Re: pg_advisory_locks in a multithreaded application context

Merlin Moncure wrote:
> On Wed, Jun 29, 2011 at 10:11 AM, Vincent Ficet
> <jean-vincent(dot)ficet(at)bull(dot)net> wrote:
>
>> Hello,
>>
>> I'm having some trouble trying to use postgresql locks. After trying
>> several options and reading the postgresql online documentation, I still
>> can't figure out what I'm doing wrong. Here's the use case:
>>
>> A multithreaded application collecting adapter firmwares on a network
>> loads data into the following table:
>>
>> CREATE TABLE firmware (
>> id SERIAL NOT NULL,
>> type CHARACTER VARYING(32),
>> version CHARACTER VARYING(30),
>> build_id INTEGER,
>> date CHARACTER VARYING(25),
>> ps_id CHARACTER VARYING(25)
>> );
>>
>> Typically, there are a few hundred adapters, but only 5 firmwares (many
>> adapters should have the same firmware if the sysadmins did their jobs
>> properly ;-) ).
>>
>> Only a single entry is required per firmware (many separate adapters can
>> share the same firmware by pointing to the appropriate firmware id field).
>> To make sure that only one entry is created per firmware, I use the
>> following trigger:
>>
>> CREATE TRIGGER firmware_pre_insert_trigger
>> BEFORE INSERT ON firmware
>> FOR EACH ROW
>> EXECUTE PROCEDURE firmware_pre_insert_trigger_cb();
>>
>> CREATE FUNCTION firmware_pre_insert_trigger_cb() RETURNS TRIGGER
>> AS $_$
>> DECLARE
>> fw_id INT;
>> BEGIN
>>
>> SELECT fw.id FROM firmware fw INTO fw_id
>> WHERE (fw.type = new.type AND
>> fw.version = new.version AND
>> fw.build_id = new.build_id AND
>> fw.date = new.date AND
>> fw.ps_id = new.ps_id);
>>
>> IF fw_id IS NULL THEN
>> -- create the non-existing firmware
>> RETURN new;
>> ELSE
>> -- skip firmware which already exists
>> RETURN NULL;
>> END IF;
>>
>> END;
>> $_$
>> LANGUAGE PLPGSQL;
>>
>> When a thread wishes to add a firmware after discovering one adapter, it
>> executes the following code:
>>
>> PERFORM pg_advisory_lock(1);
>>
>> INSERT INTO firmware (type, version, build_id, date, ps_id)
>> VALUES (chip_type, firm_version, firm_build_id, firm_date, firm_ps_id);
>>
>> PERFORM pg_advisory_unlock(1);
>>
>
> Advisory lock is not going to work here. You are releasing the lock
> before the transaction resolves and that leaves a window for second
> transaction to do the 'select' and not see the data because it hasn't
> committed yet.
>
>
>> Unfortunately, I still get duplicated entries using advisory locks, and
>> they don't seem to lock anything at all...
>>
>> On the other hand, If I use builtin locks as follows without the
>> trigger, I get deadlocks in the server logs:
>>
>> LOCK TABLE firmware IN SHARE MODE;
>>
>
> well a sharelock certainly isn't going to work. share blocks row
> exclusive (see http://www.postgresql.org/docs/current/static/explicit-locking.html#LOCKING-TABLES),
> so two transactions can simultaneously get a share lock and wait for
> each other to to resolve to get the exclusive lock on a row.
> 'EXCLUSIVE' would be better (although that would effectively serialize
> the transactions).
>
>
Thanks for the tip.

It now works fine using a SHARE UPDATE EXCLUSIVE lock in the PRE INSERT
trigger. This does not conflict with the ROW EXCLUSIVE lock which is
implicitely taken by the INSERT statement in add_firmware() function.

BTW I think there might be a thread safety issue in postgres, as I often
get a segfault when deadlocks occur (prior to applying the fix I just
described). For example, the following deadlock situation:

DETAIL: Process 7643 waits for RowExclusiveLock on relation 21060
of database 20535; blocked by process 7593.
Process 7593 waits for RowExclusiveLock on relation 21060 of
database 20535; blocked by process 7643.
Process 7643: SELECT
add_firmware('0x08003800013731aa','hca','512.1792.0',0,'0920-10-06','BL_0010030001000');
Process 7593: SELECT
add_firmware('0x08003800013734b0','hca','512.1792.0',0,'0920-10-06','BL_0010030001000');
HINT: See server log for query details.
QUERY: INSERT INTO firmware (type, version, build_id, date, ps_id)
VALUES (chip_type, firm_version, firm_build_id,
firm_date, firm_ps_id)
CONTEXT: PL/pgSQL function "add_firmware" line 31 at SQL statement

Triggers:

#0 0x0000003c46725742 in __strncpy_ssse3 () from /lib64/libc.so.6
#1 0x00007ffff5f1617f in pqParseInput3 (conn=0x7ffff0000da0) at
fe-protocol3.c:209
#2 0x00007ffff5f0cae7 in parseInput (conn=0x7ffff0000da0) at fe-exec.c:1493
#3 0x00007ffff5f0cc01 in PQgetResult (conn=0x7ffff0000da0) at
fe-exec.c:1568
#4 0x00007ffff5f0d26b in PQexecFinish (conn=0x7ffff0000da0) at
fe-exec.c:1807
#5 0x00007ffff5f0cee4 in PQexec (conn=0x7ffff0000da0,
query=0x7ffff612d340 "SELECT 1") at fe-exec.c:1648
#6 0x00007ffff612c344 in dbd_ping () from
/home/vficet/X86_64/usr/lib/dbd/libdbdpgsql.so

Stack traces often occur in different flavours, but can always be
correlated with messages such as:

DBI error -9: unexpected field count in "D" message
DBI error -9: message contents do not agree with length in message type
"T" server sent data ("D" message) without prior row description ("T"
message)

I'll try to set up a minimalist reproducer and see what can be done for
that.
Note that this happens with both postgresql 8.4.7 and 9.0.4.

Cheers,

Vincent

> merlin
>
>