[pgpool-hackers: 3177] Re: Query hang issue with extended protocol

Muhammad Usama m.usama at gmail.com
Fri Dec 7 22:51:08 JST 2018 

On Fri, Dec 7, 2018 at 5:46 PM Tatsuo Ishii <ishii at sraoss.co.jp> wrote:

> > 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
>
> Oh I ok. I was wondering because I see:
>
> 2018-12-06 12:28:16: pid 91029: LOG:  pgpool-II successfully started.
> version 4.0beta1 (torokiboshi)
>

Yes the previous log I sent was from the master branch :-)

>
> > 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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