Merlin Moncure wrote:
> On Wed, Jun 29, 2011 at 10:11 AM, Vincent Ficet
> <jean-vincent(dot)ficet(at)bull(dot)net> wrote:
>> 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 $_$
>> fw_id INT;
>> 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;
>> -- skip firmware which already exists
>> RETURN NULL;
>> END IF;
>> 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
Process 7593: SELECT
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,
CONTEXT: PL/pgSQL function "add_firmware" line 31 at SQL statement
#0 0x0000003c46725742 in __strncpy_ssse3 () from /lib64/libc.so.6
#1 0x00007ffff5f1617f in pqParseInput3 (conn=0x7ffff0000da0) at
#2 0x00007ffff5f0cae7 in parseInput (conn=0x7ffff0000da0) at fe-exec.c:1493
#3 0x00007ffff5f0cc01 in PQgetResult (conn=0x7ffff0000da0) at
#4 0x00007ffff5f0d26b in PQexecFinish (conn=0x7ffff0000da0) at
#5 0x00007ffff5f0cee4 in PQexec (conn=0x7ffff0000da0,
query=0x7ffff612d340 "SELECT 1") at fe-exec.c:1648
#6 0x00007ffff612c344 in dbd_ping () from
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"
I'll try to set up a minimalist reproducer and see what can be done for
Note that this happens with both postgresql 8.4.7 and 9.0.4.
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