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Re: Proposal for Allow postgresql.conf values to be changed via SQL [review]

From: Amit kapila <amit(dot)kapila(at)huawei(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Boszormenyi Zoltan <zb(at)cybertec(dot)at>
Cc: "'Robert Haas'" <robertmhaas(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org"<pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Proposal for Allow postgresql.conf values to be changed via SQL [review]
Date: 2013-01-13 04:49:38
Message-ID: 6C0B27F7206C9E4CA54AE035729E9C383BEB332F@szxeml509-mbx (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Sunday, January 13, 2013 12:41 AM Tom Lane wrote:
Boszormenyi Zoltan <zb(at)cybertec(dot)at> writes:
>> No, I mean the reaper(SIGNAL_ARGS) function in
>> src/backend/postmaster/postmaster.c where your patch has this:

>> *** a/src/backend/postmaster/postmaster.c
>> --- b/src/backend/postmaster/postmaster.c
>> ***************
>> *** 2552,2557 **** reaper(SIGNAL_ARGS)

> I have not been following this thread, but I happened to see this bit,
> and I have to say that I am utterly dismayed that anything like this is
> even on the table.  This screams overdesign.  We do not need a global
> lock file, much less postmaster-side cleanup.  All that's needed is a
> suitable convention about temp file names that can be written and then
> atomically renamed into place.  If we're unlucky enough to crash before
> a temp file can be renamed into place, it'll just sit there harmlessly.

I can think of below ways to generate tmp file
1. Generate session specific tmp file name during operation. This will be similar to what is 
   currently being done in OpenTemporaryFileinTablespace() to generate a tempfilename.
2. Generate global tmp file name. For this we need to maintain global counter.
3. Always generate temp file with same name "", as at a time only one
   session is allowed to operate.

In any approach, there will be chance that temp files will remain if server crashes during this command execution
which can lead to collision of temp file name next time user tries to use SET persistent command.
An appropriate error will be raised and user manually needs to delete that file.

>>>> I just noticed that you are using "%m" in format strings twice. man 3 printf says:
>>>> m      (Glibc extension.)  Print output of strerror(errno). No argument is required.
>>>> This doesn't work anywhere else, only on GLIBC systems, it means Linux.
>>>> I also like the brevity of this extension but PostgreSQL is a portable system.
>>>> You should use %s and strerror(errno) as argument explicitly.
>>> %m is used at other places in code as well.

> As far as that goes, %m is appropriate in elog/ereport (which contain
> special support for it), but not anywhere else.

In the patch, it's used in ereport, so I assume it is safe and patch doesn't need any change for %m.

With Regards,
Amit Kapila.

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