Re: Race condition in FetchTableStates() breaks synchronization of subscription tables

On Tue, 30 Jan 2024 at 07:24, Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> On Monday, January 29, 2024 9:22 PM vignesh C <vignesh21@gmail.com> wrote:
> >
> > On Fri, 26 Jan 2024 at 11:30, Alexander Lakhin <exclusion@gmail.com> wrote:
> > >
> > > Hello hackers,
> > >
> > > After determining a possible cause for intermittent failures of the
> > > test subscription/031_column_list [1], I was wondering what makes
> > > another subscription test (014_binary) fail on the buildfarm:
> > > https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=snakefly&dt=20
> > > 24-01-22%2001%3A19%3A03
> > >
> > https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mylodon&dt=202
> > > 4-01-14%2018%3A19%3A20
> > > https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=piculet&dt=202
> > > 3-12-21%2001%3A11%3A52
> > >
> > https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mylodon&dt=202
> > > 3-11-27%2001%3A42%3A39
> > >
> > > All those failures caused by a timeout when waiting for a message
> > > expected in _subscriber.log. For example, in the snakefly's case:
> > > [01:14:46.158](1.937s) ok 7 - check synced data on subscriber with
> > > custom type timed out waiting for match: (?^:ERROR: ( [A-Z0-9]+:)?
> > > incorrect binary data format) at
> > /home/bf/bf-build/piculet/HEAD/pgsql/src/test/subscription/t/014_binary.pl
> > line 269.
> > >
> > > _subscriber.log contains:
> > > 2023-12-21 01:14:46.215 UTC [410039] 014_binary.pl LOG:  statement:
> > > ALTER SUBSCRIPTION tsub REFRESH PUBLICATION;
> > > 2023-12-21 01:17:46.756 UTC [409999] ERROR:  could not receive data
> > > from WAL stream: server closed the connection unexpectedly
> > >          This probably means the server terminated abnormally
> > >          before or while processing the request.
> > > 2023-12-21 01:17:46.760 UTC [405057] LOG:  background worker "logical
> > > replication apply worker" (PID 409999) exited with exit code 1
> > > 2023-12-21 01:17:46.779 UTC [532857] LOG:  logical replication apply
> > > worker for subscription "tsub" has started ...
> > >
> > > While _subscriber.log from a successful test run contains:
> > > 2024-01-26 03:49:07.065 UTC [9726:5] 014_binary.pl LOG:  statement:
> > > ALTER SUBSCRIPTION tsub REFRESH PUBLICATION;
> > > 2024-01-26 03:49:07.075 UTC [9726:6] 014_binary.pl LOG: disconnection:
> > > session time: 0:00:00.014 user=postgres database=postgres host=[local]
> > > 2024-01-26 03:49:07.558 UTC [9729:1] LOG:  logical replication apply
> > > worker for subscription "tsub" has started
> > > 2024-01-26 03:49:07.563 UTC [9731:1] LOG:  logical replication table
> > > synchronization worker for subscription "tsub", table
> > > "test_mismatching_types" has started
> > > 2024-01-26 03:49:07.585 UTC [9731:2] ERROR:  incorrect binary data
> > > format
> > > 2024-01-26 03:49:07.585 UTC [9731:3] CONTEXT:  COPY
> > > test_mismatching_types, line 1, column a
> > >
> > > In this case, "logical replication apply worker for subscription
> > > "tsub" has started" appears just after "ALTER SUBSCRIPTION", not 3 minutes
> > later.
> > >
> > > I've managed to reproduce this failure locally by running multiple
> > > tests in parallel, and my analysis shows that it is caused by a race
> > > condition when accessing variable table_states_valid inside tablesync.c.
> > >
> > > tablesync.c does the following with table_states_valid:
> > > /*
> > >   * Callback from syscache invalidation.
> > >   */
> > > void
> > > invalidate_syncing_table_states(Datum arg, int cacheid, uint32
> > > hashvalue) {
> > >      table_states_valid = false;
> > > }
> > > ...
> > > static bool
> > > FetchTableStates(bool *started_tx)
> > > {
> > > ...
> > >      if (!table_states_valid)
> > >      {
> > > ...
> > >          /* Fetch all non-ready tables. */
> > >          rstates = GetSubscriptionRelations(MySubscription->oid,
> > > true); ...
> > >          table_states_valid = true;
> > >      }
> > >
> > > So, when syscache invalidation occurs inside the code block "if
> > > (!table_states_valid)", that invalidation is effectively ignored.
> > >
> > > In a failed case I observe the following events:
> > > 1. logical replication apply worker performs
> > >   LogicalRepApplyLoop() -> process_syncing_tables() ->
> > >   process_syncing_tables_for_apply() -> FetchTableStates() periodically.
> > >
> > > 2. ALTER SUBSCRIPTION tsub REFRESH PUBLICATION invalidates syscache
> > >   for SUBSCRIPTIONRELMAP, and that leads to calling
> > >   invalidate_syncing_table_states().
> > >
> > > 3. If the apply worker manages to fetch no non-ready tables in
> > >   FetchTableStates() and ignore "table_states_valid = false" from
> > >   invalidate_syncing_table_states(), then it just misses the invalidation
> > >   event, so it keeps working without noticing non-ready tables appeared as
> > >   the result of ALTER SUBSCRIPTION (process_syncing_tables_for_apply()
> > skips
> > >   a loop "foreach(lc, table_states_not_ready) ..." until some other event
> > >   occurs).
> > >
> > > pg_usleep(100000) added just below GetSubscriptionRelations(...)
> > > proves my analysis -- without it, I need tens of iterations (with 50
> > > tests running in
> > > parallel) to catch the failure, but with it, I get the failure on the
> > > first iteration.
> >
> > Thanks for the analysis, I was able to reproduce the issue with the steps you had
> > shared. I agree with your analysis. I added some logs to verify that the
> > invalidation was getting missed.
> >
> > I felt that this invalidation is getting ignored because we have used a boolean
> > variable here, how about changing it slightly so that table_states_invalid gets
> > incremented for every invalidation and then decrementing table_states_invalid
> > after getting the non-ready tables like in the attached patch.  I was able to
> > verify that the test passes with the attached patch.
>
> Thanks for the patch.
>
> I am not sure if counting the invalidation number is needed, as even if there
> are hundreds of invalidations outside of FetchTableStates, one FetchTableStates
> call should reset the count to 0 as it is checking the latest catalog.

Another approach I was thinking of is to reset table_states_valid
immediately in the beginning of FetchTableStates, so any new
invalidations will take care of setting table_states_valid again it
again which will be handled in the next iteration of fetching
non-ready tables like in the attached patch.

Regards,
Vigneshj