Обсуждение: Re: [COMMITTERS] pgsql: Prevent "snapshot too old" from trying to return pruned TOAST tu

On Thu, Aug 4, 2016 at 10:58 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Thu, Aug 4, 2016 at 7:23 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Robert Haas <rhaas@postgresql.org> writes:
>>> Prevent "snapshot too old" from trying to return pruned TOAST tuples.
>>
>> Looks like this patch broke the build on castoroides.  Recommend
>> changing InitToastSnapshot into a macro.  (There's a reason why
>> InitDirtySnapshot is a macro.)
>
> What is the reason?  We refuse to separate frontend and backend
> headers in any sort of principled way?

That was poorly phrased.  I'll try again: I can't see any reason for
using a macro here except that it allows frontend code to compile this
without breaking.  But that doesn't seem like a very good way of
solving that problem.  There's surely no way for a casual reader of
the code to realize that macros can be used here and inline functions
cannot, especially because this works apparently works fine on most
machines, including mine.

Here's a patch.  Is this what you want?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Aug 4, 2016 at 10:58 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> What is the reason?  We refuse to separate frontend and backend
>> headers in any sort of principled way?

> That was poorly phrased.  I'll try again: I can't see any reason for
> using a macro here except that it allows frontend code to compile this
> without breaking.

Well, the alternative would be to put "#ifndef FRONTEND" around the
static-inline function.  That's not very pretty either, and it's
inconsistent with the existing precedent (ie, InitDirtySnapshot).
Also, it presumes that a non-backend includer actually has defined
FRONTEND; that seems to be the case for pg_xlogdump but I do not
think we do that everywhere.

> Here's a patch.  Is this what you want?

OK by me.
        regards, tom lane

On Fri, Aug 5, 2016 at 11:05 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Thu, Aug 4, 2016 at 10:58 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>> What is the reason?  We refuse to separate frontend and backend
>>> headers in any sort of principled way?
>
>> That was poorly phrased.  I'll try again: I can't see any reason for
>> using a macro here except that it allows frontend code to compile this
>> without breaking.
>
> Well, the alternative would be to put "#ifndef FRONTEND" around the
> static-inline function.  That's not very pretty either, and it's
> inconsistent with the existing precedent (ie, InitDirtySnapshot).
> Also, it presumes that a non-backend includer actually has defined
> FRONTEND; that seems to be the case for pg_xlogdump but I do not
> think we do that everywhere.

That may not be pretty, but it'd be a lot more transparent.  If I see
#ifndef FRONTEND, I think, oh, that's protecting some stuff that
shouldn't be included in front-end compiles.  If I see a macro, I not
necessarily think of the fact that this may be a way of preventing
that code from being compiled in front-end compiles.

>> Here's a patch.  Is this what you want?
>
> OK by me.

OK, committed.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company