Re: 64-bit API for large object

Here is 64-bit API for large object version 2 patch.

> I checked this patch. It can be applied onto the latest master branch
> without any problems. My comments are below.
>
> 2012/9/11 Tatsuo Ishii <ishii(at)postgresql(dot)org>:
> > Ok, here is the patch to implement 64-bit API for large object, to
> > allow to use up to 4TB large objects(or 16TB if BLCKSZ changed to
> > 32KB). The patch is based on Jeremy Drake's patch posted on September
> > 23, 2005
> > (http://archives.postgresql.org/pgsql-hackers/2005-09/msg01026.php)
> > and reasonably updated/edited to adopt PostgreSQL 9.3 by Nozomi Anzai
> > for the backend part and Yugo Nagata for the rest(including
> > documentation patch).
> >
> > Here are changes made in the patch:
> >
> > 1) Frontend lo_* libpq functions(fe-lobj.c)(Yugo Nagata)
> >
> > lo_initialize() gathers backend 64-bit large object handling
> > function's oid, namely lo_lseek64, lo_tell64, lo_truncate64.
> >
> > If client calls lo_*64 functions and backend does not support them,
> > lo_*64 functions return error to caller. There might be an argument
> > since calls to lo_*64 functions can automatically be redirected to
> > 32-bit older API. I don't know this is worth the trouble though.
> >
> I think it should definitely return an error code when user tries to
> use lo_*64 functions towards the backend v9.2 or older, because
> fallback to 32bit API can raise unexpected errors if application
> intends to seek the area over than 2GB.
>
> > Currently lo_initialize() throws an error if one of oids are not
> > available. I doubt we do the same way for 64-bit functions since this
> > will make 9.3 libpq unable to access large objects stored in pre-9.2
> > PostgreSQL servers.
> >
> It seems to me the situation to split the case of pre-9.2 and post-9.3
> using a condition of "conn->sversion >= 90300".

Fixed so, and tested it by deleteing the lo_tell64's row from pg_proc.

> > To pass 64-bit integer to PQfn, PQArgBlock is used like this: int *ptr
> > is a pointer to 64-bit integer and actual data is placed somewhere
> > else. There might be other way: add new member to union u to store
> > 64-bit integer:
> >
> > typedef struct
> > {
> > int len;
> > int isint;
> > union
> > {
> > int *ptr; /* can't use void (dec compiler barfs) */
> > int integer;
> > int64 bigint; /* 64-bit integer */
> > } u;
> > } PQArgBlock;
> >
> > I'm a little bit worried about this way because PQArgBlock is a public
> > interface.
> >
> I'm inclined to add a new field for the union; that seems to me straight
> forward approach.
> For example, the manner in lo_seek64() seems to me confusable.
> It set 1 on "isint" field even though pointer is delivered actually.
>
> + argv[1].isint = 1;
> + argv[1].len = 8;
> + argv[1].u.ptr = (int *) &len;

Your proposal was not adopted per discussion.

> > Also we add new type "pg_int64":
> >
> > #ifndef NO_PG_INT64
> > #define HAVE_PG_INT64 1
> > typedef long long int pg_int64;
> > #endif
> >
> > in postgres_ext.h per suggestion from Tom Lane:
> > http://archives.postgresql.org/pgsql-hackers/2005-09/msg01062.php
> >
> I'm uncertain about context of this discussion.
>
> Does it make matter if we include <stdint.h> and use int64_t instead
> of the self defined data type?

Your proposal was not adopted per discussion.
Per discussion, endiannness translation was moved to fe-lobj.c.

> > 2) Backend lo_* functions (be-fsstubs.c)(Nozomi Anzai)
> >
> > Add lo_lseek64, lo_tell64, lo_truncate64 so that they can handle
> > 64-bit seek position and data length. loread64 and lowrite64 are not
> > added because if a program tries to read/write more than 2GB at once,
> > it would be a sign that the program need to be re-designed anyway.
> >
> I think it is a reasonable.
>
> > 3) Backend inv_api.c functions(Nozomi Anzai)
> >
> > No need to add new functions. Just extend them to handle 64-bit data.
> >
> > BTW , what will happen if older 32-bit libpq accesses large objects
> > over 2GB?
> >
> > lo_read and lo_write: they can read or write lobjs using 32-bit API as
> > long as requested read/write data length is smaller than 2GB. So I
> > think we can safely allow them to access over 2GB lobjs.
> >
> > lo_lseek: again as long as requested offset is smaller than 2GB, there
> > would be no problem.
> >
> > lo_tell:if current seek position is beyond 2GB, returns an error.
> >
> Even though iteration of lo_lseek() may move the offset to 4TB, it also
> makes unavailable to use lo_tell() to obtain the current offset, so I think
> it is reasonable behavior.
>
> However, error code is not an appropriate one.
>
> + if (INT_MAX < offset)
> + {
> + ereport(ERROR,
> + (errcode(ERRCODE_UNDEFINED_OBJECT),
> + errmsg("invalid large-object
> descriptor: %d", fd)));
> + PG_RETURN_INT32(-1);
> + }
>
> According to the manpage of lseek(2)
> EOVERFLOW
> The resulting file offset cannot be represented in an off_t.
>
> Please add a new error code such as ERRCODE_BLOB_OFFSET_OVERFLOW.

Changed the error code and error message. We added a new error code
"ERRCODE_UNDEFINED_OBJECT (22P07)".

> > 4) src/test/examples/testlo64.c added for 64-bit API example(Yugo Nagata)
> >
> > Comments and suggestions are welcome.
> >
> miscellaneous comments are below.
>
> Regression test is helpful. Even though no need to try to create 4TB large
> object, it is helpful to write some chunks around the design boundary.
> Could you add some test cases that writes some chunks around 4TB offset.

Added 64-bit lobj test items into regression test and confirmed it worked
rightly.

> Thanks,
> --
> KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>
>
>
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--
Nozomi Anzai
SRA OSS, Inc. Japan