[pgpool-hackers: 3174] Re: Query hang issue with extended protocol

Muhammad Usama m.usama at gmail.com
Fri Dec 7 20:15:48 JST 2018 

Hi Ishii-San

Sorry, forgot to mention that I was using pgpool-II version 4.0.2
(torokiboshi) but the issue is reproducible on all pgpool version

Best Regards
Muhammad Usama

On Fri, Dec 7, 2018 at 4:13 PM Muhammad Usama <m.usama at gmail.com> wrote:

> 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
>> >>
>>
>
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