Обсуждение: question about relation_open

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question about relation_open

От
Giampaolo Capelli
Дата:
Hi there,
I'm trying to write an extension to study the internals of Postgresql, and I face a server crash after calling the function relation_open.

I want to call the function ReadBufferExtended
to get the raw data of a given buffer
and I've read in some examples that I need to call relation_open
first, in order to get a Relation variable and also lock the relation.

The function relation_open returns a non NULL pointer in my extension, but then the last line in the following snippet crashes postgres.

Relation rel;
rel = relation_open(relid, AccessShareLock);

Assert(rel != NULL);
Assert(rel->rd_rel != NULL);
Assert(rel->rd_id != NULL);
elog(INFO, "rel->rd_id not null");
Oid rd_id = rel->rd_id;


Could somebody please help me understand what I'm doing wrong?
(Sorry if this is not the correct mailing list)

Re: question about relation_open

От
Rahila Syed
Дата:
Hi,


I want to call the function ReadBufferExtended
to get the raw data of a given buffer
and I've read in some examples that I need to call relation_open
first, in order to get a Relation variable and also lock the relation.

The function relation_open returns a non NULL pointer in my extension, but then the last line in the following snippet crashes postgres.
 
You mentioned that the postgres crashes. Did you mean it caused a Segmentation fault or something else like PANIC? 
If possible, can you share a core dump in that case? 

Also, can you share any errors that you see in logs when postgres crashes?

Thank you,
Rahila Syed

Re: question about relation_open

От
Giampaolo Capelli
Дата:
hi Rahila, thank you for your response,
I mean Segmentation fault, this is what I see in psql

SSL SYSCALL error: EOF detected
The connection to the server was lost. Attempting reset: Failed.
!?>

this is the content of
in /var/log/postgresql/postgresql-14-main.log when a segmentation fault happens


2025-01-14 12:30:12.092 CET [1538] LOG:  server process (PID 34594) was terminated by signal 11: Erreur de segmentation
2025-01-14 12:30:12.092 CET [1538] DETAIL:  Failed process was running: select visualize_buffers(90);
2025-01-14 12:30:12.092 CET [1538] LOG:  terminating any other active server processes
2025-01-14 12:30:12.097 CET [1538] LOG:  all server processes terminated; reinitializing
2025-01-14 12:30:12.115 CET [34608] LOG:  database system was interrupted; last known up at 2025-01-14 10:36:29 CET
2025-01-14 12:30:12.175 CET [34610] postgres@postgres FATAL:  the database system is in recovery mode
2025-01-14 12:30:12.279 CET [34608] LOG:  database system was not properly shut down; automatic recovery in progress
2025-01-14 12:30:12.281 CET [34608] LOG:  redo starts at 0/1CCF820
2025-01-14 12:30:12.281 CET [34608] LOG:  invalid record length at 0/1CCF858: wanted 24, got 0
2025-01-14 12:30:12.281 CET [34608] LOG:  redo done at 0/1CCF820 system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s
2025-01-14 12:30:12.379 CET [1538] LOG:  database system is ready to accept connections
 
I don't know how to collect a core dump of postgres, I tried with gdb, but It doesn't seem to log anything interesting 


Le mar. 14 janv. 2025 à 12:27, Rahila Syed <rahilasyed90@gmail.com> a écrit :
Hi,


I want to call the function ReadBufferExtended
to get the raw data of a given buffer
and I've read in some examples that I need to call relation_open
first, in order to get a Relation variable and also lock the relation.

The function relation_open returns a non NULL pointer in my extension, but then the last line in the following snippet crashes postgres.
 
You mentioned that the postgres crashes. Did you mean it caused a Segmentation fault or something else like PANIC? 
If possible, can you share a core dump in that case? 

Also, can you share any errors that you see in logs when postgres crashes?

Thank you,
Rahila Syed



Re: question about relation_open

От
Tom Lane
Дата:
Giampaolo Capelli <giampow@gmail.com> writes:
> The function relation_open returns a non NULL pointer in my extension, but
> then the last line in the following snippet crashes postgres.

> Relation rel;
> rel = relation_open(relid, AccessShareLock);

> Assert(rel != NULL);
> Assert(rel->rd_rel != NULL);
> Assert(rel->rd_id != NULL);
> elog(INFO, "rel->rd_id not null");
> Oid rd_id = rel->rd_id;

Hmm, I'm a little bit doubting your statement that relation_open
succeeded.  I wonder if maybe you built your extension without asserts
enabled ...

            regards, tom lane

Re: question about relation_open

От
Giampaolo Capelli
Дата:
It seems the issue was related to some misconfiguration of my postgresql server, 
I've migrated from pg 14 to 17 and now the issue has disappeared.

Also, I replaced relation_open with table_open

Le mar. 14 janv. 2025 à 15:40, Tom Lane <tgl@sss.pgh.pa.us> a écrit :
Giampaolo Capelli <giampow@gmail.com> writes:
> The function relation_open returns a non NULL pointer in my extension, but
> then the last line in the following snippet crashes postgres.

> Relation rel;
> rel = relation_open(relid, AccessShareLock);

> Assert(rel != NULL);
> Assert(rel->rd_rel != NULL);
> Assert(rel->rd_id != NULL);
> elog(INFO, "rel->rd_id not null");
> Oid rd_id = rel->rd_id;

Hmm, I'm a little bit doubting your statement that relation_open
succeeded.  I wonder if maybe you built your extension without asserts
enabled ...

                        regards, tom lane


--
Giampaolo Capelli