Re: Symbolic link breaks for postgresql.auto.conf

On 6/6/21 9:00 AM, RAJAMOHAN wrote:
> Hello all,
>
> Can someone explain how *postgresql.auto.conf *file contents gets
> updated, when running an alter system statement followed by pg_reload_conf?
>
> I am trying to keep a symbolic link for the
> *postgresql.auto.conf* outside the data directory. The link gets created
> but whenever I issue an alter system statement it gets broken.
>
>
> The main reason for this symbolic link creation is to retain the
> parameter values associated with this cluster, because we will be
> copying the entire data directory from another server to this server
> from time to time for testing purposes.
> And no issues with other files such as hba and postgresql.conf.
>
> *Operating System: Ubuntu 18.04.1 LTS
> *
> *Postgresql : 12.7*
>
> Step1:
> Created symbolic links and i am inside data directory
> lrwxrwxrwx 1 postgres postgres   41 Jun  3 05:12 postgresql.conf ->
> /etc/postgresql/12/test1/postgresql.conf
> lrwxrwxrwx 1 postgres postgres   37 Jun  3 05:12 pg_hba.conf ->
> /etc/postgresql/12/test1/pg_hba.conf
> lrwxrwxrwx 1 postgres postgres   46 Jun  6 15:19 postgresql.auto.conf ->
> /etc/postgresql/12/test1/postgresql.auto.conf
>
> Step2:
> alter system set checkpoint_timeout='2min';select pg_reload_conf();
> ALTER SYSTEM
>  pg_reload_conf
> ----------------
>  t
> (1 row)
>
> Step3:
> Symbolic link got broken
> lrwxrwxrwx 1 postgres postgres   41 Jun  3 05:12 postgresql.conf ->
> /etc/postgresql/12/test1/postgresql.conf
> lrwxrwxrwx 1 postgres postgres   37 Jun  3 05:12 pg_hba.conf ->
> /etc/postgresql/12/test1/pg_hba.conf
> lrwxrwxrwx 1 postgres postgres   46 Jun  6 15:19 postgresql.auto.conf
>

Have to believe it is due to:

~/src/backend/utils/misc/guc.c

/*
* Execute ALTER SYSTEM statement.
*
* Read the old PG_AUTOCONF_FILENAME file, merge in the new variable value,
* and write out an updated file. If the command is ALTER SYSTEM RESET
ALL,
* we can skip reading the old file and just write an empty file.
*
* An LWLock is used to serialize updates of the configuration file.
*
* In case of an error, we leave the original automatic
* configuration file (PG_AUTOCONF_FILENAME) intact.
*/

In particular:

/*
* To ensure crash safety, first write the new file data to a
temp file,
* then atomically rename it into place.
*
* If there is a temp file left over due to a previous crash,
it's okay to
* truncate and reuse it.
*/

>
>
> Thanks & Regards,
> Rajamohan.J
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com