[pgpool-hackers: 3178] Re: Query hang issue with extended protocol

Fri Dec 7 22:58:29 JST 2018

Hi Ishii-San

On Fri, Dec 7, 2018 at 6:30 PM Tatsuo Ishii <ishii at sraoss.co.jp> wrote:

> Hi Usama,
>
> I think I have found the cause. The transaction in question basically
> does these:
>
> BEGIN
> :
> (lots of extended query messages)
> :
> parse(COMMIT)
> bind(COMMIT)
> execute(COMMIT)
> Sync
> Close prepared statement (S_14) [1]
> Close prepared statement (S_11) [2]
> BEGIN <-- hang
>
> This message sequence seems to be unusual, because two close
> messages[1][2] are issued *after* a sync message (usually extended
> query protocol messages sequence including close messages are finished
> by a sync message). Pgpool-II expects a command complete message after
> the last BEGIN but unfortunately two close messages, which are
> responses of [1][2], arrived before it. While processing the close
> complete message, Pgpool-II turned off the query-in-progress flag as
> shown in the session_context. So read_kind_from_backend tried to read
> from node 1, then hangs.
>
> I was able to reproduce the hang by using attached pgproto data
> (pgproto.data).
>
> If the two close message were issued *before* the sync message,
> Pgpool-II does not hang. (see attached pgproto2.data).
>
>
Thanks for looking into this. This one seems to be a tough one.
Apparently the application getting the hang is using the JDBC driver which
is sending this
unusual message sequence.

Thanks
Kind Regards
Muhammad Usama

> > Hi Ishii-San
> >
> > Please find the attached the debug log and pgpool.conf
> >
> > The output of the stack trace, session context and query context is
> > pasted below
> >
> > *--STACK-TRACE--*
> > #0  0x00007f3d5d218c53 in __select_nocancel () at
> > ../sysdeps/unix/syscall-template.S:81
> > #1  0x000000000042e086 in pool_check_fd (cp=cp at entry=0x7f3d5efb43a0) at
> > protocol/pool_process_query.c:696
> > #2  0x000000000042e356 in pool_check_fd (cp=cp at entry=0x7f3d5efb43a0) at
> > protocol/pool_process_query.c:728
> > #3  0x000000000045fafc in pool_read (cp=0x7f3d5efb43a0,
> > buf=buf at entry=0x7fffbc4830b5,
> > len=len at entry=1) at utils/pool_stream.c:166
> > #4  0x000000000043303b in read_kind_from_backend
> > (frontend=frontend at entry=0x2314e98,
> > backend=backend at entry=0x7f3d5efaf750,
> > decided_kind=decided_kind at entry=0x7fffbc4834ba
> > "D")
> >     at protocol/pool_process_query.c:3379
> > #5  0x000000000043f402 in ProcessBackendResponse
> > (frontend=frontend at entry=0x2314e98,
> > backend=backend at entry=0x7f3d5efaf750, state=state at entry=0x7fffbc483524,
> >     num_fields=num_fields at entry=0x7fffbc483522) at
> > protocol/pool_proto_modules.c:2663
> > #6  0x0000000000431ea7 in pool_process_query (frontend=0x2314e98,
> > backend=0x7f3d5efaf750, reset_request=reset_request at entry=0) at
> > protocol/pool_process_query.c:320
> > #7  0x000000000042cd49 in do_child (fds=fds at entry=0x23134b0) at
> > protocol/child.c:388
> > #8  0x00000000004090a5 in fork_a_child (fds=0x23134b0, id=0) at
> > main/pgpool_main.c:659
> > #9  0x000000000040fb73 in PgpoolMain (discard_status=discard_status at entry
> =0
> > '\000', clear_memcache_oidmaps=clear_memcache_oidmaps at entry=0 '\000') at
> > main/pgpool_main.c:378
> > #10 0x0000000000407777 in main (argc=<optimized out>, argv=<optimized
> out>)
> > at main/main.c:349
> >
> > *--SESSION CONTEXT--*
> > (gdb) print *session_context
> > $4 = {process_context = 0x77c260 <process_context_d>, frontend =
> 0x2314e98,
> > backend = 0x7f3d5efaf750, in_progress = 0 '\000',
> > doing_extended_query_message = 0 '\000',
> >   need_to_restore_where_to_send = 0 '\000', where_to_send_save = "\001",
> > '\000' <repeats 126 times>, command_success = 1 '\001',
> writing_transaction
> > = 0 '\000',
> >   failed_transaction = 0 '\000', skip_reading_from_backends = 0 '\000',
> > ignore_till_sync = 0 '\000', transaction_isolation = POOL_UNKNOWN,
> > query_context = 0x248d418,
> >   memory_context = 0x22f2558, uncompleted_message = 0x231aeb8,
> message_list
> > = {capacity = 64, size = 63, sent_messages = 0x2351f70},
> > load_balance_node_id = 0,
> >   mismatch_ntuples = 0 '\000', ntuples = {0, -1, 0 <repeats 126 times>},
> > reset_context = 0 '\000', query_cache_array = 0x0, num_selects = 0,
> > pending_messages = 0x231b020,
> >   previous_message_exists = 0 '\000', previous_message = {type =
> > POOL_EXECUTE, contents = 0x24704f0 "SS_13", contents_len = 5, query =
> > "COMMIT", '\000' <repeats 1017 times>,
> >     statement = "\000_46", '\000' <repeats 123 times>, portal = '\000'
> > <repeats 127 times>, is_rows_returned = 0 '\000',
> not_forward_to_frontend =
> > 0 '\000', node_ids = {0, -1},
> >     query_context = 0x242ff68}, major = 3, minor = 0}
> >
> > *--QEURY-TRACE--*
> > (gdb) print *session_context->query_context
> > $5 = {original_query = 0x248e0e8 "BEGIN", rewritten_query = 0x0,
> > original_length = 6, rewritten_length = -1, parse_tree = 0x248e038,
> > rewritten_parse_tree = 0x0,
> >   where_to_send = "\001", '\000' <repeats 126 times>,
> > virtual_master_node_id = 0, query_state = {POOL_UNPARSED <repeats 128
> > times>}, is_cache_safe = 0 '\000', temp_cache = 0x0,
> >   is_multi_statement = 0 '\000', dboid = 0, query_w_hex = 0x0,
> > is_parse_error = 0 '\000', num_original_params = -1,
> > pg_terminate_backend_conn = 0x0, skip_cache_commit = 0 '\000',
> >   memory_context = 0x7f3d5efbff20}
> >
> > Thanks
> > Best Regards
> > Muhammad Usama
> >
> >
> > On Fri, Dec 7, 2018 at 3:49 AM Tatsuo Ishii <ishii at sraoss.co.jp> wrote:
> >
> >> Hi Usama,
> >>
> >> I need:
> >>
> >> - exact Pgpool-II version
> >> - pgpool.conf
> >> - gdb print command result of *pool_session_context when Pgpool-II hangs
> >>
> >> Also a full debug log will be helpful.
> >>
> >> Best regards,
> >> --
> >> Tatsuo Ishii
> >> SRA OSS, Inc. Japan
> >> English: http://www.sraoss.co.jp/index_en.php
> >> Japanese:http://www.sraoss.co.jp
> >>
> >> > Hi Ishii San
> >> >
> >> > Thanks for looking into this, As discussed over the call please find
> the
> >> > attached pgpool log and stack trace for the stuck issue.
> >> > I am currently not able to trim down the test case so sorry for
> throwing
> >> a
> >> > huge log file.
> >> > There is no special configuration on PostgreSQL server side and this
> >> issue
> >> > only happens when the load balance node and the primary node are same
> >> > and as you can see from the stack trace the stuck happens in the
> >> > read_kind_from_backend() function and the reason for that is
> >> > the function is trying to read response from the backend node
> 1(standby)
> >> > while it has only sent the query to the backend node 0 (primary)
> >> >
> >> > --stack-trace--
> >> > #0  0x00007f1775216c53 in __select_nocancel () at
> >> > ../sysdeps/unix/syscall-template.S:81
> >> > #1  0x000000000042def6 in pool_check_fd (cp=cp at entry=0x7f1776fb2328)
> at
> >> > protocol/pool_process_query.c:696
> >> > #2  0x000000000042e1c6 in pool_check_fd (cp=cp at entry=0x7f1776fb2328)
> at
> >> > protocol/pool_process_query.c:728
> >> > #3  0x000000000045f92c in pool_read (cp=0x7f1776fb2328,
> >> > buf=buf at entry=0x7ffebb5f2345,
> >> > len=len at entry=1) at utils/pool_stream.c:166
> >> > #4  0x0000000000432eab in read_kind_from_backend
> >> > (frontend=frontend at entry=0x2292e88,
> >> > backend=backend at entry=0x7f1776fad6d8,
> >> > decided_kind=decided_kind at entry=0x7ffebb5f274a
> >> > "D")
> >> >     at protocol/pool_process_query.c:3379
> >> > #5  0x000000000043f232 in ProcessBackendResponse
> >> > (frontend=frontend at entry=0x2292e88,
> >> > backend=backend at entry=0x7f1776fad6d8, state=state at entry
> =0x7ffebb5f27b4,
> >> >     num_fields=num_fields at entry=0x7ffebb5f27b2) at
> >> > protocol/pool_proto_modules.c:2653
> >> > #6  0x0000000000431d17 in pool_process_query (frontend=0x2292e88,
> >> > backend=0x7f1776fad6d8, reset_request=reset_request at entry=0) at
> >> > protocol/pool_process_query.c:320
> >> > #7  0x000000000042cbb9 in do_child (fds=fds at entry=0x2291370) at
> >> > protocol/child.c:387
> >> > #8  0x00000000004090a5 in fork_a_child (fds=0x2291370, id=2) at
> >> > main/pgpool_main.c:659
> >> > #9  0x000000000040fb73 in PgpoolMain
> (discard_status=discard_status at entry
> >> =0
> >> > '\000', clear_memcache_oidmaps=clear_memcache_oidmaps at entry=0
> '\000') at
> >> > main/pgpool_main.c:378
> >> > #10 0x0000000000407777 in main (argc=<optimized out>, argv=<optimized
> >> out>)
> >> > at main/main.c:361
> >> >
> >> > Thanks
> >> > Best Regards
> >> > Muhammad Usama
> >> >
> >> >
> >> >
> >> > On Thu, Dec 6, 2018 at 3:52 AM Tatsuo Ishii <ishii at sraoss.co.jp>
> wrote:
> >> >
> >> >> > Hi Ishii San
> >> >> >
> >> >> > I was investigating the issue faced by one of Pgpool-II user who
> >> reported
> >> >> > that after executing a certain transaction (large amount of data
> >> DELETE /
> >> >> > INSERT / UPDATE, COMMIT), the next execution of the BEGIN statement
> >> >> > hangs(using the extended query protocol).
> >> >> >
> >> >> > I think the problem is in
> pool_is_node_to_be_sent_in_current_query()
> >> >> > function. The issue is in streaming replication mode, when
> >> >> > pool_is_query_in_progress is not set, the function always returns
> >> true.
> >> >> Now
> >> >> > consider that we have two backend servers 1 primary and one
> standby.
> >> And
> >> >> > for a particular session the primary node gets selected as a load
> >> balance
> >> >> > node. So effectively that session should only be sending queries
> and
> >> >> > reading responses from primary server alone. But with the current
> >> >> > implementation of pool_is_node_to_be_sent_in_current_query() the
> >> >> > VALID_BACKEND macro will return true for standby node as well when
> the
> >> >> > query in progress flag is not set(which is wrong when standby node
> is
> >> not
> >> >> > selected for load balancing). and if this happens in let say
> >> >> > read_kind_from_backend() function we will get a stuck, since the
> >> >> > read_kind_from_backend() will be trying to read the response from
> >> standby
> >> >> > node while we have never sent any query to that node.
> >> >> >
> >> >> >
> >> >> > Can you please take a look at the attached patch, I am not sure if
> it
> >> >> will
> >> >> > have some side effects or not.
> >> >>
> >> >> Unfortunately the patch would not help and probably would break many
> >> >> cases which are working today.
> >> >>
> >> >> For the scenario you mentioned is already handled by
> >> >> pool_pending_message_* functions defined in
> >> >> context/query_context.c. When a message, for example parse, is sent
> >> >> from frontend and forwarded to backends, the info is recorded in an
> >> >> FIFO queue by pool_pending_message_add() along with info which of
> >> >> backend the message is forwarded to. When the parse message will be
> >> >> replied back with "parse complete message" from the backend,
> >> >> read_kind_message_from_backend() can know from which of backend it
> >> >> should read a message by looking at the head of the queue.  Once the
> >> >> message is retrieved (in this example parse complete message), an
> >> >> entry in the queue (in this example parse message) is pulled out and
> >> >> removed.
> >> >>
> >> >> This mechanism usually works well unless backend sends unpredicted
> >> >> message. Recently one of such a case was reported:
> >> >> https://www.pgpool.net/mantisbt/view.php?id=448
> >> >>
> >> >> In the report PostgreSQL sends an error message caused by
> >> >> idle_in_transaction_timeout setting which Pgpool-II does not expect.
> >> >>
> >> >> Does the user has any uncommon setting in postgresql.conf?
> >> >>
> >> >> Best regards,
> >> >> --
> >> >> Tatsuo Ishii
> >> >> SRA OSS, Inc. Japan
> >> >> English: http://www.sraoss.co.jp/index_en.php
> >> >> Japanese:http://www.sraoss.co.jp
> >> >>
> >>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.sraoss.jp/pipermail/pgpool-hackers/attachments/20181207/84a9ba1c/attachment-0001.html>