inv_api.c

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/*-------------------------------------------------------------------------
 *
 * inv_api.c
 *    routines for manipulating inversion fs large objects. This file
 *    contains the user-level large object application interface routines.
 *
 *
 * Note: we access pg_largeobject.data using its C struct declaration.
 * This is safe because it immediately follows pageno which is an int4 field,
 * and therefore the data field will always be 4-byte aligned, even if it
 * is in the short 1-byte-header format.  We have to detoast it since it's
 * quite likely to be in compressed or short format.  We also need to check
 * for NULLs, since initdb will mark loid and pageno but not data as NOT NULL.
 *
 * Note: many of these routines leak memory in CurrentMemoryContext, as indeed
 * does most of the backend code.  We expect that CurrentMemoryContext will
 * be a short-lived context.  Data that must persist across function calls
 * is kept either in CacheMemoryContext (the Relation structs) or in the
 * memory context given to inv_open (for LargeObjectDesc structs).
 *
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/large_object/inv_api.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <limits.h>
 
#include "access/detoast.h"
#include "access/genam.h"
#include "access/htup_details.h"
#include "access/sysattr.h"
#include "access/table.h"
#include "access/xact.h"
#include "catalog/dependency.h"
#include "catalog/indexing.h"
#include "catalog/objectaccess.h"
#include "catalog/pg_largeobject.h"
#include "catalog/pg_largeobject_metadata.h"
#include "libpq/libpq-fs.h"
#include "miscadmin.h"
#include "storage/large_object.h"
#include "utils/acl.h"
#include "utils/fmgroids.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
 
 
/*
 * GUC: backwards-compatibility flag to suppress LO permission checks
 */
bool        lo_compat_privileges;
 
/*
 * All accesses to pg_largeobject and its index make use of a single
 * Relation reference.  To guarantee that the relcache entry remains
 * in the cache, on the first reference inside a subtransaction, we
 * execute a slightly klugy maneuver to assign ownership of the
 * Relation reference to TopTransactionResourceOwner.
 */
static Relation lo_heap_r = NULL;
static Relation lo_index_r = NULL;
 
 
/*
 * Open pg_largeobject and its index, if not already done in current xact
 */
static void
open_lo_relation(void)
{
    ResourceOwner currentOwner;
 
    if (lo_heap_r && lo_index_r)
        return;                 /* already open in current xact */
 
    /* Arrange for the top xact to own these relation references */
    currentOwner = CurrentResourceOwner;
    CurrentResourceOwner = TopTransactionResourceOwner;
 
    /* Use RowExclusiveLock since we might either read or write */
    if (lo_heap_r == NULL)
        lo_heap_r = table_open(LargeObjectRelationId, RowExclusiveLock);
    if (lo_index_r == NULL)
        lo_index_r = index_open(LargeObjectLOidPNIndexId, RowExclusiveLock);
 
    CurrentResourceOwner = currentOwner;
}
 
/*
 * Clean up at main transaction end
 */
void
close_lo_relation(bool isCommit)
{
    if (lo_heap_r || lo_index_r)
    {
        /*
         * Only bother to close if committing; else abort cleanup will handle
         * it
         */
        if (isCommit)
        {
            ResourceOwner currentOwner;
 
            currentOwner = CurrentResourceOwner;
            CurrentResourceOwner = TopTransactionResourceOwner;
 
            if (lo_index_r)
                index_close(lo_index_r, NoLock);
            if (lo_heap_r)
                table_close(lo_heap_r, NoLock);
 
            CurrentResourceOwner = currentOwner;
        }
        lo_heap_r = NULL;
        lo_index_r = NULL;
    }
}
 
 
/*
 * Same as pg_largeobject.c's LargeObjectExists(), except snapshot to
 * read with can be specified.
 */
static bool
myLargeObjectExists(Oid loid, Snapshot snapshot)
{
    Relation    pg_lo_meta;
    ScanKeyData skey[1];
    SysScanDesc sd;
    HeapTuple   tuple;
    bool        retval = false;
 
    ScanKeyInit(&skey[0],
                Anum_pg_largeobject_metadata_oid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(loid));
 
    pg_lo_meta = table_open(LargeObjectMetadataRelationId,
                            AccessShareLock);
 
    sd = systable_beginscan(pg_lo_meta,
                            LargeObjectMetadataOidIndexId, true,
                            snapshot, 1, skey);
 
    tuple = systable_getnext(sd);
    if (HeapTupleIsValid(tuple))
        retval = true;
 
    systable_endscan(sd);
 
    table_close(pg_lo_meta, AccessShareLock);
 
    return retval;
}
 
 
/*
 * Extract data field from a pg_largeobject tuple, detoasting if needed
 * and verifying that the length is sane.  Returns data pointer (a bytea *),
 * data length, and an indication of whether to pfree the data pointer.
 */
static void
getdatafield(Form_pg_largeobject tuple,
             bytea **pdatafield,
             int *plen,
             bool *pfreeit)
{
    bytea      *datafield;
    int         len;
    bool        freeit;
 
    datafield = &(tuple->data); /* see note at top of file */
    freeit = false;
    if (VARATT_IS_EXTENDED(datafield))
    {
        datafield = (bytea *)
            detoast_attr((struct varlena *) datafield);
        freeit = true;
    }
    len = VARSIZE(datafield) - VARHDRSZ;
    if (len < 0 || len > LOBLKSIZE)
        ereport(ERROR,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("pg_largeobject entry for OID %u, page %d has invalid data field size %d",
                        tuple->loid, tuple->pageno, len)));
    *pdatafield = datafield;
    *plen = len;
    *pfreeit = freeit;
}
 
 
/*
 *  inv_create -- create a new large object
 *
 *  Arguments:
 *    lobjId - OID to use for new large object, or InvalidOid to pick one
 *
 *  Returns:
 *    OID of new object
 *
 * If lobjId is not InvalidOid, then an error occurs if the OID is already
 * in use.
 */
Oid
inv_create(Oid lobjId)
{
    Oid         lobjId_new;
 
    /*
     * Create a new largeobject with empty data pages
     */
    lobjId_new = LargeObjectCreate(lobjId);
 
    /*
     * dependency on the owner of largeobject
     *
     * The reason why we use LargeObjectRelationId instead of
     * LargeObjectMetadataRelationId here is to provide backward compatibility
     * to the applications which utilize a knowledge about internal layout of
     * system catalogs. OID of pg_largeobject_metadata and loid of
     * pg_largeobject are same value, so there are no actual differences here.
     */
    recordDependencyOnOwner(LargeObjectRelationId,
                            lobjId_new, GetUserId());
 
    /* Post creation hook for new large object */
    InvokeObjectPostCreateHook(LargeObjectRelationId, lobjId_new, 0);
 
    /*
     * Advance command counter to make new tuple visible to later operations.
     */
    CommandCounterIncrement();
 
    return lobjId_new;
}
 
/*
 *  inv_open -- access an existing large object.
 *
 * Returns a large object descriptor, appropriately filled in.
 * The descriptor and subsidiary data are allocated in the specified
 * memory context, which must be suitably long-lived for the caller's
 * purposes.  If the returned descriptor has a snapshot associated
 * with it, the caller must ensure that it also lives long enough,
 * e.g. by calling RegisterSnapshotOnOwner
 */
LargeObjectDesc *
inv_open(Oid lobjId, int flags, MemoryContext mcxt)
{
    LargeObjectDesc *retval;
    Snapshot    snapshot = NULL;
    int         descflags = 0;
 
    /*
     * Historically, no difference is made between (INV_WRITE) and (INV_WRITE
     * | INV_READ), the caller being allowed to read the large object
     * descriptor in either case.
     */
    if (flags & INV_WRITE)
        descflags |= IFS_WRLOCK | IFS_RDLOCK;
    if (flags & INV_READ)
        descflags |= IFS_RDLOCK;
 
    if (descflags == 0)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid flags for opening a large object: %d",
                        flags)));
 
    /* Get snapshot.  If write is requested, use an instantaneous snapshot. */
    if (descflags & IFS_WRLOCK)
        snapshot = NULL;
    else
        snapshot = GetActiveSnapshot();
 
    /* Can't use LargeObjectExists here because we need to specify snapshot */
    if (!myLargeObjectExists(lobjId, snapshot))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("large object %u does not exist", lobjId)));
 
    /* Apply permission checks, again specifying snapshot */
    if ((descflags & IFS_RDLOCK) != 0)
    {
        if (!lo_compat_privileges &&
            pg_largeobject_aclcheck_snapshot(lobjId,
                                             GetUserId(),
                                             ACL_SELECT,
                                             snapshot) != ACLCHECK_OK)
            ereport(ERROR,
                    (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                     errmsg("permission denied for large object %u",
                            lobjId)));
    }
    if ((descflags & IFS_WRLOCK) != 0)
    {
        if (!lo_compat_privileges &&
            pg_largeobject_aclcheck_snapshot(lobjId,
                                             GetUserId(),
                                             ACL_UPDATE,
                                             snapshot) != ACLCHECK_OK)
            ereport(ERROR,
                    (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                     errmsg("permission denied for large object %u",
                            lobjId)));
    }
 
    /* OK to create a descriptor */
    retval = (LargeObjectDesc *) MemoryContextAlloc(mcxt,
                                                    sizeof(LargeObjectDesc));
    retval->id = lobjId;
    retval->offset = 0;
    retval->flags = descflags;
 
    /* caller sets if needed, not used by the functions in this file */
    retval->subid = InvalidSubTransactionId;
 
    /*
     * The snapshot (if any) is just the currently active snapshot.  The
     * caller will replace it with a longer-lived copy if needed.
     */
    retval->snapshot = snapshot;
 
    return retval;
}
 
/*
 * Closes a large object descriptor previously made by inv_open(), and
 * releases the long-term memory used by it.
 */
void
inv_close(LargeObjectDesc *obj_desc)
{
    Assert(PointerIsValid(obj_desc));
    pfree(obj_desc);
}
 
/*
 * Destroys an existing large object (not to be confused with a descriptor!)
 *
 * Note we expect caller to have done any required permissions check.
 */
int
inv_drop(Oid lobjId)
{
    ObjectAddress object;
 
    /*
     * Delete any comments and dependencies on the large object
     */
    object.classId = LargeObjectRelationId;
    object.objectId = lobjId;
    object.objectSubId = 0;
    performDeletion(&object, DROP_CASCADE, 0);
 
    /*
     * Advance command counter so that tuple removal will be seen by later
     * large-object operations in this transaction.
     */
    CommandCounterIncrement();
 
    /* For historical reasons, we always return 1 on success. */
    return 1;
}
 
/*
 * Determine size of a large object
 *
 * NOTE: LOs can contain gaps, just like Unix files.  We actually return
 * the offset of the last byte + 1.
 */
static uint64
inv_getsize(LargeObjectDesc *obj_desc)
{
    uint64      lastbyte = 0;
    ScanKeyData skey[1];
    SysScanDesc sd;
    HeapTuple   tuple;
 
    Assert(PointerIsValid(obj_desc));
 
    open_lo_relation();
 
    ScanKeyInit(&skey[0],
                Anum_pg_largeobject_loid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(obj_desc->id));
 
    sd = systable_beginscan_ordered(lo_heap_r, lo_index_r,
                                    obj_desc->snapshot, 1, skey);
 
    /*
     * Because the pg_largeobject index is on both loid and pageno, but we
     * constrain only loid, a backwards scan should visit all pages of the
     * large object in reverse pageno order.  So, it's sufficient to examine
     * the first valid tuple (== last valid page).
     */
    tuple = systable_getnext_ordered(sd, BackwardScanDirection);
    if (HeapTupleIsValid(tuple))
    {
        Form_pg_largeobject data;
        bytea      *datafield;
        int         len;
        bool        pfreeit;
 
        if (HeapTupleHasNulls(tuple))   /* paranoia */
            elog(ERROR, "null field found in pg_largeobject");
        data = (Form_pg_largeobject) GETSTRUCT(tuple);
        getdatafield(data, &datafield, &len, &pfreeit);
        lastbyte = (uint64) data->pageno * LOBLKSIZE + len;
        if (pfreeit)
            pfree(datafield);
    }
 
    systable_endscan_ordered(sd);
 
    return lastbyte;
}
 
int64
inv_seek(LargeObjectDesc *obj_desc, int64 offset, int whence)
{
    int64       newoffset;
 
    Assert(PointerIsValid(obj_desc));
 
    /*
     * We allow seek/tell if you have either read or write permission, so no
     * need for a permission check here.
     */
 
    /*
     * Note: overflow in the additions is possible, but since we will reject
     * negative results, we don't need any extra test for that.
     */
    switch (whence)
    {
        case SEEK_SET:
            newoffset = offset;
            break;
        case SEEK_CUR:
            newoffset = obj_desc->offset + offset;
            break;
        case SEEK_END:
            newoffset = inv_getsize(obj_desc) + offset;
            break;
        default:
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("invalid whence setting: %d", whence)));
            newoffset = 0;      /* keep compiler quiet */
            break;
    }
 
    /*
     * use errmsg_internal here because we don't want to expose INT64_FORMAT
     * in translatable strings; doing better is not worth the trouble
     */
    if (newoffset < 0 || newoffset > MAX_LARGE_OBJECT_SIZE)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg_internal("invalid large object seek target: " INT64_FORMAT,
                                 newoffset)));
 
    obj_desc->offset = newoffset;
    return newoffset;
}
 
int64
inv_tell(LargeObjectDesc *obj_desc)
{
    Assert(PointerIsValid(obj_desc));
 
    /*
     * We allow seek/tell if you have either read or write permission, so no
     * need for a permission check here.
     */
 
    return obj_desc->offset;
}
 
int
inv_read(LargeObjectDesc *obj_desc, char *buf, int nbytes)
{
    int         nread = 0;
    int64       n;
    int64       off;
    int         len;
    int32       pageno = (int32) (obj_desc->offset / LOBLKSIZE);
    uint64      pageoff;
    ScanKeyData skey[2];
    SysScanDesc sd;
    HeapTuple   tuple;
 
    Assert(PointerIsValid(obj_desc));
    Assert(buf != NULL);
 
    if ((obj_desc->flags & IFS_RDLOCK) == 0)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied for large object %u",
                        obj_desc->id)));
 
    if (nbytes <= 0)
        return 0;
 
    open_lo_relation();
 
    ScanKeyInit(&skey[0],
                Anum_pg_largeobject_loid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(obj_desc->id));
 
    ScanKeyInit(&skey[1],
                Anum_pg_largeobject_pageno,
                BTGreaterEqualStrategyNumber, F_INT4GE,
                Int32GetDatum(pageno));
 
    sd = systable_beginscan_ordered(lo_heap_r, lo_index_r,
                                    obj_desc->snapshot, 2, skey);
 
    while ((tuple = systable_getnext_ordered(sd, ForwardScanDirection)) != NULL)
    {
        Form_pg_largeobject data;
        bytea      *datafield;
        bool        pfreeit;
 
        if (HeapTupleHasNulls(tuple))   /* paranoia */
            elog(ERROR, "null field found in pg_largeobject");
        data = (Form_pg_largeobject) GETSTRUCT(tuple);
 
        /*
         * We expect the indexscan will deliver pages in order.  However,
         * there may be missing pages if the LO contains unwritten "holes". We
         * want missing sections to read out as zeroes.
         */
        pageoff = ((uint64) data->pageno) * LOBLKSIZE;
        if (pageoff > obj_desc->offset)
        {
            n = pageoff - obj_desc->offset;
            n = (n <= (nbytes - nread)) ? n : (nbytes - nread);
            MemSet(buf + nread, 0, n);
            nread += n;
            obj_desc->offset += n;
        }
 
        if (nread < nbytes)
        {
            Assert(obj_desc->offset >= pageoff);
            off = (int) (obj_desc->offset - pageoff);
            Assert(off >= 0 && off < LOBLKSIZE);
 
            getdatafield(data, &datafield, &len, &pfreeit);
            if (len > off)
            {
                n = len - off;
                n = (n <= (nbytes - nread)) ? n : (nbytes - nread);
                memcpy(buf + nread, VARDATA(datafield) + off, n);
                nread += n;
                obj_desc->offset += n;
            }
            if (pfreeit)
                pfree(datafield);
        }
 
        if (nread >= nbytes)
            break;
    }
 
    systable_endscan_ordered(sd);
 
    return nread;
}
 
int
inv_write(LargeObjectDesc *obj_desc, const char *buf, int nbytes)
{
    int         nwritten = 0;
    int         n;
    int         off;
    int         len;
    int32       pageno = (int32) (obj_desc->offset / LOBLKSIZE);
    ScanKeyData skey[2];
    SysScanDesc sd;
    HeapTuple   oldtuple;
    Form_pg_largeobject olddata;
    bool        neednextpage;
    bytea      *datafield;
    bool        pfreeit;
    union
    {
        bytea       hdr;
        /* this is to make the union big enough for a LO data chunk: */
        char        data[LOBLKSIZE + VARHDRSZ];
        /* ensure union is aligned well enough: */
        int32       align_it;
    }           workbuf;
    char       *workb = VARDATA(&workbuf.hdr);
    HeapTuple   newtup;
    Datum       values[Natts_pg_largeobject];
    bool        nulls[Natts_pg_largeobject];
    bool        replace[Natts_pg_largeobject];
    CatalogIndexState indstate;
 
    Assert(PointerIsValid(obj_desc));
    Assert(buf != NULL);
 
    /* enforce writability because snapshot is probably wrong otherwise */
    if ((obj_desc->flags & IFS_WRLOCK) == 0)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied for large object %u",
                        obj_desc->id)));
 
    if (nbytes <= 0)
        return 0;
 
    /* this addition can't overflow because nbytes is only int32 */
    if ((nbytes + obj_desc->offset) > MAX_LARGE_OBJECT_SIZE)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid large object write request size: %d",
                        nbytes)));
 
    open_lo_relation();
 
    indstate = CatalogOpenIndexes(lo_heap_r);
 
    ScanKeyInit(&skey[0],
                Anum_pg_largeobject_loid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(obj_desc->id));
 
    ScanKeyInit(&skey[1],
                Anum_pg_largeobject_pageno,
                BTGreaterEqualStrategyNumber, F_INT4GE,
                Int32GetDatum(pageno));
 
    sd = systable_beginscan_ordered(lo_heap_r, lo_index_r,
                                    obj_desc->snapshot, 2, skey);
 
    oldtuple = NULL;
    olddata = NULL;
    neednextpage = true;
 
    while (nwritten < nbytes)
    {
        /*
         * If possible, get next pre-existing page of the LO.  We expect the
         * indexscan will deliver these in order --- but there may be holes.
         */
        if (neednextpage)
        {
            if ((oldtuple = systable_getnext_ordered(sd, ForwardScanDirection)) != NULL)
            {
                if (HeapTupleHasNulls(oldtuple))    /* paranoia */
                    elog(ERROR, "null field found in pg_largeobject");
                olddata = (Form_pg_largeobject) GETSTRUCT(oldtuple);
                Assert(olddata->pageno >= pageno);
            }
            neednextpage = false;
        }
 
        /*
         * If we have a pre-existing page, see if it is the page we want to
         * write, or a later one.
         */
        if (olddata != NULL && olddata->pageno == pageno)
        {
            /*
             * Update an existing page with fresh data.
             *
             * First, load old data into workbuf
             */
            getdatafield(olddata, &datafield, &len, &pfreeit);
            memcpy(workb, VARDATA(datafield), len);
            if (pfreeit)
                pfree(datafield);
 
            /*
             * Fill any hole
             */
            off = (int) (obj_desc->offset % LOBLKSIZE);
            if (off > len)
                MemSet(workb + len, 0, off - len);
 
            /*
             * Insert appropriate portion of new data
             */
            n = LOBLKSIZE - off;
            n = (n <= (nbytes - nwritten)) ? n : (nbytes - nwritten);
            memcpy(workb + off, buf + nwritten, n);
            nwritten += n;
            obj_desc->offset += n;
            off += n;
            /* compute valid length of new page */
            len = (len >= off) ? len : off;
            SET_VARSIZE(&workbuf.hdr, len + VARHDRSZ);
 
            /*
             * Form and insert updated tuple
             */
            memset(values, 0, sizeof(values));
            memset(nulls, false, sizeof(nulls));
            memset(replace, false, sizeof(replace));
            values[Anum_pg_largeobject_data - 1] = PointerGetDatum(&workbuf);
            replace[Anum_pg_largeobject_data - 1] = true;
            newtup = heap_modify_tuple(oldtuple, RelationGetDescr(lo_heap_r),
                                       values, nulls, replace);
            CatalogTupleUpdateWithInfo(lo_heap_r, &newtup->t_self, newtup,
                                       indstate);
            heap_freetuple(newtup);
 
            /*
             * We're done with this old page.
             */
            oldtuple = NULL;
            olddata = NULL;
            neednextpage = true;
        }
        else
        {
            /*
             * Write a brand new page.
             *
             * First, fill any hole
             */
            off = (int) (obj_desc->offset % LOBLKSIZE);
            if (off > 0)
                MemSet(workb, 0, off);
 
            /*
             * Insert appropriate portion of new data
             */
            n = LOBLKSIZE - off;
            n = (n <= (nbytes - nwritten)) ? n : (nbytes - nwritten);
            memcpy(workb + off, buf + nwritten, n);
            nwritten += n;
            obj_desc->offset += n;
            /* compute valid length of new page */
            len = off + n;
            SET_VARSIZE(&workbuf.hdr, len + VARHDRSZ);
 
            /*
             * Form and insert updated tuple
             */
            memset(values, 0, sizeof(values));
            memset(nulls, false, sizeof(nulls));
            values[Anum_pg_largeobject_loid - 1] = ObjectIdGetDatum(obj_desc->id);
            values[Anum_pg_largeobject_pageno - 1] = Int32GetDatum(pageno);
            values[Anum_pg_largeobject_data - 1] = PointerGetDatum(&workbuf);
            newtup = heap_form_tuple(lo_heap_r->rd_att, values, nulls);
            CatalogTupleInsertWithInfo(lo_heap_r, newtup, indstate);
            heap_freetuple(newtup);
        }
        pageno++;
    }
 
    systable_endscan_ordered(sd);
 
    CatalogCloseIndexes(indstate);
 
    /*
     * Advance command counter so that my tuple updates will be seen by later
     * large-object operations in this transaction.
     */
    CommandCounterIncrement();
 
    return nwritten;
}
 
void
inv_truncate(LargeObjectDesc *obj_desc, int64 len)
{
    int32       pageno = (int32) (len / LOBLKSIZE);
    int32       off;
    ScanKeyData skey[2];
    SysScanDesc sd;
    HeapTuple   oldtuple;
    Form_pg_largeobject olddata;
    union
    {
        bytea       hdr;
        /* this is to make the union big enough for a LO data chunk: */
        char        data[LOBLKSIZE + VARHDRSZ];
        /* ensure union is aligned well enough: */
        int32       align_it;
    }           workbuf;
    char       *workb = VARDATA(&workbuf.hdr);
    HeapTuple   newtup;
    Datum       values[Natts_pg_largeobject];
    bool        nulls[Natts_pg_largeobject];
    bool        replace[Natts_pg_largeobject];
    CatalogIndexState indstate;
 
    Assert(PointerIsValid(obj_desc));
 
    /* enforce writability because snapshot is probably wrong otherwise */
    if ((obj_desc->flags & IFS_WRLOCK) == 0)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied for large object %u",
                        obj_desc->id)));
 
    /*
     * use errmsg_internal here because we don't want to expose INT64_FORMAT
     * in translatable strings; doing better is not worth the trouble
     */
    if (len < 0 || len > MAX_LARGE_OBJECT_SIZE)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg_internal("invalid large object truncation target: " INT64_FORMAT,
                                 len)));
 
    open_lo_relation();
 
    indstate = CatalogOpenIndexes(lo_heap_r);
 
    /*
     * Set up to find all pages with desired loid and pageno >= target
     */
    ScanKeyInit(&skey[0],
                Anum_pg_largeobject_loid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(obj_desc->id));
 
    ScanKeyInit(&skey[1],
                Anum_pg_largeobject_pageno,
                BTGreaterEqualStrategyNumber, F_INT4GE,
                Int32GetDatum(pageno));
 
    sd = systable_beginscan_ordered(lo_heap_r, lo_index_r,
                                    obj_desc->snapshot, 2, skey);
 
    /*
     * If possible, get the page the truncation point is in. The truncation
     * point may be beyond the end of the LO or in a hole.
     */
    olddata = NULL;
    if ((oldtuple = systable_getnext_ordered(sd, ForwardScanDirection)) != NULL)
    {
        if (HeapTupleHasNulls(oldtuple))    /* paranoia */
            elog(ERROR, "null field found in pg_largeobject");
        olddata = (Form_pg_largeobject) GETSTRUCT(oldtuple);
        Assert(olddata->pageno >= pageno);
    }
 
    /*
     * If we found the page of the truncation point we need to truncate the
     * data in it.  Otherwise if we're in a hole, we need to create a page to
     * mark the end of data.
     */
    if (olddata != NULL && olddata->pageno == pageno)
    {
        /* First, load old data into workbuf */
        bytea      *datafield;
        int         pagelen;
        bool        pfreeit;
 
        getdatafield(olddata, &datafield, &pagelen, &pfreeit);
        memcpy(workb, VARDATA(datafield), pagelen);
        if (pfreeit)
            pfree(datafield);
 
        /*
         * Fill any hole
         */
        off = len % LOBLKSIZE;
        if (off > pagelen)
            MemSet(workb + pagelen, 0, off - pagelen);
 
        /* compute length of new page */
        SET_VARSIZE(&workbuf.hdr, off + VARHDRSZ);
 
        /*
         * Form and insert updated tuple
         */
        memset(values, 0, sizeof(values));
        memset(nulls, false, sizeof(nulls));
        memset(replace, false, sizeof(replace));
        values[Anum_pg_largeobject_data - 1] = PointerGetDatum(&workbuf);
        replace[Anum_pg_largeobject_data - 1] = true;
        newtup = heap_modify_tuple(oldtuple, RelationGetDescr(lo_heap_r),
                                   values, nulls, replace);
        CatalogTupleUpdateWithInfo(lo_heap_r, &newtup->t_self, newtup,
                                   indstate);
        heap_freetuple(newtup);
    }
    else
    {
        /*
         * If the first page we found was after the truncation point, we're in
         * a hole that we'll fill, but we need to delete the later page
         * because the loop below won't visit it again.
         */
        if (olddata != NULL)
        {
            Assert(olddata->pageno > pageno);
            CatalogTupleDelete(lo_heap_r, &oldtuple->t_self);
        }
 
        /*
         * Write a brand new page.
         *
         * Fill the hole up to the truncation point
         */
        off = len % LOBLKSIZE;
        if (off > 0)
            MemSet(workb, 0, off);
 
        /* compute length of new page */
        SET_VARSIZE(&workbuf.hdr, off + VARHDRSZ);
 
        /*
         * Form and insert new tuple
         */
        memset(values, 0, sizeof(values));
        memset(nulls, false, sizeof(nulls));
        values[Anum_pg_largeobject_loid - 1] = ObjectIdGetDatum(obj_desc->id);
        values[Anum_pg_largeobject_pageno - 1] = Int32GetDatum(pageno);
        values[Anum_pg_largeobject_data - 1] = PointerGetDatum(&workbuf);
        newtup = heap_form_tuple(lo_heap_r->rd_att, values, nulls);
        CatalogTupleInsertWithInfo(lo_heap_r, newtup, indstate);
        heap_freetuple(newtup);
    }
 
    /*
     * Delete any pages after the truncation point.  If the initial search
     * didn't find a page, then of course there's nothing more to do.
     */
    if (olddata != NULL)
    {
        while ((oldtuple = systable_getnext_ordered(sd, ForwardScanDirection)) != NULL)
        {
            CatalogTupleDelete(lo_heap_r, &oldtuple->t_self);
        }
    }
 
    systable_endscan_ordered(sd);
 
    CatalogCloseIndexes(indstate);
 
    /*
     * Advance command counter so that tuple updates will be seen by later
     * large-object operations in this transaction.
     */
    CommandCounterIncrement();
}