Re: [HACKERS] More race conditions in logical replication

On 06/07/17 18:20, Petr Jelinek wrote:
> On 06/07/17 17:33, Petr Jelinek wrote:
>> On 03/07/17 01:54, Tom Lane wrote:
>>> I noticed a recent failure that looked suspiciously like a race condition:
>>>
>>> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hornet&dt=2017-07-02%2018%3A02%3A07
>>>
>>> The critical bit in the log file is
>>>
>>> error running SQL: 'psql:<stdin>:1: ERROR:  could not drop the replication slot "tap_sub" on publisher
>>> DETAIL:  The error was: ERROR:  replication slot "tap_sub" is active for PID 3866790'
>>> while running 'psql -XAtq -d port=59543 host=/tmp/QpCJtafT7R dbname='postgres' -f - -v ON_ERROR_STOP=1' with sql
'DROPSUBSCRIPTION tap_sub' at
/home/nm/farm/xlc64/HEAD/pgsql.build/src/test/subscription/../../../src/test/perl/PostgresNode.pmline 1198.
 
>>>
>>> After poking at it a bit, I found that I can cause several different
>>> failures of this ilk in the subscription tests by injecting delays at
>>> the points where a slot's active_pid is about to be cleared, as in the
>>> attached patch (which also adds some extra printouts for debugging
>>> purposes; none of that is meant for commit).  It seems clear that there
>>> is inadequate interlocking going on when we kill and restart a logical
>>> rep worker: we're trying to start a new one before the old one has
>>> gotten out of the slot.
>>>
>>
>> Thanks for the test case.
>>
>> It's not actually that we start new worker fast. It's that we try to
>> drop the slot right after worker process was killed but if the code that
>> clears slot's active_pid takes too long, it still looks like it's being
>> used. I am quite sure it's possible to make this happen for physical
>> replication as well when using slots.
>>
>> This is not something that can be solved by locking on subscriber. ISTM
>> we need to make pg_drop_replication_slot behave more nicely, like making
>> it wait for the slot to become available (either by default or as an
>> option).
>>
>> I'll have to think about how to do it without rewriting half of
>> replication slots or reimplementing lock queue though because the
>> replication slots don't use normal catalog access so there is no object
>> locking with wait queue. We could use some latch wait with small timeout
>> but that seems ugly as that function can be called by user without
>> having dropped the slot before so the wait can be quite long (as in
>> "forever").
>>
> 
> Naive fix would be something like attached. But as I said, it's not
> exactly pretty.
> 

So best idea I could come up with is to make use of the new condition
variable API. That lets us wait for variable which can be set per slot.

It's not backportable however, I am not sure how big problem that is
considering the lack of complaints until now (maybe we could backport
using the ugly timeout version?).

The attached patch is a prototype of such solution and there are some
race conditions (variable can get signaled before the waiting process
starts sleeping for it). I am mainly sending it to get feedback on the
approach.

-- 
  Petr Jelinek                  http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services

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