[HACKERS] ERROR: MultiXactId 3268957 has not been created yet -- apparent wraparound after missused pg_resetxlogs
Hi, I’m facing a problem with a PostgreSQL 9.6.2 reporting this error when selecting a table ERROR: MultiXactId 3268957 has not been created yet -- apparent wraparound The root cause is not a Postgres bug but a buggy person that used pg_resetxlogs obviously without reading or understanding the documentation. I’m trying to extract data from the db to create a new one ;-) But pg_dump logically end with the same error. I’ve seen the row with t_max = 3268957 page_inspect, I might use it to extract row from the page, but this will not be quite easy. Is there a way to change the t_max to a value that won’t fire the error, with this row. Could pg_filedump be used in this problem configuration. As usual, xlogs where deleted even stored in a directory named pg_wal L ( I made the change before 10 ) and xlogs reseted by the same person that commented the backup months ago. Hopefully, this is not a production database :-D but the user want to start qualification of the new application version today .. no luck for him. Any advice of how to extract data is welcome. At least, there is something to learn, renaming pg_xlog to pg_wal won’t stop everybody :-/ Regards. Alain Radix
Re: [HACKERS] Requesting external_pid_file with postgres -C when not initialized lead to coredump
Testing that external_pid_file was not null didn't seem necessary to me
because it's initialized to '' and background workers could only start
after the startup piece of code.
If external_pid_file is set to null while the server run, then I prefer to
not hide the problem as it would only be reported at the server shutdown
I tested with ALTER SYSTEM SET but couldn't set external_pid_file to null,
only to ''
This another good reason to use '' instead of null, because it can be set
with ALTER SYSTEM SET even if I see no reason to ;-)
Using strlen(external_pid_file) instead of external_pid_file[0] seems more
readable to me.
I agree that it cost a few cpu cycles more.
I'm also unsure that testing if(A && B) would always be compiled to A being
tested before B so that it won't always protect from testing
external_pid_file[0] with external_pid_file being null.
To ensure that, I would prefer :
if(! external_pid_file){
if(external_pid_file[0])
But, I see it at useless protective code against an impossible situation.
Maybe I'm wrong, but I don't see how it could happen.
About parameters being null, as they're reported as '' in pg_settings and
ALTER SYSTEM SET can only set them to '', I consider this should be avoided.
I used all parameters reported to '' in pg_settings after initdb and found
only external_pid_file to crash postgres -C
Alain
PS : Thanks for pgBackRest ;-)
On 22 June 2016 at 18:51, Tom Lane wrote:
> alain radix writes:
> > Another effect of the patch is that it become possible to start a cluster
> > with external_pid_file='' in postgresql.conf
> > It would have that same behavior as many other parameters.
>
> I'd still be inclined to do that with something along the lines of
>
> - if (external_pid_file)
> + if (external_pid_file && external_pid_file[0])
>
> rather than changing the default per se.
>
> regards, tom lane
>
--
Alain Radix
Re: [HACKERS] Requesting external_pid_file with postgres -C when not initialized lead to coredump
Hi Tom, The actual fix do the job for preventing coredump. Effectively, reporting "" instead of (null) would be more consistent with the values found in pg_settings. Another effect of the patch is that it become possible to start a cluster with external_pid_file='' in postgresql.conf It would have that same behavior as many other parameters. I started a 9.6 beta with the following lines in postgresql.conf and the only one that reported a problem is external_pid_file external_pid_file = '' # write an extra PID file unix_socket_group = '' # (change requires restart) bonjour_name = '' # defaults to the computer name ssl_ca_file = ''# (change requires restart) ssl_crl_file = '' # (change requires restart) krb_server_keyfile = '' shared_preload_libraries = '' # (change requires restart) synchronous_standby_names = '' # standby servers that provide sync rep cluster_name = '' # added to process titles if nonempty default_tablespace = '' # a tablespace name, '' uses the default temp_tablespaces = '' # a list of tablespace names, '' uses local_preload_libraries = '' session_preload_libraries = '' It seems to me that the general behavior it to consider an empty string as an unset parameter in postgresql.conf, why should external_pid_file be an exception ? I would prefer postgresql -C to report an empty string to be consistent with pg_settings in a backport of the fix. I also would prefer external_pid_file to behave as other parameters in the next versions. Regards. Alain On 22 June 2016 at 17:24, Tom Lane wrote: > alain radix writes: > > Here is a new patch with postmaster.c modification so that it check > about a > > empty string instead of a null pointer. > > Why should we adopt this, when there is already a better (more complete) > fix in git? > > I'm not sold on the wisdom of modifying the semantics of > external_pid_file, which is what this patch effectively does. I don't > know if anything outside our core code looks at that variable, but if > anything does, this would create issues for it. > > I'm even less sold on the wisdom of changing every other GUC that > has a null bootstrap default, which is what we would also have to do > in order to avoid the identical misbehavior for other -C cases. > > > The meaning is no more to avoid a core dump as you've done a change for > > that but to have the same result with postgres -C as with a request to > > pg_settings, an empty string when the parameter is not set. > > Well, maybe we should make -C print an empty string not "(nil)" for > such cases. You're right that we don't make a distinction between > NULL and "" in other code paths that display a string GUC, so doing > so here is a bit inconsistent. > > regards, tom lane > -- Alain Radix
Re: [HACKERS] Requesting external_pid_file with postgres -C when not initialized lead to coredump
Yes Michael, You're right, I aw this yesterday but couldn't make a patch. Here is a new patch with postmaster.c modification so that it check about a empty string instead of a null pointer. The meaning is no more to avoid a core dump as you've done a change for that but to have the same result with postgres -C as with a request to pg_settings, an empty string when the parameter is not set. It seems a lot better to me that different ways to request the same parameter should return the same answer. Actually, setting from a request to pg_settings an empty string in postgresql.conf for external_pid_file would lead to an error for the postmaster. To values of parameter should be the same reported in pg_settings and postgres -C and should be a valid settings in postgresql.conf So, that if you create a postgresql.conf from pg_settings or postresql -C,, it would would be a valid one. So, the patch make an empty string valid in all places and shouldn't cause problem with existing installations. Regards. Alain Radix On 21 June 2016 at 23:45, Michael Paquier wrote: > On Wed, Jun 22, 2016 at 2:02 AM, Alvaro Herrera > wrote: > > alain radix wrote: > >> So, here is my first patch for PostgreSQL. > > > > Looking forward to further ones, > > A comment on this patch: this is an incorrect approach anyway. > PostmasterMain relies on this value being NULL to decide if this PID > file should be written or not, so this patch applied as-is would > result in a message outputted to stderr if the logic is not changed > there. > -- > Michael > -- Alain Radix external_pid_file.patch Description: Binary data -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] Requesting external_pid_file with postgres -C when not initialized lead to coredump
Hi, I faced a coredump when reading the value of the parameter "external_pid_file" when it was not initialized in postgresql.conf This came from the value not being specified to be initialized to en empty string in guc.c in the ConfigureNamesString array. the behavior can easily been tested with the following commands : initdb test postgres -D test -C external_pid_file I faced the problem with version 9.3, 9.5 and 9.6 beta 1 This seems to come from a long time ago. I wrote a patch ( with help from Stéphane Schildknecht ) to correct the problem with a proper initialization. The patch also removed a useless initialization of cluster_name to save a little memory. So, here is my first patch for PostgreSQL. Best regards. -- Alain Radix external_pid_file.patch Description: Binary data -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
