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

From: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
To: tgl(at)sss(dot)pgh(dot)pa(dot)us (Tom Lane)
Cc: pgsql-interfaces(at)postgreSQL(dot)org, pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: [HACKERS] Convert PGconn, PGresult to opaque types?
Date: 1998-08-24 01:21:56
Message-ID: 199808240121.VAA29023@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-interfaces
> In last week's episode, I wrote:
> > I have dealt with this by splitting libpq-fe.h into two files, leaving
> > only exportable information in libpq-fe.h and moving the internals into
> > a new file libpq-int.h.  This should solve Ewan's problem and reduce
> > application-code namespace pollution in general.
> > 
> > (If I were really being fascist about this I would've moved the
> > definitions of struct pg_conn and friends into libpq-int.h, leaving
> > only an opaque "struct pg_conn" declaration to be seen by applications.
> > But I'm afraid that that will break too many applications.  Perhaps we
> > can take that step in some future release.)
> 
> Today I am wondering whether it wouldn't be a good idea to just go ahead
> and move those struct declarations into libpq-int.h.  Then a pointer to
> PGconn would be truly an opaque type for applications; all they'd see is
> 
> 	typedef struct pg_conn * PGconn;
> 
> 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.

Not really sure what the advantage would be, aside from cleanliness.

> 
> I think it is a real good idea to do this eventually, because it will
> help ensure that applications don't assume things they shouldn't about
> what is in a PGconn, and it will allow us to modify the PGconn structure
> without breaking binary compatibility of applications across libpq
> releases.  This is a critical consideration now that libpq is released
> as a shared library on Unix and will be available in DLL form for
> Windows.  People tend to think that they can update shared libraries
> without recompiling the programs that use the libraries.
> 
> What occurs to me today is that if we intend to do it eventually, we
> may as well do it *now*.  Waiting will just give more programmers the
> opportunity to write bad code, I think.  And we are going to break
> binary compatibility of libpq in 6.4 anyway --- the contents of PGconn
> have changed since last time.  So anyone who's not relying on the
> accessor functions is already in trouble.
> 
> The downside is that we will probably break at least some applications
> at source-code level.  They weren't playing by the rules, but they'll
> be annoyed anyway.  (Anyone who's too lazy to fix their code can just
> import libpq-int.h instead of libpq-fe.h, but one hopes not too many
> people will take that way out.)
> 
> I know that both psql and libpgtcl are not playing entirely by the rules,
> and will need to be fixed.  I can handle that.  I'm not volunteering
> to fix any code outside the distribution, however.
> 
> Comments?  Any strong reasons *not* to do this?
> 
> 			regards, tom lane
> 
> 


-- 
Bruce Momjian                          |  830 Blythe Avenue
maillist(at)candle(dot)pha(dot)pa(dot)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)

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