verify_gin.c

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/*-------------------------------------------------------------------------
 *
 * verify_gin.c
 *      Verifies the integrity of GIN indexes based on invariants.
 *
 *
 * GIN index verification checks a number of invariants:
 *
 * - consistency: Paths in GIN graph have to contain consistent keys: tuples
 *   on parent pages consistently include tuples from children pages.
 *
 * - graph invariants: Each internal page must have at least one downlink, and
 *   can reference either only leaf pages or only internal pages.
 *
 *
 * Copyright (c) 2016-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    contrib/amcheck/verify_gin.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/gin_private.h"
#include "access/nbtree.h"
#include "catalog/pg_am.h"
#include "utils/memutils.h"
#include "utils/rel.h"
#include "verify_common.h"
#include "string.h"
 
/*
 * GinScanItem represents one item of depth-first scan of the index.
 */
typedef struct GinScanItem
{
    int         depth;
    IndexTuple  parenttup;
    BlockNumber parentblk;
    BlockNumber blkno;
    struct GinScanItem *next;
} GinScanItem;
 
/*
 * GinPostingTreeScanItem represents one item of a depth-first posting tree scan.
 */
typedef struct GinPostingTreeScanItem
{
    int         depth;
    ItemPointerData parentkey;
    BlockNumber parentblk;
    BlockNumber blkno;
    struct GinPostingTreeScanItem *next;
} GinPostingTreeScanItem;
 
 
PG_FUNCTION_INFO_V1(gin_index_check);
 
static void gin_check_parent_keys_consistency(Relation rel,
                                              Relation heaprel,
                                              void *callback_state, bool readonly);
static void check_index_page(Relation rel, Buffer buffer, BlockNumber blockNo);
static IndexTuple gin_refind_parent(Relation rel,
                                    BlockNumber parentblkno,
                                    BlockNumber childblkno,
                                    BufferAccessStrategy strategy);
static ItemId PageGetItemIdCareful(Relation rel, BlockNumber block, Page page,
                                   OffsetNumber offset);
 
/*
 * gin_index_check(index regclass)
 *
 * Verify integrity of GIN index.
 *
 * Acquires AccessShareLock on heap & index relations.
 */
Datum
gin_index_check(PG_FUNCTION_ARGS)
{
    Oid         indrelid = PG_GETARG_OID(0);
 
    amcheck_lock_relation_and_check(indrelid,
                                    GIN_AM_OID,
                                    gin_check_parent_keys_consistency,
                                    AccessShareLock,
                                    NULL);
 
    PG_RETURN_VOID();
}
 
/*
 * Read item pointers from leaf entry tuple.
 *
 * Returns a palloc'd array of ItemPointers. The number of items is returned
 * in *nitems.
 */
static ItemPointer
ginReadTupleWithoutState(IndexTuple itup, int *nitems)
{
    Pointer     ptr = GinGetPosting(itup);
    int         nipd = GinGetNPosting(itup);
    ItemPointer ipd;
    int         ndecoded;
 
    if (GinItupIsCompressed(itup))
    {
        if (nipd > 0)
        {
            ipd = ginPostingListDecode(ptr, &ndecoded);
            if (nipd != ndecoded)
                elog(ERROR, "number of items mismatch in GIN entry tuple, %d in tuple header, %d decoded",
                     nipd, ndecoded);
        }
        else
            ipd = palloc(0);
    }
    else
    {
        ipd = palloc_array(ItemPointerData, nipd);
        memcpy(ipd, ptr, sizeof(ItemPointerData) * nipd);
    }
    *nitems = nipd;
    return ipd;
}
 
/*
 * Scans through a posting tree (given by the root), and verifies that the keys
 * on a child keys are consistent with the parent.
 *
 * Allocates a separate memory context and scans through posting tree graph.
 */
static void
gin_check_posting_tree_parent_keys_consistency(Relation rel, BlockNumber posting_tree_root)
{
    BufferAccessStrategy strategy = GetAccessStrategy(BAS_BULKREAD);
    GinPostingTreeScanItem *stack;
    MemoryContext mctx;
    MemoryContext oldcontext;
 
    int         leafdepth;
 
    mctx = AllocSetContextCreate(CurrentMemoryContext,
                                 "posting tree check context",
                                 ALLOCSET_DEFAULT_SIZES);
    oldcontext = MemoryContextSwitchTo(mctx);
 
    /*
     * We don't know the height of the tree yet, but as soon as we encounter a
     * leaf page, we will set 'leafdepth' to its depth.
     */
    leafdepth = -1;
 
    /* Start the scan at the root page */
    stack = palloc0_object(GinPostingTreeScanItem);
    stack->depth = 0;
    ItemPointerSetInvalid(&stack->parentkey);
    stack->parentblk = InvalidBlockNumber;
    stack->blkno = posting_tree_root;
 
    elog(DEBUG3, "processing posting tree at blk %u", posting_tree_root);
 
    while (stack)
    {
        GinPostingTreeScanItem *stack_next;
        Buffer      buffer;
        Page        page;
        OffsetNumber i,
                    maxoff;
        BlockNumber rightlink;
 
        CHECK_FOR_INTERRUPTS();
 
        buffer = ReadBufferExtended(rel, MAIN_FORKNUM, stack->blkno,
                                    RBM_NORMAL, strategy);
        LockBuffer(buffer, GIN_SHARE);
        page = BufferGetPage(buffer);
 
        Assert(GinPageIsData(page));
 
        /* Check that the tree has the same height in all branches */
        if (GinPageIsLeaf(page))
        {
            ItemPointerData minItem;
            int         nlist;
            ItemPointerData *list;
            char        tidrange_buf[MAXPGPATH];
 
            ItemPointerSetMin(&minItem);
 
            elog(DEBUG1, "page blk: %u, type leaf", stack->blkno);
 
            if (leafdepth == -1)
                leafdepth = stack->depth;
            else if (stack->depth != leafdepth)
                ereport(ERROR,
                        (errcode(ERRCODE_INDEX_CORRUPTED),
                         errmsg("index \"%s\": internal pages traversal encountered leaf page unexpectedly on block %u",
                                RelationGetRelationName(rel), stack->blkno)));
            list = GinDataLeafPageGetItems(page, &nlist, minItem);
 
            if (nlist > 0)
                snprintf(tidrange_buf, sizeof(tidrange_buf),
                         "%d tids (%u, %u) - (%u, %u)",
                         nlist,
                         ItemPointerGetBlockNumberNoCheck(&list[0]),
                         ItemPointerGetOffsetNumberNoCheck(&list[0]),
                         ItemPointerGetBlockNumberNoCheck(&list[nlist - 1]),
                         ItemPointerGetOffsetNumberNoCheck(&list[nlist - 1]));
            else
                snprintf(tidrange_buf, sizeof(tidrange_buf), "0 tids");
 
            if (stack->parentblk != InvalidBlockNumber)
                elog(DEBUG3, "blk %u: parent %u highkey (%u, %u), %s",
                     stack->blkno,
                     stack->parentblk,
                     ItemPointerGetBlockNumberNoCheck(&stack->parentkey),
                     ItemPointerGetOffsetNumberNoCheck(&stack->parentkey),
                     tidrange_buf);
            else
                elog(DEBUG3, "blk %u: root leaf, %s",
                     stack->blkno,
                     tidrange_buf);
 
            if (stack->parentblk != InvalidBlockNumber &&
                ItemPointerGetOffsetNumberNoCheck(&stack->parentkey) != InvalidOffsetNumber &&
                nlist > 0 && ItemPointerCompare(&stack->parentkey, &list[nlist - 1]) < 0)
                ereport(ERROR,
                        (errcode(ERRCODE_INDEX_CORRUPTED),
                         errmsg("index \"%s\": tid exceeds parent's high key in postingTree leaf on block %u",
                                RelationGetRelationName(rel), stack->blkno)));
        }
        else
        {
            LocationIndex pd_lower;
            ItemPointerData bound;
            int         lowersize;
 
            /*
             * Check that tuples in each page are properly ordered and
             * consistent with parent high key
             */
            maxoff = GinPageGetOpaque(page)->maxoff;
            rightlink = GinPageGetOpaque(page)->rightlink;
 
            elog(DEBUG1, "page blk: %u, type data, maxoff %d", stack->blkno, maxoff);
 
            if (stack->parentblk != InvalidBlockNumber)
                elog(DEBUG3, "blk %u: internal posting tree page with %u items, parent %u highkey (%u, %u)",
                     stack->blkno, maxoff, stack->parentblk,
                     ItemPointerGetBlockNumberNoCheck(&stack->parentkey),
                     ItemPointerGetOffsetNumberNoCheck(&stack->parentkey));
            else
                elog(DEBUG3, "blk %u: root internal posting tree page with %u items",
                     stack->blkno, maxoff);
 
            /*
             * A GIN posting tree internal page stores PostingItems in the
             * 'lower' part of the page. The 'upper' part is unused. The
             * number of elements is stored in the opaque area (maxoff). Make
             * sure the size of the 'lower' part agrees with 'maxoff'
             *
             * We didn't set pd_lower until PostgreSQL version 9.4, so if this
             * check fails, it could also be because the index was
             * binary-upgraded from an earlier version. That was a long time
             * ago, though, so let's warn if it doesn't match.
             */
            pd_lower = ((PageHeader) page)->pd_lower;
            lowersize = pd_lower - MAXALIGN(SizeOfPageHeaderData);
            if ((lowersize - MAXALIGN(sizeof(ItemPointerData))) / sizeof(PostingItem) != maxoff)
                ereport(ERROR,
                        (errcode(ERRCODE_INDEX_CORRUPTED),
                         errmsg("index \"%s\" has unexpected pd_lower %u in posting tree block %u with maxoff %u)",
                                RelationGetRelationName(rel), pd_lower, stack->blkno, maxoff)));
 
            /*
             * Before the PostingItems, there's one ItemPointerData in the
             * 'lower' part that stores the page's high key.
             */
            bound = *GinDataPageGetRightBound(page);
 
            /*
             * Gin page right bound has a sane value only when not a highkey
             * on the rightmost page (at a given level). For the rightmost
             * page does not store the highkey explicitly, and the value is
             * infinity.
             */
            if (ItemPointerIsValid(&stack->parentkey) &&
                rightlink != InvalidBlockNumber &&
                !ItemPointerEquals(&stack->parentkey, &bound))
                ereport(ERROR,
                        (errcode(ERRCODE_INDEX_CORRUPTED),
                         errmsg("index \"%s\": posting tree page's high key (%u, %u) doesn't match the downlink on block %u (parent blk %u, key (%u, %u))",
                                RelationGetRelationName(rel),
                                ItemPointerGetBlockNumberNoCheck(&bound),
                                ItemPointerGetOffsetNumberNoCheck(&bound),
                                stack->blkno, stack->parentblk,
                                ItemPointerGetBlockNumberNoCheck(&stack->parentkey),
                                ItemPointerGetOffsetNumberNoCheck(&stack->parentkey))));
 
            for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
            {
                GinPostingTreeScanItem *ptr;
                PostingItem *posting_item = GinDataPageGetPostingItem(page, i);
 
                /* ItemPointerGetOffsetNumber expects a valid pointer */
                if (!(i == maxoff &&
                      rightlink == InvalidBlockNumber))
                    elog(DEBUG3, "key (%u, %u) -> %u",
                         ItemPointerGetBlockNumber(&posting_item->key),
                         ItemPointerGetOffsetNumber(&posting_item->key),
                         BlockIdGetBlockNumber(&posting_item->child_blkno));
                else
                    elog(DEBUG3, "key (%u, %u) -> %u",
                         0, 0, BlockIdGetBlockNumber(&posting_item->child_blkno));
 
                if (i == maxoff && rightlink == InvalidBlockNumber)
                {
                    /*
                     * The rightmost item in the tree level has (0, 0) as the
                     * key
                     */
                    if (ItemPointerGetBlockNumberNoCheck(&posting_item->key) != 0 ||
                        ItemPointerGetOffsetNumberNoCheck(&posting_item->key) != 0)
                        ereport(ERROR,
                                (errcode(ERRCODE_INDEX_CORRUPTED),
                                 errmsg("index \"%s\": rightmost posting tree page (blk %u) has unexpected last key (%u, %u)",
                                        RelationGetRelationName(rel),
                                        stack->blkno,
                                        ItemPointerGetBlockNumberNoCheck(&posting_item->key),
                                        ItemPointerGetOffsetNumberNoCheck(&posting_item->key))));
                }
                else if (i != FirstOffsetNumber)
                {
                    PostingItem *previous_posting_item = GinDataPageGetPostingItem(page, i - 1);
 
                    if (ItemPointerCompare(&posting_item->key, &previous_posting_item->key) < 0)
                        ereport(ERROR,
                                (errcode(ERRCODE_INDEX_CORRUPTED),
                                 errmsg("index \"%s\" has wrong tuple order in posting tree, block %u, offset %u",
                                        RelationGetRelationName(rel), stack->blkno, i)));
                }
 
                /*
                 * Check if this tuple is consistent with the downlink in the
                 * parent.
                 */
                if (i == maxoff && ItemPointerIsValid(&stack->parentkey) &&
                    ItemPointerCompare(&stack->parentkey, &posting_item->key) < 0)
                    ereport(ERROR,
                            (errcode(ERRCODE_INDEX_CORRUPTED),
                             errmsg("index \"%s\": posting item exceeds parent's high key in postingTree internal page on block %u offset %u",
                                    RelationGetRelationName(rel),
                                    stack->blkno, i)));
 
                /* This is an internal page, recurse into the child. */
                ptr = palloc_object(GinPostingTreeScanItem);
                ptr->depth = stack->depth + 1;
 
                /*
                 * The rightmost parent key is always invalid item pointer.
                 * Its value is 'Infinity' and not explicitly stored.
                 */
                ptr->parentkey = posting_item->key;
                ptr->parentblk = stack->blkno;
                ptr->blkno = BlockIdGetBlockNumber(&posting_item->child_blkno);
                ptr->next = stack->next;
                stack->next = ptr;
            }
        }
        UnlockReleaseBuffer(buffer);
 
        /* Step to next item in the queue */
        stack_next = stack->next;
        pfree(stack);
        stack = stack_next;
    }
 
    MemoryContextSwitchTo(oldcontext);
    MemoryContextDelete(mctx);
}
 
/*
 * Main entry point for GIN checks.
 *
 * Allocates memory context and scans through the whole GIN graph.
 */
static void
gin_check_parent_keys_consistency(Relation rel,
                                  Relation heaprel,
                                  void *callback_state,
                                  bool readonly)
{
    BufferAccessStrategy strategy = GetAccessStrategy(BAS_BULKREAD);
    GinScanItem *stack;
    MemoryContext mctx;
    MemoryContext oldcontext;
    GinState    state;
    int         leafdepth;
 
    mctx = AllocSetContextCreate(CurrentMemoryContext,
                                 "amcheck consistency check context",
                                 ALLOCSET_DEFAULT_SIZES);
    oldcontext = MemoryContextSwitchTo(mctx);
    initGinState(&state, rel);
 
    /*
     * We don't know the height of the tree yet, but as soon as we encounter a
     * leaf page, we will set 'leafdepth' to its depth.
     */
    leafdepth = -1;
 
    /* Start the scan at the root page */
    stack = palloc0_object(GinScanItem);
    stack->depth = 0;
    stack->parenttup = NULL;
    stack->parentblk = InvalidBlockNumber;
    stack->blkno = GIN_ROOT_BLKNO;
 
    while (stack)
    {
        GinScanItem *stack_next;
        Buffer      buffer;
        Page        page;
        OffsetNumber i,
                    maxoff,
                    prev_attnum;
        IndexTuple  prev_tuple;
        BlockNumber rightlink;
 
        CHECK_FOR_INTERRUPTS();
 
        buffer = ReadBufferExtended(rel, MAIN_FORKNUM, stack->blkno,
                                    RBM_NORMAL, strategy);
        LockBuffer(buffer, GIN_SHARE);
        page = BufferGetPage(buffer);
        maxoff = PageGetMaxOffsetNumber(page);
        rightlink = GinPageGetOpaque(page)->rightlink;
 
        /* Do basic sanity checks on the page headers */
        check_index_page(rel, buffer, stack->blkno);
 
        elog(DEBUG3, "processing entry tree page at blk %u, maxoff: %u", stack->blkno, maxoff);
 
        /*
         * It's possible that the page was split since we looked at the
         * parent, so that we didn't missed the downlink of the right sibling
         * when we scanned the parent.  If so, add the right sibling to the
         * stack now.
         */
        if (stack->parenttup != NULL)
        {
            GinNullCategory parent_key_category;
            Datum       parent_key = gintuple_get_key(&state,
                                                      stack->parenttup,
                                                      &parent_key_category);
            OffsetNumber parent_key_attnum = gintuple_get_attrnum(&state, stack->parenttup);
            ItemId      iid = PageGetItemIdCareful(rel, stack->blkno,
                                                   page, maxoff);
            IndexTuple  idxtuple = (IndexTuple) PageGetItem(page, iid);
            OffsetNumber page_max_key_attnum = gintuple_get_attrnum(&state, idxtuple);
            GinNullCategory page_max_key_category;
            Datum       page_max_key = gintuple_get_key(&state, idxtuple, &page_max_key_category);
 
            if (rightlink != InvalidBlockNumber &&
                ginCompareAttEntries(&state, page_max_key_attnum, page_max_key,
                                     page_max_key_category, parent_key_attnum,
                                     parent_key, parent_key_category) < 0)
            {
                /* split page detected, install right link to the stack */
                GinScanItem *ptr;
 
                elog(DEBUG3, "split detected for blk: %u, parent blk: %u", stack->blkno, stack->parentblk);
 
                ptr = palloc_object(GinScanItem);
                ptr->depth = stack->depth;
                ptr->parenttup = CopyIndexTuple(stack->parenttup);
                ptr->parentblk = stack->parentblk;
                ptr->blkno = rightlink;
                ptr->next = stack->next;
                stack->next = ptr;
            }
        }
 
        /* Check that the tree has the same height in all branches */
        if (GinPageIsLeaf(page))
        {
            if (leafdepth == -1)
                leafdepth = stack->depth;
            else if (stack->depth != leafdepth)
                ereport(ERROR,
                        (errcode(ERRCODE_INDEX_CORRUPTED),
                         errmsg("index \"%s\": internal pages traversal encountered leaf page unexpectedly on block %u",
                                RelationGetRelationName(rel), stack->blkno)));
        }
 
        /*
         * Check that tuples in each page are properly ordered and consistent
         * with parent high key
         */
        prev_tuple = NULL;
        prev_attnum = InvalidAttrNumber;
        for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
        {
            ItemId      iid = PageGetItemIdCareful(rel, stack->blkno, page, i);
            IndexTuple  idxtuple = (IndexTuple) PageGetItem(page, iid);
            OffsetNumber current_attnum = gintuple_get_attrnum(&state, idxtuple);
            GinNullCategory current_key_category;
            Datum       current_key;
 
            if (MAXALIGN(ItemIdGetLength(iid)) != MAXALIGN(IndexTupleSize(idxtuple)))
                ereport(ERROR,
                        (errcode(ERRCODE_INDEX_CORRUPTED),
                         errmsg("index \"%s\" has inconsistent tuple sizes, block %u, offset %u",
                                RelationGetRelationName(rel), stack->blkno, i)));
 
            current_key = gintuple_get_key(&state, idxtuple, &current_key_category);
 
            /*
             * Compare the entry to the preceding one.
             *
             * Don't check for high key on the rightmost inner page, as this
             * key is not really stored explicitly.
             *
             * The entries may be for different attributes, so make sure to
             * use ginCompareAttEntries for comparison.
             */
            if ((i != FirstOffsetNumber) &&
                !(i == maxoff && rightlink == InvalidBlockNumber && !GinPageIsLeaf(page)))
            {
                Datum       prev_key;
                GinNullCategory prev_key_category;
 
                prev_key = gintuple_get_key(&state, prev_tuple, &prev_key_category);
                if (ginCompareAttEntries(&state, prev_attnum, prev_key,
                                         prev_key_category, current_attnum,
                                         current_key, current_key_category) >= 0)
                    ereport(ERROR,
                            (errcode(ERRCODE_INDEX_CORRUPTED),
                             errmsg("index \"%s\" has wrong tuple order on entry tree page, block %u, offset %u, rightlink %u",
                                    RelationGetRelationName(rel), stack->blkno, i, rightlink)));
            }
 
            /*
             * Check if this tuple is consistent with the downlink in the
             * parent.
             */
            if (stack->parenttup &&
                i == maxoff)
            {
                GinNullCategory parent_key_category;
                OffsetNumber parent_key_attnum = gintuple_get_attrnum(&state, stack->parenttup);
                Datum       parent_key = gintuple_get_key(&state,
                                                          stack->parenttup,
                                                          &parent_key_category);
 
                if (ginCompareAttEntries(&state, current_attnum, current_key,
                                         current_key_category, parent_key_attnum,
                                         parent_key, parent_key_category) > 0)
                {
                    /*
                     * There was a discrepancy between parent and child
                     * tuples. We need to verify it is not a result of
                     * concurrent call of gistplacetopage(). So, lock parent
                     * and try to find downlink for current page. It may be
                     * missing due to concurrent page split, this is OK.
                     */
                    pfree(stack->parenttup);
                    stack->parenttup = gin_refind_parent(rel, stack->parentblk,
                                                         stack->blkno, strategy);
 
                    /* We found it - make a final check before failing */
                    if (!stack->parenttup)
                        elog(NOTICE, "Unable to find parent tuple for block %u on block %u due to concurrent split",
                             stack->blkno, stack->parentblk);
                    else
                    {
                        parent_key_attnum = gintuple_get_attrnum(&state, stack->parenttup);
                        parent_key = gintuple_get_key(&state,
                                                      stack->parenttup,
                                                      &parent_key_category);
 
                        /*
                         * Check if it is properly adjusted. If succeed,
                         * proceed to the next key.
                         */
                        if (ginCompareAttEntries(&state, current_attnum, current_key,
                                                 current_key_category, parent_key_attnum,
                                                 parent_key, parent_key_category) > 0)
                            ereport(ERROR,
                                    (errcode(ERRCODE_INDEX_CORRUPTED),
                                     errmsg("index \"%s\" has inconsistent records on page %u offset %u",
                                            RelationGetRelationName(rel), stack->blkno, i)));
                    }
                }
            }
 
            /* If this is an internal page, recurse into the child */
            if (!GinPageIsLeaf(page))
            {
                GinScanItem *ptr;
 
                ptr = palloc_object(GinScanItem);
                ptr->depth = stack->depth + 1;
                /* last tuple in layer has no high key */
                if (i == maxoff && rightlink == InvalidBlockNumber)
                    ptr->parenttup = NULL;
                else
                    ptr->parenttup = CopyIndexTuple(idxtuple);
                ptr->parentblk = stack->blkno;
                ptr->blkno = GinGetDownlink(idxtuple);
                ptr->next = stack->next;
                stack->next = ptr;
            }
            /* If this item is a pointer to a posting tree, recurse into it */
            else if (GinIsPostingTree(idxtuple))
            {
                BlockNumber rootPostingTree = GinGetPostingTree(idxtuple);
 
                gin_check_posting_tree_parent_keys_consistency(rel, rootPostingTree);
            }
            else
            {
                ItemPointer ipd;
                int         nipd;
 
                ipd = ginReadTupleWithoutState(idxtuple, &nipd);
 
                for (int j = 0; j < nipd; j++)
                {
                    if (!OffsetNumberIsValid(ItemPointerGetOffsetNumber(&ipd[j])))
                        ereport(ERROR,
                                (errcode(ERRCODE_INDEX_CORRUPTED),
                                 errmsg("index \"%s\": posting list contains invalid heap pointer on block %u",
                                        RelationGetRelationName(rel), stack->blkno)));
                }
                pfree(ipd);
            }
 
            prev_tuple = CopyIndexTuple(idxtuple);
            prev_attnum = current_attnum;
        }
 
        UnlockReleaseBuffer(buffer);
 
        /* Step to next item in the queue */
        stack_next = stack->next;
        if (stack->parenttup)
            pfree(stack->parenttup);
        pfree(stack);
        stack = stack_next;
    }
 
    MemoryContextSwitchTo(oldcontext);
    MemoryContextDelete(mctx);
}
 
/*
 * Verify that a freshly-read page looks sane.
 */
static void
check_index_page(Relation rel, Buffer buffer, BlockNumber blockNo)
{
    Page        page = BufferGetPage(buffer);
 
    /*
     * ReadBuffer verifies that every newly-read page passes
     * PageHeaderIsValid, which means it either contains a reasonably sane
     * page header or is all-zero.  We have to defend against the all-zero
     * case, however.
     */
    if (PageIsNew(page))
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("index \"%s\" contains unexpected zero page at block %u",
                        RelationGetRelationName(rel),
                        BufferGetBlockNumber(buffer)),
                 errhint("Please REINDEX it.")));
 
    /*
     * Additionally check that the special area looks sane.
     */
    if (PageGetSpecialSize(page) != MAXALIGN(sizeof(GinPageOpaqueData)))
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("index \"%s\" contains corrupted page at block %u",
                        RelationGetRelationName(rel),
                        BufferGetBlockNumber(buffer)),
                 errhint("Please REINDEX it.")));
 
    if (GinPageIsDeleted(page))
    {
        if (!GinPageIsLeaf(page))
            ereport(ERROR,
                    (errcode(ERRCODE_INDEX_CORRUPTED),
                     errmsg("index \"%s\" has deleted internal page %u",
                            RelationGetRelationName(rel), blockNo)));
        if (PageGetMaxOffsetNumber(page) > InvalidOffsetNumber)
            ereport(ERROR,
                    (errcode(ERRCODE_INDEX_CORRUPTED),
                     errmsg("index \"%s\" has deleted page %u with tuples",
                            RelationGetRelationName(rel), blockNo)));
    }
    else if (PageGetMaxOffsetNumber(page) > MaxIndexTuplesPerPage)
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("index \"%s\" has page %u with exceeding count of tuples",
                        RelationGetRelationName(rel), blockNo)));
}
 
/*
 * Try to re-find downlink pointing to 'blkno', in 'parentblkno'.
 *
 * If found, returns a palloc'd copy of the downlink tuple. Otherwise,
 * returns NULL.
 */
static IndexTuple
gin_refind_parent(Relation rel, BlockNumber parentblkno,
                  BlockNumber childblkno, BufferAccessStrategy strategy)
{
    Buffer      parentbuf;
    Page        parentpage;
    OffsetNumber o,
                parent_maxoff;
    IndexTuple  result = NULL;
 
    parentbuf = ReadBufferExtended(rel, MAIN_FORKNUM, parentblkno, RBM_NORMAL,
                                   strategy);
 
    LockBuffer(parentbuf, GIN_SHARE);
    parentpage = BufferGetPage(parentbuf);
 
    if (GinPageIsLeaf(parentpage))
    {
        UnlockReleaseBuffer(parentbuf);
        return result;
    }
 
    parent_maxoff = PageGetMaxOffsetNumber(parentpage);
    for (o = FirstOffsetNumber; o <= parent_maxoff; o = OffsetNumberNext(o))
    {
        ItemId      p_iid = PageGetItemIdCareful(rel, parentblkno, parentpage, o);
        IndexTuple  itup = (IndexTuple) PageGetItem(parentpage, p_iid);
 
        if (GinGetDownlink(itup) == childblkno)
        {
            /* Found it! Make copy and return it */
            result = CopyIndexTuple(itup);
            break;
        }
    }
 
    UnlockReleaseBuffer(parentbuf);
 
    return result;
}
 
static ItemId
PageGetItemIdCareful(Relation rel, BlockNumber block, Page page,
                     OffsetNumber offset)
{
    ItemId      itemid = PageGetItemId(page, offset);
 
    if (ItemIdGetOffset(itemid) + ItemIdGetLength(itemid) >
        BLCKSZ - MAXALIGN(sizeof(GinPageOpaqueData)))
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("line pointer points past end of tuple space in index \"%s\"",
                        RelationGetRelationName(rel)),
                 errdetail_internal("Index tid=(%u,%u) lp_off=%u, lp_len=%u lp_flags=%u.",
                                    block, offset, ItemIdGetOffset(itemid),
                                    ItemIdGetLength(itemid),
                                    ItemIdGetFlags(itemid))));
 
    /*
     * Verify that line pointer isn't LP_REDIRECT or LP_UNUSED or LP_DEAD,
     * since GIN never uses all three.  Verify that line pointer has storage,
     * too.
     */
    if (ItemIdIsRedirected(itemid) || !ItemIdIsUsed(itemid) ||
        ItemIdIsDead(itemid) || ItemIdGetLength(itemid) == 0)
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("invalid line pointer storage in index \"%s\"",
                        RelationGetRelationName(rel)),
                 errdetail_internal("Index tid=(%u,%u) lp_off=%u, lp_len=%u lp_flags=%u.",
                                    block, offset, ItemIdGetOffset(itemid),
                                    ItemIdGetLength(itemid),
                                    ItemIdGetFlags(itemid))));
 
    return itemid;
}