be-fsstubs.c

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/*-------------------------------------------------------------------------
 *
 * be-fsstubs.c
 *    Builtin functions for open/close/read/write operations on large objects
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/libpq/be-fsstubs.c
 *
 * NOTES
 *    This should be moved to a more appropriate place.  It is here
 *    for lack of a better place.
 *
 *    These functions store LargeObjectDesc structs in a private MemoryContext,
 *    which means that large object descriptors hang around until we destroy
 *    the context at transaction end.  It'd be possible to prolong the lifetime
 *    of the context so that LO FDs are good across transactions (for example,
 *    we could release the context only if we see that no FDs remain open).
 *    But we'd need additional state in order to do the right thing at the
 *    end of an aborted transaction.  FDs opened during an aborted xact would
 *    still need to be closed, since they might not be pointing at valid
 *    relations at all.  Locking semantics are also an interesting problem
 *    if LOs stay open across transactions.  For now, we'll stick with the
 *    existing documented semantics of LO FDs: they're only good within a
 *    transaction.
 *
 *    As of PostgreSQL 8.0, much of the angst expressed above is no longer
 *    relevant, and in fact it'd be pretty easy to allow LO FDs to stay
 *    open across transactions.  (Snapshot relevancy would still be an issue.)
 *    However backwards compatibility suggests that we should stick to the
 *    status quo.
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>

#include "libpq/be-fsstubs.h"
#include "libpq/libpq-fs.h"
#include "miscadmin.h"
#include "storage/fd.h"
#include "storage/large_object.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/memutils.h"

/*
 * compatibility flag for permission checks
 */
bool        lo_compat_privileges;

/* define this to enable debug logging */
/* #define FSDB 1 */
/* chunk size for lo_import/lo_export transfers */
#define BUFSIZE         8192

/*
 * LO "FD"s are indexes into the cookies array.
 *
 * A non-null entry is a pointer to a LargeObjectDesc allocated in the
 * LO private memory context "fscxt".  The cookies array itself is also
 * dynamically allocated in that context.  Its current allocated size is
 * cookies_len entries, of which any unused entries will be NULL.
 */
static LargeObjectDesc **cookies = NULL;
static int  cookies_size = 0;

static MemoryContext fscxt = NULL;

#define CreateFSContext() \
    do { \
        if (fscxt == NULL) \
            fscxt = AllocSetContextCreate(TopMemoryContext, \
                                          "Filesystem", \
                                          ALLOCSET_DEFAULT_SIZES); \
    } while (0)


static int  newLOfd(LargeObjectDesc *lobjCookie);
static void deleteLOfd(int fd);
static Oid  lo_import_internal(text *filename, Oid lobjOid);


/*****************************************************************************
 *  File Interfaces for Large Objects
 *****************************************************************************/

Datum
be_lo_open(PG_FUNCTION_ARGS)
{
    Oid         lobjId = PG_GETARG_OID(0);
    int32       mode = PG_GETARG_INT32(1);
    LargeObjectDesc *lobjDesc;
    int         fd;

#if FSDB
    elog(DEBUG4, "lo_open(%u,%d)", lobjId, mode);
#endif

    CreateFSContext();

    lobjDesc = inv_open(lobjId, mode, fscxt);

    if (lobjDesc == NULL)
    {                           /* lookup failed */
#if FSDB
        elog(DEBUG4, "could not open large object %u", lobjId);
#endif
        PG_RETURN_INT32(-1);
    }

    fd = newLOfd(lobjDesc);

    PG_RETURN_INT32(fd);
}

Datum
be_lo_close(PG_FUNCTION_ARGS)
{
    int32       fd = PG_GETARG_INT32(0);

    if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("invalid large-object descriptor: %d", fd)));

#if FSDB
    elog(DEBUG4, "lo_close(%d)", fd);
#endif

    inv_close(cookies[fd]);

    deleteLOfd(fd);

    PG_RETURN_INT32(0);
}


/*****************************************************************************
 *  Bare Read/Write operations --- these are not fmgr-callable!
 *
 *  We assume the large object supports byte oriented reads and seeks so
 *  that our work is easier.
 *
 *****************************************************************************/

int
lo_read(int fd, char *buf, int len)
{
    int         status;
    LargeObjectDesc *lobj;

    if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("invalid large-object descriptor: %d", fd)));
    lobj = cookies[fd];

    /* We don't bother to check IFS_RDLOCK, since it's always set */

    /* Permission checks --- first time through only */
    if ((lobj->flags & IFS_RD_PERM_OK) == 0)
    {
        if (!lo_compat_privileges &&
            pg_largeobject_aclcheck_snapshot(lobj->id,
                                             GetUserId(),
                                             ACL_SELECT,
                                             lobj->snapshot) != ACLCHECK_OK)
            ereport(ERROR,
                    (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                     errmsg("permission denied for large object %u",
                            lobj->id)));
        lobj->flags |= IFS_RD_PERM_OK;
    }

    status = inv_read(lobj, buf, len);

    return status;
}

int
lo_write(int fd, const char *buf, int len)
{
    int         status;
    LargeObjectDesc *lobj;

    if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("invalid large-object descriptor: %d", fd)));
    lobj = cookies[fd];

    if ((lobj->flags & IFS_WRLOCK) == 0)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("large object descriptor %d was not opened for writing",
                        fd)));

    /* Permission checks --- first time through only */
    if ((lobj->flags & IFS_WR_PERM_OK) == 0)
    {
        if (!lo_compat_privileges &&
            pg_largeobject_aclcheck_snapshot(lobj->id,
                                             GetUserId(),
                                             ACL_UPDATE,
                                             lobj->snapshot) != ACLCHECK_OK)
            ereport(ERROR,
                    (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                     errmsg("permission denied for large object %u",
                            lobj->id)));
        lobj->flags |= IFS_WR_PERM_OK;
    }

    status = inv_write(lobj, buf, len);

    return status;
}

Datum
be_lo_lseek(PG_FUNCTION_ARGS)
{
    int32       fd = PG_GETARG_INT32(0);
    int32       offset = PG_GETARG_INT32(1);
    int32       whence = PG_GETARG_INT32(2);
    int64       status;

    if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("invalid large-object descriptor: %d", fd)));

    status = inv_seek(cookies[fd], offset, whence);

    /* guard against result overflow */
    if (status != (int32) status)
        ereport(ERROR,
                (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
                 errmsg("lo_lseek result out of range for large-object descriptor %d",
                        fd)));

    PG_RETURN_INT32((int32) status);
}

Datum
be_lo_lseek64(PG_FUNCTION_ARGS)
{
    int32       fd = PG_GETARG_INT32(0);
    int64       offset = PG_GETARG_INT64(1);
    int32       whence = PG_GETARG_INT32(2);
    int64       status;

    if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("invalid large-object descriptor: %d", fd)));

    status = inv_seek(cookies[fd], offset, whence);

    PG_RETURN_INT64(status);
}

Datum
be_lo_creat(PG_FUNCTION_ARGS)
{
    Oid         lobjId;

    /*
     * We don't actually need to store into fscxt, but create it anyway to
     * ensure that AtEOXact_LargeObject knows there is state to clean up
     */
    CreateFSContext();

    lobjId = inv_create(InvalidOid);

    PG_RETURN_OID(lobjId);
}

Datum
be_lo_create(PG_FUNCTION_ARGS)
{
    Oid         lobjId = PG_GETARG_OID(0);

    /*
     * We don't actually need to store into fscxt, but create it anyway to
     * ensure that AtEOXact_LargeObject knows there is state to clean up
     */
    CreateFSContext();

    lobjId = inv_create(lobjId);

    PG_RETURN_OID(lobjId);
}

Datum
be_lo_tell(PG_FUNCTION_ARGS)
{
    int32       fd = PG_GETARG_INT32(0);
    int64       offset;

    if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("invalid large-object descriptor: %d", fd)));

    offset = inv_tell(cookies[fd]);

    /* guard against result overflow */
    if (offset != (int32) offset)
        ereport(ERROR,
                (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
                 errmsg("lo_tell result out of range for large-object descriptor %d",
                        fd)));

    PG_RETURN_INT32((int32) offset);
}

Datum
be_lo_tell64(PG_FUNCTION_ARGS)
{
    int32       fd = PG_GETARG_INT32(0);
    int64       offset;

    if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("invalid large-object descriptor: %d", fd)));

    offset = inv_tell(cookies[fd]);

    PG_RETURN_INT64(offset);
}

Datum
be_lo_unlink(PG_FUNCTION_ARGS)
{
    Oid         lobjId = PG_GETARG_OID(0);

    /* Must be owner of the largeobject */
    if (!lo_compat_privileges &&
        !pg_largeobject_ownercheck(lobjId, GetUserId()))
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be owner of large object %u", lobjId)));

    /*
     * If there are any open LO FDs referencing that ID, close 'em.
     */
    if (fscxt != NULL)
    {
        int         i;

        for (i = 0; i < cookies_size; i++)
        {
            if (cookies[i] != NULL && cookies[i]->id == lobjId)
            {
                inv_close(cookies[i]);
                deleteLOfd(i);
            }
        }
    }

    /*
     * inv_drop does not create a need for end-of-transaction cleanup and
     * hence we don't need to have created fscxt.
     */
    PG_RETURN_INT32(inv_drop(lobjId));
}

/*****************************************************************************
 *  Read/Write using bytea
 *****************************************************************************/

Datum
be_loread(PG_FUNCTION_ARGS)
{
    int32       fd = PG_GETARG_INT32(0);
    int32       len = PG_GETARG_INT32(1);
    bytea      *retval;
    int         totalread;

    if (len < 0)
        len = 0;

    retval = (bytea *) palloc(VARHDRSZ + len);
    totalread = lo_read(fd, VARDATA(retval), len);
    SET_VARSIZE(retval, totalread + VARHDRSZ);

    PG_RETURN_BYTEA_P(retval);
}

Datum
be_lowrite(PG_FUNCTION_ARGS)
{
    int32       fd = PG_GETARG_INT32(0);
    bytea      *wbuf = PG_GETARG_BYTEA_PP(1);
    int         bytestowrite;
    int         totalwritten;

    bytestowrite = VARSIZE_ANY_EXHDR(wbuf);
    totalwritten = lo_write(fd, VARDATA_ANY(wbuf), bytestowrite);
    PG_RETURN_INT32(totalwritten);
}

/*****************************************************************************
 *   Import/Export of Large Object
 *****************************************************************************/

/*
 * lo_import -
 *    imports a file as an (inversion) large object.
 */
Datum
be_lo_import(PG_FUNCTION_ARGS)
{
    text       *filename = PG_GETARG_TEXT_PP(0);

    PG_RETURN_OID(lo_import_internal(filename, InvalidOid));
}

/*
 * lo_import_with_oid -
 *    imports a file as an (inversion) large object specifying oid.
 */
Datum
be_lo_import_with_oid(PG_FUNCTION_ARGS)
{
    text       *filename = PG_GETARG_TEXT_PP(0);
    Oid         oid = PG_GETARG_OID(1);

    PG_RETURN_OID(lo_import_internal(filename, oid));
}

static Oid
lo_import_internal(text *filename, Oid lobjOid)
{
    int         fd;
    int         nbytes,
                tmp PG_USED_FOR_ASSERTS_ONLY;
    char        buf[BUFSIZE];
    char        fnamebuf[MAXPGPATH];
    LargeObjectDesc *lobj;
    Oid         oid;

#ifndef ALLOW_DANGEROUS_LO_FUNCTIONS
    if (!superuser())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be superuser to use server-side lo_import()"),
                 errhint("Anyone can use the client-side lo_import() provided by libpq.")));
#endif

    CreateFSContext();

    /*
     * open the file to be read in
     */
    text_to_cstring_buffer(filename, fnamebuf, sizeof(fnamebuf));
    fd = OpenTransientFile(fnamebuf, O_RDONLY | PG_BINARY, S_IRWXU);
    if (fd < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open server file \"%s\": %m",
                        fnamebuf)));

    /*
     * create an inversion object
     */
    oid = inv_create(lobjOid);

    /*
     * read in from the filesystem and write to the inversion object
     */
    lobj = inv_open(oid, INV_WRITE, fscxt);

    while ((nbytes = read(fd, buf, BUFSIZE)) > 0)
    {
        tmp = inv_write(lobj, buf, nbytes);
        Assert(tmp == nbytes);
    }

    if (nbytes < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not read server file \"%s\": %m",
                        fnamebuf)));

    inv_close(lobj);
    CloseTransientFile(fd);

    return oid;
}

/*
 * lo_export -
 *    exports an (inversion) large object.
 */
Datum
be_lo_export(PG_FUNCTION_ARGS)
{
    Oid         lobjId = PG_GETARG_OID(0);
    text       *filename = PG_GETARG_TEXT_PP(1);
    int         fd;
    int         nbytes,
                tmp;
    char        buf[BUFSIZE];
    char        fnamebuf[MAXPGPATH];
    LargeObjectDesc *lobj;
    mode_t      oumask;

#ifndef ALLOW_DANGEROUS_LO_FUNCTIONS
    if (!superuser())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be superuser to use server-side lo_export()"),
                 errhint("Anyone can use the client-side lo_export() provided by libpq.")));
#endif

    CreateFSContext();

    /*
     * open the inversion object (no need to test for failure)
     */
    lobj = inv_open(lobjId, INV_READ, fscxt);

    /*
     * open the file to be written to
     *
     * Note: we reduce backend's normal 077 umask to the slightly friendlier
     * 022. This code used to drop it all the way to 0, but creating
     * world-writable export files doesn't seem wise.
     */
    text_to_cstring_buffer(filename, fnamebuf, sizeof(fnamebuf));
    oumask = umask(S_IWGRP | S_IWOTH);
    fd = OpenTransientFile(fnamebuf, O_CREAT | O_WRONLY | O_TRUNC | PG_BINARY,
                           S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
    umask(oumask);
    if (fd < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not create server file \"%s\": %m",
                        fnamebuf)));

    /*
     * read in from the inversion file and write to the filesystem
     */
    while ((nbytes = inv_read(lobj, buf, BUFSIZE)) > 0)
    {
        tmp = write(fd, buf, nbytes);
        if (tmp != nbytes)
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not write server file \"%s\": %m",
                            fnamebuf)));
    }

    CloseTransientFile(fd);
    inv_close(lobj);

    PG_RETURN_INT32(1);
}

/*
 * lo_truncate -
 *    truncate a large object to a specified length
 */
static void
lo_truncate_internal(int32 fd, int64 len)
{
    LargeObjectDesc *lobj;

    if (fd < 0 || fd >= cookies_size || cookies[fd] == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("invalid large-object descriptor: %d", fd)));
    lobj = cookies[fd];

    if ((lobj->flags & IFS_WRLOCK) == 0)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("large object descriptor %d was not opened for writing",
                        fd)));

    /* Permission checks --- first time through only */
    if ((lobj->flags & IFS_WR_PERM_OK) == 0)
    {
        if (!lo_compat_privileges &&
            pg_largeobject_aclcheck_snapshot(lobj->id,
                                             GetUserId(),
                                             ACL_UPDATE,
                                             lobj->snapshot) != ACLCHECK_OK)
            ereport(ERROR,
                    (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                     errmsg("permission denied for large object %u",
                            lobj->id)));
        lobj->flags |= IFS_WR_PERM_OK;
    }

    inv_truncate(lobj, len);
}

Datum
be_lo_truncate(PG_FUNCTION_ARGS)
{
    int32       fd = PG_GETARG_INT32(0);
    int32       len = PG_GETARG_INT32(1);

    lo_truncate_internal(fd, len);
    PG_RETURN_INT32(0);
}

Datum
be_lo_truncate64(PG_FUNCTION_ARGS)
{
    int32       fd = PG_GETARG_INT32(0);
    int64       len = PG_GETARG_INT64(1);

    lo_truncate_internal(fd, len);
    PG_RETURN_INT32(0);
}

/*
 * AtEOXact_LargeObject -
 *       prepares large objects for transaction commit
 */
void
AtEOXact_LargeObject(bool isCommit)
{
    int         i;

    if (fscxt == NULL)
        return;                 /* no LO operations in this xact */

    /*
     * Close LO fds and clear cookies array so that LO fds are no longer good.
     * On abort we skip the close step.
     */
    for (i = 0; i < cookies_size; i++)
    {
        if (cookies[i] != NULL)
        {
            if (isCommit)
                inv_close(cookies[i]);
            deleteLOfd(i);
        }
    }

    /* Needn't actually pfree since we're about to zap context */
    cookies = NULL;
    cookies_size = 0;

    /* Release the LO memory context to prevent permanent memory leaks. */
    MemoryContextDelete(fscxt);
    fscxt = NULL;

    /* Give inv_api.c a chance to clean up, too */
    close_lo_relation(isCommit);
}

/*
 * AtEOSubXact_LargeObject
 *      Take care of large objects at subtransaction commit/abort
 *
 * Reassign LOs created/opened during a committing subtransaction
 * to the parent subtransaction.  On abort, just close them.
 */
void
AtEOSubXact_LargeObject(bool isCommit, SubTransactionId mySubid,
                        SubTransactionId parentSubid)
{
    int         i;

    if (fscxt == NULL)          /* no LO operations in this xact */
        return;

    for (i = 0; i < cookies_size; i++)
    {
        LargeObjectDesc *lo = cookies[i];

        if (lo != NULL && lo->subid == mySubid)
        {
            if (isCommit)
                lo->subid = parentSubid;
            else
            {
                /*
                 * Make sure we do not call inv_close twice if it errors out
                 * for some reason.  Better a leak than a crash.
                 */
                deleteLOfd(i);
                inv_close(lo);
            }
        }
    }
}

/*****************************************************************************
 *  Support routines for this file
 *****************************************************************************/

static int
newLOfd(LargeObjectDesc *lobjCookie)
{
    int         i,
                newsize;

    /* Try to find a free slot */
    for (i = 0; i < cookies_size; i++)
    {
        if (cookies[i] == NULL)
        {
            cookies[i] = lobjCookie;
            return i;
        }
    }

    /* No free slot, so make the array bigger */
    if (cookies_size <= 0)
    {
        /* First time through, arbitrarily make 64-element array */
        i = 0;
        newsize = 64;
        cookies = (LargeObjectDesc **)
            MemoryContextAllocZero(fscxt, newsize * sizeof(LargeObjectDesc *));
        cookies_size = newsize;
    }
    else
    {
        /* Double size of array */
        i = cookies_size;
        newsize = cookies_size * 2;
        cookies = (LargeObjectDesc **)
            repalloc(cookies, newsize * sizeof(LargeObjectDesc *));
        MemSet(cookies + cookies_size, 0,
               (newsize - cookies_size) * sizeof(LargeObjectDesc *));
        cookies_size = newsize;
    }

    Assert(cookies[i] == NULL);
    cookies[i] = lobjCookie;
    return i;
}

static void
deleteLOfd(int fd)
{
    cookies[fd] = NULL;
}

/*****************************************************************************
 *  Wrappers oriented toward SQL callers
 *****************************************************************************/

/*
 * Read [offset, offset+nbytes) within LO; when nbytes is -1, read to end.
 */
static bytea *
lo_get_fragment_internal(Oid loOid, int64 offset, int32 nbytes)
{
    LargeObjectDesc *loDesc;
    int64       loSize;
    int64       result_length;
    int         total_read PG_USED_FOR_ASSERTS_ONLY;
    bytea      *result = NULL;

    /*
     * We don't actually need to store into fscxt, but create it anyway to
     * ensure that AtEOXact_LargeObject knows there is state to clean up
     */
    CreateFSContext();

    loDesc = inv_open(loOid, INV_READ, fscxt);

    /* Permission check */
    if (!lo_compat_privileges &&
        pg_largeobject_aclcheck_snapshot(loDesc->id,
                                         GetUserId(),
                                         ACL_SELECT,
                                         loDesc->snapshot) != ACLCHECK_OK)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied for large object %u",
                        loDesc->id)));

    /*
     * Compute number of bytes we'll actually read, accommodating nbytes == -1
     * and reads beyond the end of the LO.
     */
    loSize = inv_seek(loDesc, 0, SEEK_END);
    if (loSize > offset)
    {
        if (nbytes >= 0 && nbytes <= loSize - offset)
            result_length = nbytes; /* request is wholly inside LO */
        else
            result_length = loSize - offset;    /* adjust to end of LO */
    }
    else
        result_length = 0;      /* request is wholly outside LO */

    /*
     * A result_length calculated from loSize may not fit in a size_t.  Check
     * that the size will satisfy this and subsequently-enforced size limits.
     */
    if (result_length > MaxAllocSize - VARHDRSZ)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("large object read request is too large")));

    result = (bytea *) palloc(VARHDRSZ + result_length);

    inv_seek(loDesc, offset, SEEK_SET);
    total_read = inv_read(loDesc, VARDATA(result), result_length);
    Assert(total_read == result_length);
    SET_VARSIZE(result, result_length + VARHDRSZ);

    inv_close(loDesc);

    return result;
}

/*
 * Read entire LO
 */
Datum
be_lo_get(PG_FUNCTION_ARGS)
{
    Oid         loOid = PG_GETARG_OID(0);
    bytea      *result;

    result = lo_get_fragment_internal(loOid, 0, -1);

    PG_RETURN_BYTEA_P(result);
}

/*
 * Read range within LO
 */
Datum
be_lo_get_fragment(PG_FUNCTION_ARGS)
{
    Oid         loOid = PG_GETARG_OID(0);
    int64       offset = PG_GETARG_INT64(1);
    int32       nbytes = PG_GETARG_INT32(2);
    bytea      *result;

    if (nbytes < 0)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("requested length cannot be negative")));

    result = lo_get_fragment_internal(loOid, offset, nbytes);

    PG_RETURN_BYTEA_P(result);
}

/*
 * Create LO with initial contents given by a bytea argument
 */
Datum
be_lo_from_bytea(PG_FUNCTION_ARGS)
{
    Oid         loOid = PG_GETARG_OID(0);
    bytea      *str = PG_GETARG_BYTEA_PP(1);
    LargeObjectDesc *loDesc;
    int         written PG_USED_FOR_ASSERTS_ONLY;

    CreateFSContext();

    loOid = inv_create(loOid);
    loDesc = inv_open(loOid, INV_WRITE, fscxt);
    written = inv_write(loDesc, VARDATA_ANY(str), VARSIZE_ANY_EXHDR(str));
    Assert(written == VARSIZE_ANY_EXHDR(str));
    inv_close(loDesc);

    PG_RETURN_OID(loOid);
}

/*
 * Update range within LO
 */
Datum
be_lo_put(PG_FUNCTION_ARGS)
{
    Oid         loOid = PG_GETARG_OID(0);
    int64       offset = PG_GETARG_INT64(1);
    bytea      *str = PG_GETARG_BYTEA_PP(2);
    LargeObjectDesc *loDesc;
    int         written PG_USED_FOR_ASSERTS_ONLY;

    CreateFSContext();

    loDesc = inv_open(loOid, INV_WRITE, fscxt);

    /* Permission check */
    if (!lo_compat_privileges &&
        pg_largeobject_aclcheck_snapshot(loDesc->id,
                                         GetUserId(),
                                         ACL_UPDATE,
                                         loDesc->snapshot) != ACLCHECK_OK)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied for large object %u",
                        loDesc->id)));

    inv_seek(loDesc, offset, SEEK_SET);
    written = inv_write(loDesc, VARDATA_ANY(str), VARSIZE_ANY_EXHDR(str));
    Assert(written == VARSIZE_ANY_EXHDR(str));
    inv_close(loDesc);

    PG_RETURN_VOID();
}