Re: [INTERFACES] Re: [HACKERS] Convert PGconn, PGresult to opaque types?

"TL" == Tom Lane <tgl@sss.pgh.pa.us> writes:

(sorry, attributions are lost here)

Basically this would force applications to use the accessor functions
as recommended in the documentation, and not touch fields of a PGconn
object directly. (Ditto for PGresult.)

I am scared about external stuff like php. If they use it, and we
release something that doesn't work with their stuff, we are cooked
until they upgrade.

TL>
TL> But if they are using any direct references to fields of the PGconn
TL> struct, their stuff *already* won't work with 6.4. Admittedly it'd
TL> most likely only take a recompile to fix, and not code changes
TL> (however trivial). But if they'd been using only the documented API,
TL> ie using the accessor functions and not directly touching the struct,
TL> then a new shared library or DLL could be plopped right in without even
TL> a recompile of calling applications.
TL>
TL> Is the PHP source code available? It wouldn't take much work to check
TL> whether it will compile without a definition for struct pg_conn.

The PHP source is available from http://www.php.net/. From a quick
look through it, it does access the PGconn structure directly. Stuff
like (this is from the file php3.0.2a/functions/pgsql.c):

lo_read((PGconn *)pgsql->conn, pgsql->lofd, buf, 8192))

However, the whole PostgreSQL-specific stuff is only 1400 lines worth,
and the PHP guys are reputed to very active, so I don't think a change
should pose too much of a problem if they are forewarned.

[I have CCed the PHP/PostgreSQL module developers]

--
Eric Marsden
emarsden @ mail.dotcom.fr
It's elephants all the way down

Goran Thyni <goran@bildbasen.se> writes:

Bruce Momjian wrote:

Basically this would force applications to use the accessor functions
as recommended in the documentation, and not touch fields of a PGconn
object directly. (Ditto for PGresult.)

I am scared about external stuff like php. If they use it, and we
release something that doesn't work with their stuff, we are cooked
until they upgrade.

But if they are using any direct references to fields of the PGconn
struct, their stuff *already* won't work with 6.4. Admittedly it'd
most likely only take a recompile to fix, and not code changes
(however trivial). But if they'd been using only the documented API,
ie using the accessor functions and not directly touching the struct,
then a new shared library or DLL could be plopped right in without even
a recompile of calling applications.

Is the PHP source code available? It wouldn't take much work to check
whether it will compile without a definition for struct pg_conn.

I think Tom is aiming for thread-safeness which can't be done as long as
external stuff insists on accessing global structs inside libpq.

This is not a thread-safeness issue, it's an issue of being able to
promise binary compatibility across versions. Before the days of shared
libraries, source-code compatibility across versions was Good Enough,
because users had to rebuild their apps anyway to drop in a new version
of a library. Nowadays, people who don't even *have* the source of an
app still expect that they can shove in a new version of a shared library
that the app depends on. And that's a good thing, if it fixes some bugs
or adds new features; but it only works if the library's API is fully
binary compatible across releases. Hiding all but the simplest, most
stable structs is a necessary restriction if you hope to achieve that.

I made the wrong choice on this years ago with libjpeg (in self-defense,
that was before anyone had heard of shared libraries for Unix): I
exposed as part of the library's API a large parameter struct that I
knew would need to change with every new library version. At the time
it didn't seem like a problem, but I've learned to regret it. I see
people complaining all the time that their apps compiled against
libjpeg v6a stop working when they drop in v6b instead. Learn
from my bad example ;-)

Basically my feeling is that we will want to do this eventually, and
the pain level can only get worse the longer we put it off.

I am convinced. Hide the structure members, and lets go through beta
like that. If we have problems, we can supply a patch to expose the
structure members, with the knowledge there will be no workaround patch
for 6.5. They have to fix it by then.

-- 
Bruce Momjian                          |  830 Blythe Avenue
maillist@candle.pha.pa.us              |  Drexel Hill, Pennsylvania 19026
  +  If your life is a hard drive,     |  (610) 353-9879(w)
  +  Christ can be your backup.        |  (610) 853-3000(h)

On 24 Aug 1998, Eric Marsden wrote:

"TL" == Tom Lane <tgl@sss.pgh.pa.us> writes:

(sorry, attributions are lost here)

Basically this would force applications to use the accessor functions
as recommended in the documentation, and not touch fields of a PGconn
object directly. (Ditto for PGresult.)

I am scared about external stuff like php. If they use it, and we
release something that doesn't work with their stuff, we are cooked
until they upgrade.

TL>
TL> But if they are using any direct references to fields of the PGconn
TL> struct, their stuff *already* won't work with 6.4. Admittedly it'd
TL> most likely only take a recompile to fix, and not code changes
TL> (however trivial). But if they'd been using only the documented API,
TL> ie using the accessor functions and not directly touching the struct,
TL> then a new shared library or DLL could be plopped right in without even
TL> a recompile of calling applications.
TL>
TL> Is the PHP source code available? It wouldn't take much work to check
TL> whether it will compile without a definition for struct pg_conn.

The PHP source is available from http://www.php.net/. From a quick
look through it, it does access the PGconn structure directly. Stuff
like (this is from the file php3.0.2a/functions/pgsql.c):

lo_read((PGconn *)pgsql->conn, pgsql->lofd, buf, 8192))

Shouldn't be a problem. In this case, pgsql is is pointer to a struct

typedef struct pgLofp {
PGconn *conn;
int lofd;
} pgLofp;

so, pgsql->conn is a pointer to a PGconn struct to pass to lo_read, but
the code doesn't touch or refer anywhere to the members of that struct.

Anyway, I'll try to keep an eye on what's happening on pgsql-hackers list.

However, the whole PostgreSQL-specific stuff is only 1400 lines worth,
and the PHP guys are reputed to very active, so I don't think a change
should pose too much of a problem if they are forewarned.

Jouni Ahto

Bruce Momjian <maillist@candle.pha.pa.us> writes:

I am convinced. Hide the structure members, and lets go through beta
like that. If we have problems, we can supply a patch to expose the
structure members, with the knowledge there will be no workaround patch
for 6.5. They have to fix it by then.

Actually, the "patch" will just be to #include libpq-int.h along with
lipq-fe.h. I arranged for libpq-int.h to be one of the installed header
files, with the idea that some people would rather do that than fix
their code properly. It's their choice: if they want to depend on libpq
internals and have a greater risk of their code breaking across
releases, they can. I just want to discourage that a little bit, and
make it real clear what's considered supported API and what's considered
internal detail.

regards, tom lane