_search found a suitable new block, return that.
     * Otherwise, search as usual.
     */
    if (search_slot != -1)
        return fsm_get_heap_blk(addr, search_slot);
    else
        return fsm_search(rel, search_cat);
}
 
/*
 * RecordPageWithFreeSpace - update info about a page.
 *
 * Note that if the new spaceAvail value is higher than the old value stored
 * in the FSM, the space might not become visible to searchers until the next
 * FreeSpaceMapVacuum call, which updates the upper level pages.
 */
void
RecordPageWithFreeSpace(Relation rel, BlockNumber heapBlk, Size spaceAvail)
{
    int         new_cat = fsm_space_avail_to_cat(spaceAvail);
    FSMAddress  addr;
    uint16      slot;
 
    /* Get the location of the FSM byte representing the heap block */
    addr = fsm_get_location(heapBlk, &slot);
 
    fsm_set_and_search(rel, addr, slot, new_cat, 0);
}
 
/*
 * XLogRecordPageWithFreeSpace - like RecordPageWithFreeSpace, for use in
 *      WAL replay
 */
void
XLogRecordPageWithFreeSpace(RelFileLocator rlocator, BlockNumber heapBlk,
                            Size spaceAvail)
{
    int         new_cat = fsm_space_avail_to_cat(spaceAvail);
    FSMAddress  addr;
    uint16      slot;
    BlockNumber blkno;
    Buffer      buf;
    Page        page;
 
    /* Get the location of the FSM byte representing the heap block */
    addr = fsm_get_location(heapBlk, &slot);
    blkno = fsm_logical_to_physical(addr);
 
    /* If the page doesn't exist already, extend */
    buf = XLogReadBufferExtended(rlocator, FSM_FORKNUM, blkno,
                                 RBM_ZERO_ON_ERROR, InvalidBuffer);
    LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 
    page = BufferGetPage(buf);
    if (PageIsNew(page))
        PageInit(page, BLCKSZ, 0);
 
    if (fsm_set_avail(page, slot, new_cat))
        MarkBufferDirtyHint(buf, false);
    UnlockReleaseBuffer(buf);
}
 
/*
 * GetRecordedFreeSpace - return the amount of free space on a particular page,
 *      according to the FSM.
 */
Size
GetRecordedFreeSpace(Relation rel, BlockNumber heapBlk)
{
    FSMAddress  addr;
    uint16      slot;
    Buffer      buf;
    uint8       cat;
 
    /* Get the location of the FSM byte representing the heap block */
    addr = fsm_get_location(heapBlk, &slot);
 
    buf = fsm_readbuf(rel, addr, false);
    if (!BufferIsValid(buf))
        return 0;
    cat = fsm_get_avail(BufferGetPage(buf), slot);
    ReleaseBuffer(buf);
 
    return fsm_space_cat_to_avail(cat);
}
 
/*
 * FreeSpaceMapPrepareTruncateRel - prepare for truncation of a relation.
 *
 * nblocks is the new size of the heap.
 *
 * Return the number of blocks of new FSM.
 * If it's InvalidBlockNumber, there is nothing to truncate;
 * otherwise the caller is responsible for calling smgrtruncate()
 * to truncate the FSM pages, and FreeSpaceMapVacuumRange()
 * to update upper-level pages in the FSM.
 */
BlockNumber
FreeSpaceMapPrepareTruncateRel(Relation rel, BlockNumber nblocks)
{
    BlockNumber new_nfsmblocks;
    FSMAddress  first_removed_address;
    uint16      first_removed_slot;
    Buffer      buf;
 
    /*
     * If no FSM has been created yet for this relation, there's nothing to
     * truncate.
     */
    if (!smgrexists(RelationGetSmgr(rel), FSM_FORKNUM))
        return InvalidBlockNumber;
 
    /* Get the location in the FSM of the first removed heap block */
    first_removed_address = fsm_get_location(nblocks, &first_removed_slot);
 
    /*
     * Zero out the tail of the last remaining FSM page. If the slot
     * representing the first removed heap block is at a page boundary, as the
     * first slot on the FSM page that first_removed_address points to, we can
     * just truncate that page altogether.
     */
    if (first_removed_slot > 0)
    {
        buf = fsm_readbuf(rel, first_removed_address, false);
        if (!BufferIsValid(buf))
            return InvalidBlockNumber;  /* nothing to do; the FSM was already
                                         * smaller */
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 
        /* NO EREPORT(ERROR) from here till changes are logged */
        START_CRIT_SECTION();
 
        fsm_truncate_avail(BufferGetPage(buf), first_removed_slot);
 
        /*
         * Truncation of a relation is WAL-logged at a higher-level, and we
         * will be called at WAL replay. But if checksums are enabled, we need
         * to still write a WAL record to protect against a torn page, if the
         * page is flushed to disk before the truncation WAL record. We cannot
         * use MarkBufferDirtyHint here, because that will not dirty the page
         * during recovery.
         */
        MarkBufferDirty(buf);
        if (!InRecovery && RelationNeedsWAL(rel) && XLogHintBitIsNeeded())
            log_newpage_buffer(buf, false);
 
        END_CRIT_SECTION();
 
        UnlockReleaseBuffer(buf);
 
        new_nfsmblocks = fsm_logical_to_physical(first_removed_address) + 1;
    }
    else
    {
        new_nfsmblocks = fsm_logical_to_physical(first_removed_address);
        if (smgrnblocks(RelationGetSmgr(rel), FSM_FORKNUM) <= new_nfsmblocks)
            return InvalidBlockNumber;  /* nothing to do; the FSM was already
                                         * smaller */
    }
 
    return new_nfsmblocks;
}
 
/*
 * FreeSpaceMapVacuum - update upper-level pages in the rel's FSM
 *
 * We assume that the bottom-level pages have already been updated with
 * new free-space information.
 */
void
FreeSpaceMapVacuum(Relation rel)
{
    bool        dummy;
 
    /* Recursively scan the tree, starting at the root */
    (void) fsm_vacuum_page(rel, FSM_ROOT_ADDRESS,
                           (BlockNumber) 0, InvalidBlockNumber,
                           &dummy);
}
 
/*
 * FreeSpaceMapVacuumRange - update upper-level pages in the rel's FSM
 *
 * As above, but assume that only heap pages between start and end-1 inclusive
 * have new free-space information, so update only the upper-level slots
 * covering that block range.  end == InvalidBlockNumber is equivalent to
 * "all the rest of the relation".
 */
void
FreeSpaceMapVacuumRange(Relation rel, BlockNumber start, BlockNumber end)
{
    bool        dummy;
 
    /* Recursively scan the tree, starting at the root */
    if (end > start)
        (void) fsm_vacuum_page(rel, FSM_ROOT_ADDRESS, start, end, &dummy);
}
 
/******** Internal routines ********/
 
/*
 * Return category corresponding x bytes of free space
 */
static uint8
fsm_space_avail_to_cat(Size avail)
{
    int         cat;
 
    Assert(avail < BLCKSZ);
 
    if (avail >= MaxFSMRequestSize)
        return 255;
 
    cat = avail / FSM_CAT_STEP;
 
    /*
     * The highest category, 255, is reserved for MaxFSMRequestSize bytes or
     * more.
     */
    if (cat > 254)
        cat = 254;
 
    return (uint8) cat;
}
 
/*
 * Return the lower bound of the range of free space represented by given
 * category.
 */
static Size
fsm_space_cat_to_avail(uint8 cat)
{
    /* The highest category represents exactly MaxFSMRequestSize bytes. */
    if (cat == 255)
        return MaxFSMRequestSize;
    else
        return cat * FSM_CAT_STEP;
}
 
/*
 * Which category does a page need to have, to accommodate x bytes of data?
 * While fsm_space_avail_to_cat() rounds down, this needs to round up.
 */
static uint8
fsm_space_needed_to_cat(Size needed)
{
    int         cat;
 
    /* Can't ask for more space than the highest category represents */
    if (needed > MaxFSMRequestSize)
        elog(ERROR, "invalid FSM request size %zu", needed);
 
    if (needed == 0)
        return 1;
 
    cat = (needed + FSM_CAT_STEP - 1) / FSM_CAT_STEP;
 
    if (cat > 255)
        cat = 255;
 
    return (uint8) cat;
}
 
/*
 * Returns the physical block number of a FSM page
 */
static BlockNumber
fsm_logical_to_physical(FSMAddress addr)
{
    BlockNumber pages;
    int         leafno;
    int         l;
 
    /*
     * Calculate the logical page number of the first leaf page below the
     * given page.
     */
    leafno = addr.logpageno;
    for (l = 0; l < addr.level; l++)
        leafno *= SlotsPerFSMPage;
 
    /* Count upper level nodes required to address the leaf page */
    pages = 0;
    for (l = 0; l < FSM_TREE_DEPTH; l++)
    {
        pages += leafno + 1;
        leafno /= SlotsPerFSMPage;
    }
 
    /*
     * If the page we were asked for wasn't at the bottom level, subtract the
     * additional lower level pages we counted above.
     */
    pages -= addr.level;
 
    /* Turn the page count into 0-based block number */
    return pages - 1;
}
 
/*
 * Return the FSM location corresponding to given heap block.
 */
static FSMAddress
fsm_get_location(BlockNumber heapblk, uint16 *slot)
{
    FSMAddress  addr;
 
    addr.level = FSM_BOTTOM_LEVEL;
    addr.logpageno = heapblk / SlotsPerFSMPage;
    *slot = heapblk % SlotsPerFSMPage;
 
    return addr;
}
 
/*
 * Return the heap block number corresponding to given location in the FSM.
 */
static BlockNumber
fsm_get_heap_blk(FSMAddress addr, uint16 slot)
{
    Assert(addr.level == FSM_BOTTOM_LEVEL);
    return ((unsigned int) addr.logpageno) * SlotsPerFSMPage + slot;
}
 
/*
 * Given a logical address of a child page, get the logical page number of
 * the parent, and the slot within the parent corresponding to the child.
 */
static FSMAddress
fsm_get_parent(FSMAddress child, uint16 *slot)
{
    FSMAddress  parent;
 
    Assert(child.level < FSM_ROOT_LEVEL);
 
    parent.level = child.level + 1;
    parent.logpageno = child.logpageno / SlotsPerFSMPage;
    *slot = child.logpageno % SlotsPerFSMPage;
 
    return parent;
}
 
/*
 * Given a logical address of a parent page and a slot number, get the
 * logical address of the corresponding child page.
 */
static FSMAddress
fsm_get_child(FSMAddress parent, uint16 slot)
{
    FSMAddress  child;
 
    Assert(parent.level > FSM_BOTTOM_LEVEL);
 
    child.level = parent.level - 1;
    child.logpageno = parent.logpageno * SlotsPerFSMPage + slot;
 
    return child;
}
 
/*
 * Read a FSM page.
 *
 * If the page doesn't exist, InvalidBuffer is returned, or if 'extend' is
 * true, the FSM file is extended.
 */
static Buffer
fsm_readbuf(Relation rel, FSMAddress addr, bool extend)
{
    BlockNumber blkno = fsm_logical_to_physical(addr);
    Buffer      buf;
    SMgrRelation reln;
 
    /*
     * Caution: re-using this smgr pointer could fail if the relcache entry
     * gets closed.  It's safe as long as we only do smgr-level operations
     * between here and the last use of the pointer.
     */
    reln = RelationGetSmgr(rel);
 
    /*
     * If we haven't cached the size of the FSM yet, check it first.  Also
     * recheck if the requested block seems to be past end, since our cached
     * value might be stale.  (We send smgr inval messages on truncation, but
     * not on extension.)
     */
    if (reln->smgr_cached_nblocks[FSM_FORKNUM] == InvalidBlockNumber ||
        blkno >= reln->smgr_cached_nblocks[FSM_FORKNUM])
    {
        /* Invalidate the cache so smgrnblocks asks the kernel. */
        reln->smgr_cached_nblocks[FSM_FORKNUM] = InvalidBlockNumber;
        if (smgrexists(reln, FSM_FORKNUM))
            smgrnblocks(reln, FSM_FORKNUM);
        else
            reln->smgr_cached_nblocks[FSM_FORKNUM] = 0;
    }
 
    /*
     * For reading we use ZERO_ON_ERROR mode, and initialize the page if
     * necessary.  The FSM information is not accurate anyway, so it's better
     * to clear corrupt pages than error out. Since the FSM changes are not
     * WAL-logged, the so-called torn page problem on crash can lead to pages
     * with corrupt headers, for example.
     *
     * We use the same path below to initialize pages when extending the
     * relation, as a concurrent extension can end up with vm_extend()
     * returning an already-initialized page.
     */
    if (blkno >= reln->smgr_cached_nblocks[FSM_FORKNUM])
    {
        if (extend)
            buf = fsm_extend(rel, blkno + 1);
        else
            return InvalidBuffer;
    }
    else
        buf = ReadBufferExtended(rel, FSM_FORKNUM, blkno, RBM_ZERO_ON_ERROR, NULL);
 
    /*
     * Initializing the page when needed is trickier than it looks, because of
     * the possibility of multiple backends doing this concurrently, and our
     * desire to not uselessly take the buffer lock in the normal path where
     * the page is OK.  We must take the lock to initialize the page, so
     * recheck page newness after we have the lock, in case someone else
     * already did it.  Also, because we initially check PageIsNew with no
     * lock, it's possible to fall through and return the buffer while someone
     * else is still initializing the page (i.e., we might see pd_upper as set
     * but other page header fields are still zeroes).  This is harmless for
     * callers that will take a buffer lock themselves, but some callers
     * inspect the page without any lock at all.  The latter is OK only so
     * long as it doesn't depend on the page header having correct contents.
     * Current usage is safe because PageGetContents() does not require that.
     */
    if (PageIsNew(BufferGetPage(buf)))
    {
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
        if (PageIsNew(BufferGetPage(buf)))
            PageInit(BufferGetPage(buf), BLCKSZ, 0);
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
    }
    return buf;
}
 
/*
 * Ensure that the FSM fork is at least fsm_nblocks long, extending
 * it if necessary with empty pages. And by empty, I mean pages filled
 * with zeros, meaning there's no free space.
 */
static Buffer
fsm_extend(Relation rel, BlockNumber fsm_nblocks)
{
    return ExtendBufferedRelTo(BMR_REL(rel), FSM_FORKNUM, NULL,
                               EB_CREATE_FORK_IF_NEEDED |
                               EB_CLEAR_SIZE_CACHE,
                               fsm_nblocks,
                               RBM_ZERO_ON_ERROR);
}
 
/*
 * Set value in given FSM page and slot.
 *
 * If minValue > 0, the updated page is also searched for a page with at
 * least minValue of free space. If one is found, its slot number is
 * returned, -1 otherwise.
 */
static int
fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
                   uint8 newValue, uint8 minValue)
{
    Buffer      buf;
    Page        page;
    int         newslot = -1;
 
    buf = fsm_readbuf(rel, addr, true);
    LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 
    page = BufferGetPage(buf);
 
    if (fsm_set_avail(page, slot, newValue))
        MarkBufferDirtyHint(buf, false);
 
    if (minValue != 0)
    {
        /* Search while we still hold the lock */
        newslot = fsm_search_avail(buf, minValue,
                                   addr.level == FSM_BOTTOM_LEVEL,
                                   true);
    }
 
    UnlockReleaseBuffer(buf);
 
    return newslot;
}
 
/*
 * Search the tree for a heap page with at least min_cat of free space
 */
static BlockNumber
fsm_search(Relation rel, uint8 min_cat)
{
    int         restarts = 0;
    FSMAddress  addr = FSM_ROOT_ADDRESS;
 
    for (;;)
    {
        int         slot;
        Buffer      buf;
        uint8       max_avail = 0;
 
        /* Read the FSM page. */
        buf = fsm_readbuf(rel, addr, false);
 
        /* Search within the page */
        if (BufferIsValid(buf))
        {
            LockBuffer(buf, BUFFER_LOCK_SHARE);
            slot = fsm_search_avail(buf, min_cat,
                                    (addr.level == FSM_BOTTOM_LEVEL),
                                    false);
            if (slot == -1)
                max_avail = fsm_get_max_avail(BufferGetPage(buf));
            UnlockReleaseBuffer(buf);
        }
        else
            slot = -1;
 
        if (slot != -1)
        {
            /*
             * Descend the tree, or return the found block if we're at the
             * bottom.
             */
            if (addr.level == FSM_BOTTOM_LEVEL)
                return fsm_get_heap_blk(addr, slot);
 
            addr = fsm_get_child(addr, slot);
        }
        else if (addr.level == FSM_ROOT_LEVEL)
        {
            /*
             * At the root, failure means there's no page with enough free
             * space in the FSM. Give up.
             */
            return InvalidBlockNumber;
        }
        else
        {
            uint16      parentslot;
            FSMAddress  parent;
 
            /*
             * At lower level, failure can happen if the value in the upper-
             * level node didn't reflect the value on the lower page. Update
             * the upper node, to avoid falling into the same trap again, and
             * start over.
             *
             * There's a race condition here, if another backend updates this
             * page right after we release it, and gets the lock on the parent
             * page before us. We'll then update the parent page with the now
             * stale information we had. It's OK, because it should happen
             * rarely, and will be fixed by the next vacuum.
             */
            parent = fsm_get_parent(addr, &parentslot);
            fsm_set_and_search(rel, parent, parentslot, max_avail, 0);
 
            /*
             * If the upper pages are badly out of date, we might need to loop
             * quite a few times, updating them as we go. Any inconsistencies
             * should eventually be corrected and the loop should end. Looping
             * indefinitely is nevertheless scary, so provide an emergency
             * valve.
             */
            if (restarts++ > 10000)
                return InvalidBlockNumber;
 
            /* Start search all over from the root */
            addr = FSM_ROOT_ADDRESS;
        }
    }
}
 
 
/*
 * Recursive guts of FreeSpaceMapVacuum
 *
 * Examine the FSM page indicated by addr, as well as its children, updating
 * upper-level nodes that cover the heap block range from start to end-1.
 * (It's okay if end is beyond the actual end of the map.)
 * Return the maximum freespace value on this page.
 *
 * If addr is past the end of the FSM, set *eof_p to true and return 0.
 *
 * This traverses the tree in depth-first order.  The tree is stored
 * physically in depth-first order, so this should be pretty I/O efficient.
 */
static uint8
fsm_vacuum_page(Relation rel, FSMAddress addr,
                BlockNumber start, BlockNumber end,
                bool *eof_p)
{
    Buffer      buf;
    Page        page;
    uint8       max_avail;
 
    /* Read the page if it exists, or return EOF */
    buf = fsm_readbuf(rel, addr, false);
    if (!BufferIsValid(buf))
    {
        *eof_p = true;
        return 0;
    }
    else
        *eof_p = false;
 
    page = BufferGetPage(buf);
 
    /*
     * If we're above the bottom level, recurse into children, and fix the
     * information stored about them at this level.
     */
    if (addr.level > FSM_BOTTOM_LEVEL)
    {
        FSMAddress  fsm_start,
                    fsm_end;
        uint16      fsm_start_slot,
                    fsm_end_slot;
        int         slot,
                    start_slot,
                    end_slot;
        bool        eof = false;
 
        /*
         * Compute the range of slots we need to update on this page, given
         * the requested range of heap blocks to consider.  The first slot to
         * update is the one covering the "start" block, and the last slot is
         * the one covering "end - 1".  (Some of this work will be duplicated
         * in each recursive call, but it's cheap enough to not worry about.)
         */
        fsm_start = fsm_get_location(start, &fsm_start_slot);
        fsm_end = fsm_get_location(end - 1, &fsm_end_slot);
 
        while (fsm_start.level < addr.level)
        {
            fsm_start = fsm_get_parent(fsm_start, &fsm_start_slot);
            fsm_end = fsm_get_parent(fsm_end, &fsm_end_slot);
        }
        Assert(fsm_start.level == addr.level);
 
        if (fsm_start.logpageno == addr.logpageno)
            start_slot = fsm_start_slot;
        else if (fsm_start.logpageno > addr.logpageno)
            start_slot = SlotsPerFSMPage;   /* shouldn't get here... */
        else
            start_slot = 0;
 
        if (fsm_end.logpageno == addr.logpageno)
            end_slot = fsm_end_slot;
        else if (fsm_end.logpageno > addr.logpageno)
            end_slot = SlotsPerFSMPage - 1;
        else
            end_slot = -1;      /* shouldn't get here... */
 
        for (slot = start_slot; slot <= end_slot; slot++)
        {
            int         child_avail;
 
            CHECK_FOR_INTERRUPTS();
 
            /* After we hit end-of-file, just clear the rest of the slots */
            if (!eof)
                child_avail = fsm_vacuum_page(rel, fsm_get_child(addr, slot),
                                              start, end,
                                              &eof);
            else
                child_avail = 0;
 
            /* Update information about the child */
            if (fsm_get_avail(page, slot) != child_avail)
            {
                LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
                fsm_set_avail(page, slot, child_avail);
                MarkBufferDirtyHint(buf, false);
                LockBuffer(buf, BUFFER_LOCK_UNLOCK);
            }
        }
    }
 
    /* Now get the maximum value on the page, to return to caller */
    max_avail = fsm_get_max_avail(page);
 
    /*
     * Reset the next slot pointer. This encourages the use of low-numbered
     * pages, increasing the chances that a later vacuum can truncate the
     * relation.  We don't bother with a lock here, nor with marking the page
     * dirty if it wasn't already, since this is just a hint.
     */
    ((FSMPage) PageGetContents(page))->fp_next_slot = 0;
 
    ReleaseBuffer(buf);
 
    return max_avail;
}