bufmask.c

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/*-------------------------------------------------------------------------
 *
 * bufmask.c
 *    Routines for buffer masking. Used to mask certain bits
 *    in a page which can be different when the WAL is generated
 *    and when the WAL is applied.
 *
 * Portions Copyright (c) 2016-2025, PostgreSQL Global Development Group
 *
 * Contains common routines required for masking a page.
 *
 * IDENTIFICATION
 *    src/backend/access/common/bufmask.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/bufmask.h"
 
/*
 * mask_page_lsn_and_checksum
 *
 * In consistency checks, the LSN of the two pages compared will likely be
 * different because of concurrent operations when the WAL is generated and
 * the state of the page when WAL is applied. Also, mask out checksum as
 * masking anything else on page means checksum is not going to match as well.
 */
void
mask_page_lsn_and_checksum(Page page)
{
    PageHeader  phdr = (PageHeader) page;
 
    PageXLogRecPtrSet(phdr->pd_lsn, (uint64) MASK_MARKER);
    phdr->pd_checksum = MASK_MARKER;
}
 
/*
 * mask_page_hint_bits
 *
 * Mask hint bits in PageHeader. We want to ignore differences in hint bits,
 * since they can be set without emitting any WAL.
 */
void
mask_page_hint_bits(Page page)
{
    PageHeader  phdr = (PageHeader) page;
 
    /* Ignore prune_xid (it's like a hint-bit) */
    phdr->pd_prune_xid = MASK_MARKER;
 
    /* Ignore PD_PAGE_FULL and PD_HAS_FREE_LINES flags, they are just hints. */
    PageClearFull(page);
    PageClearHasFreeLinePointers(page);
 
    /*
     * During replay, if the page LSN has advanced past our XLOG record's LSN,
     * we don't mark the page all-visible. See heap_xlog_visible() for
     * details.
     */
    PageClearAllVisible(page);
}
 
/*
 * mask_unused_space
 *
 * Mask the unused space of a page between pd_lower and pd_upper.
 */
void
mask_unused_space(Page page)
{
    int         pd_lower = ((PageHeader) page)->pd_lower;
    int         pd_upper = ((PageHeader) page)->pd_upper;
    int         pd_special = ((PageHeader) page)->pd_special;
 
    /* Sanity check */
    if (pd_lower > pd_upper || pd_special < pd_upper ||
        pd_lower < SizeOfPageHeaderData || pd_special > BLCKSZ)
    {
        elog(ERROR, "invalid page pd_lower %u pd_upper %u pd_special %u",
             pd_lower, pd_upper, pd_special);
    }
 
    memset(page + pd_lower, MASK_MARKER, pd_upper - pd_lower);
}
 
/*
 * mask_lp_flags
 *
 * In some index AMs, line pointer flags can be modified on the primary
 * without emitting any WAL record.
 */
void
mask_lp_flags(Page page)
{
    OffsetNumber offnum,
                maxoff;
 
    maxoff = PageGetMaxOffsetNumber(page);
    for (offnum = FirstOffsetNumber;
         offnum <= maxoff;
         offnum = OffsetNumberNext(offnum))
    {
        ItemId      itemId = PageGetItemId(page, offnum);
 
        if (ItemIdIsUsed(itemId))
            itemId->lp_flags = LP_UNUSED;
    }
}
 
/*
 * mask_page_content
 *
 * In some index AMs, the contents of deleted pages need to be almost
 * completely ignored.
 */
void
mask_page_content(Page page)
{
    /* Mask Page Content */
    memset(page + SizeOfPageHeaderData, MASK_MARKER,
           BLCKSZ - SizeOfPageHeaderData);
 
    /* Mask pd_lower and pd_upper */
    memset(&((PageHeader) page)->pd_lower, MASK_MARKER,
           sizeof(uint16));
    memset(&((PageHeader) page)->pd_upper, MASK_MARKER,
           sizeof(uint16));
}