Bug in libpq large-object interface

Uh ... I forgot to send the code I referred to. Here it is again:

Hi PostgreSQL people in general and Tatsuo in particular,

I'm using V 7.0 on a Linux machine and I believe I have found a bug in the
large object interface provided by libpq. The code below will reproduce it, I
hope. Basically it creates a large object, writes six 'a' characters to it,
then closes it. Then, in another transaction, it opens the object, seeks to
position 1 from the start, writes a 'b', then seeks to position 3 from the
start and writes another 'b'. Then it closes the object and COMMITs the
transaction. Finally, in a further separate transaction, it calls lo_export
to write out the resulting object to a file testloseek.c.lobj I find this
file, instead of containing the string 'ababaa' as expected, contains
'^(at)b^@baa' where ^@ is ASCII NUL.

Compile with something like

gcc -o testloseek testloseek.c -lpq

The program sets the PQtrace to STDOUT and writes messages to STDERR, so run
it with STDOUT redirected to a log file.

This is a C version of a basic regression test of guile-pg, my Guile language
bindings for libpq. You may recall I reported a similar bug a year or so ago,
and I believed it was then fixed by Tatsuo, after a couple of iterations. I'm
sorry to be the bearer of bad news ...

Please reply to me directly since I'm not on the list.

Thanks
Ian

#include <stdio.h>
#include "libpq-fe.h"
#include "libpq/libpq-fs.h"

void exec_cmd(PGconn *conn, char *str);

main (int argc, char *argv[])
{
PGconn *conn;
int lobj_fd;
char buf[256];
int ret, i;
Oid lobj_id;

conn = PQconnectdb("dbname=test");
if (PQstatus(conn) != CONNECTION_OK) {
fprintf(stderr, "Can't connect to backend.\n");
fprintf(stderr, "ERROR: %s\n", PQerrorMessage(conn));
exit(1);
}
exec_cmd(conn, "BEGIN TRANSACTION");
PQtrace (conn, stdout);
if ((lobj_id = lo_creat(conn, INV_READ | INV_WRITE)) < 0) {
fprintf(stderr, "Can't create lobj.\n");
fprintf(stderr, "ERROR: %s\n", PQerrorMessage(conn));
exit(1);
}
fprintf(stderr, "lo_creat() returned OID %ld.\n", lobj_id);
if ((lobj_fd = lo_open(conn, lobj_id, INV_READ | INV_WRITE)) < 0) {
fprintf(stderr, "Can't open lobj.\n");
fprintf(stderr, "ERROR: %s\n", PQerrorMessage(conn));
exit(1);
}
fprintf(stderr, "lo_open returned fd = %d.\n", lobj_fd);
if ((ret = lo_write(conn, lobj_fd, "aaaaaa", 6)) != 6) {
fprintf(stderr, "Can't write lobj.\n");
fprintf(stderr, "ERROR: %s\n", PQerrorMessage(conn));
exit(1);
}
ret = lo_close(conn, lobj_fd);
printf("lo_close returned %d.\n", ret);
exec_cmd(conn, "END TRANSACTION");

exec_cmd(conn, "BEGIN TRANSACTION");
if ((lobj_fd = lo_open(conn, lobj_id, INV_READ | INV_WRITE)) < 0) {
fprintf(stderr, "Can't open lobj.\n");
fprintf(stderr, "ERROR: %s\n", PQerrorMessage(conn));
exit(1);
}
fprintf(stderr, "lo_open returned fd = %d.\n", lobj_fd);
if (ret)
fprintf(stderr, "Error message: %s\n", PQerrorMessage(conn));
if ((ret = lo_lseek(conn, lobj_fd, 1, 0)) != 1) {
fprintf(stderr, "error (%d) lseeking in large object.\n", ret);
fprintf(stderr, "ERROR: %s\n", PQerrorMessage(conn));
exit(1);
}
if ((ret = lo_write(conn, lobj_fd, "b", 1)) != 1) {
fprintf(stderr, "Can't write lobj.\n");
fprintf(stderr, "ERROR: %s\n", PQerrorMessage(conn));
exit(1);
}
if ((ret = lo_lseek(conn, lobj_fd, 3, 0)) != 3) {
fprintf(stderr, "error (%d) lseeking in large object.\n", ret);
fprintf(stderr, "ERROR: %s\n", PQerrorMessage(conn));
exit(1);
}
if ((ret = lo_write(conn, lobj_fd, "b", 1)) != 1) {
fprintf(stderr, "Can't write lobj.\n");
fprintf(stderr, "ERROR: %s\n", PQerrorMessage(conn));
exit(1);
}
ret = lo_close(conn, lobj_fd);
printf("lo_close returned %d.\n", ret);
if (ret)
fprintf(stderr, "Error message: %s\n", PQerrorMessage(conn));
PQuntrace(conn);
exec_cmd(conn, "END TRANSACTION");

exec_cmd(conn, "BEGIN TRANSACTION");
ret = lo_export(conn, lobj_id, "testloseek.c.lobj");
printf("lo_export returned %d.\n", ret);
if (ret != 1)
fprintf(stderr, "Error message: %s\n", PQerrorMessage(conn));
exec_cmd(conn, "END TRANSACTION");
exit(0);
}

void exec_cmd(PGconn *conn, char *str)
{
PGresult *res;

if ((res = PQexec(conn, str)) == NULL) {
fprintf(stderr, "Error executing %s.\n", str);
fprintf(stderr, "Error message: %s\n", PQerrorMessage(conn));
exit(1);
}
if (PQresultStatus(res) != PGRES_COMMAND_OK) {
fprintf(stderr, "Error executing %s.\n", str);
fprintf(stderr, "Error message: %s\n", PQerrorMessage(conn));
PQclear(res);
exit(1);
}
PQclear(res);
}

--
Ian Grant, Computer Lab., New Museums Site, Pembroke Street, Cambridge
Phone: +44 1223 334420 Personal e-mail: iang at pobox dot com