PostgreSQL 11.3 hangs forever during DEALLOCATE statement

Hi,

We have a distributed system (all CentOS 7 with PG 11.3 compiled from 
sources) that makes use of pg_fdw to forward changes to a local table to 
a remote server.

Today we noticed that one process got stuck with wait_event_type 
'Extension'. Attaching GDB to the process on the local server (see 
further below) showed that the extension in question was actually 
pg_fdw. Further inspection on the remote server showed that the 
connection was stuck in a DEALLOCATE call that never returned.

*** Local server ***

Blocked statement:

voip=# select 

pid,wait_event_type,wait_event,datid,datname,backend_start,xact_start,query_start,state_change,backend_xid,backend_xmin,backend_type,substring(query,1,50)

from pg_stat_activity where state <> 'idle';
   pid  | wait_event_type | wait_event | datid | datname | 
backend_start         |          xact_start           | 
query_start          |         state_change          | backend_xid | 
backend_xmin |  backend_type  | substring

-------+-----------------+------------+-------+---------+-------------------------------+-------------------------------+-------------------------------+-------------------
------------+-------------+--------------+----------------+----------------------------------------------------
  25301 | Extension       | Extension  | 16689 | voip    | 2019-06-11 
11:47:32.75992+02  | 2019-06-11 12:24:46.554381+02 | 2019-06-11 
12:24:46.55486+02  | 2019-06-11 12:24:46.55486+02  | 12198484 |     
12198483 | client backend | UPDATE importer.config SET value='f' WHERE ....


The table table updated in the query has a trigger function that 
synchronizes the changes to a remote table:


                      Table "importer.config"
   Column  |  Type  | Collation | Nullable | Default
----------+--------+-----------+----------+-------------------------------------------------
  id       | bigint |           | not null | 
nextval('importer.config_new_id_seq'::regclass)
  param    | text   |           | not null |
  value    | text   |           |          |
  hostname | text   |           | not null |
  filetype | text   |           | not null |
Indexes:
     "config_param_hostname_filetype_key" UNIQUE CONSTRAINT, btree 
(param, hostname, filetype)
Triggers:
     importer_config_trigger AFTER INSERT OR DELETE OR UPDATE ON 
importer.config FOR EACH STATEMENT EXECUTE PROCEDURE 
notify_importer_config_change()
     sync_importer_config AFTER INSERT OR DELETE OR UPDATE OR TRUNCATE 
ON importer.config FOR EACH STATEMENT EXECUTE PROCEDURE 
importer.trig_sync_importer_config()

voip=# \dfS+ importer.trig_sync_importer_config()

BEGIN
DELETE FROM remote_qdc_importer.config;
INSERT INTO remote_qdc_importer.config SELECT * FROM importer.config;
RETURN NEW;
END;


Remote table definition:

voip=# \d remote_qdc_importer.config;
                   Foreign table "remote_qdc_importer.config"
   Column  |  Type  | Collation | Nullable | Default |       FDW options
----------+--------+-----------+----------+---------+--------------------------
  id       | bigint |           | not null |         | (column_name 'id')
  param    | text   |           | not null |         | (column_name 'param')
  value    | text   |           |          |         | (column_name 'value')
  hostname | text   |           | not null |         | (column_name 
'hostname')
  filetype | text   |           | not null |         | (column_name 
'filetype')
Server: qdc
FDW options: (schema_name 'importer', table_name 'config')

Backtrace on the local server (PID 25301)


#0  0x00007fe8a32cd463 in __epoll_wait_nocancel () from /lib64/libc.so.6
(gdb) bt
#0  0x00007fe8a32cd463 in __epoll_wait_nocancel () from /lib64/libc.so.6
#1  0x000000000070c81e in WaitEventSetWaitBlock (nevents=1, 
occurred_events=0x7ffd7bfc0fc0, cur_timeout=-1, set=0x24f1900) at 
latch.c:1048
#2  WaitEventSetWait (set=set@entry=0x24f1900, timeout=timeout@entry=-1, 
occurred_events=occurred_events@entry=0x7ffd7bfc0fc0, 
nevents=nevents@entry=1,
     wait_event_info=wait_event_info@entry=117440512) at latch.c:1000
#3  0x000000000070cc57 in WaitLatchOrSocket (latch=0x7fe8967e9fc4, 
wakeEvents=wakeEvents@entry=3, sock=26, timeout=timeout@entry=-1,
     wait_event_info=wait_event_info@entry=117440512) at latch.c:385
#4  0x00007fe89a9dbc03 in pgfdw_get_result (conn=0x2880170, 
query=0x2348c90 "DELETE FROM importer.config") at connection.c:548
#5  0x00007fe89a9d5a9e in execute_dml_stmt (node=0x24d4710, 
node=0x24d4710) at postgres_fdw.c:3787
#6  postgresIterateDirectModify (node=0x24d4710) at postgres_fdw.c:2553
#7  0x000000000061fabc in ForeignNext (node=node@entry=0x24d4710) at 
nodeForeignscan.c:52
#8  0x00000000005fe2e5 in ExecScanFetch (recheckMtd=0x61f9f0 
<ForeignRecheck>, accessMtd=0x61fa80 <ForeignNext>, node=0x24d4710) at 
execScan.c:95
#9  ExecScan (node=0x24d4710, accessMtd=0x61fa80 <ForeignNext>, 
recheckMtd=0x61f9f0 <ForeignRecheck>) at execScan.c:162
#10 0x00000000006186bb in ExecProcNode (node=0x24d4710) at 
../../../src/include/executor/executor.h:247
#11 ExecModifyTable (pstate=0x24d3db0) at nodeModifyTable.c:2025
#12 0x00000000005f638a in ExecProcNode (node=0x24d3db0) at 
../../../src/include/executor/executor.h:247
#13 ExecutePlan (execute_once=<optimized out>, dest=0xccd060 
<spi_printtupDR>, direction=<optimized out>, numberTuples=0, 
sendTuples=false, operation=CMD_DELETE,
     use_parallel_mode=<optimized out>, planstate=0x24d3db0, 
estate=0x24d3850) at execMain.c:1723
#14 standard_ExecutorRun (queryDesc=0x2aa39d0, direction=<optimized 
out>, count=0, execute_once=<optimized out>) at execMain.c:364
#15 0x00007fe89b23ff85 in pgss_ExecutorRun (queryDesc=0x2aa39d0, 
direction=ForwardScanDirection, count=0, execute_once=<optimized out>) 
at pg_stat_statements.c:892
#16 0x0000000000626969 in _SPI_pquery (tcount=0, fire_triggers=true, 
queryDesc=0x2aa39d0) at spi.c:2501
#17 _SPI_execute_plan (plan=plan@entry=0x1e73820, 
paramLI=paramLI@entry=0x0, snapshot=snapshot@entry=0x0, 
crosscheck_snapshot=crosscheck_snapshot@entry=0x0,
     read_only=read_only@entry=false, 
fire_triggers=fire_triggers@entry=true, tcount=tcount@entry=0) at spi.c:2263
#18 0x0000000000626d68 in SPI_execute_plan_with_paramlist 
(plan=0x1e73820, params=0x0, read_only=<optimized out>, tcount=0) at 
spi.c:540
#19 0x00007fe80c6b16ef in exec_stmt_execsql 
(estate=estate@entry=0x7ffd7bfc18e0, stmt=stmt@entry=0x1e67470) at 
pl_exec.c:4115
#20 0x00007fe80c6b2deb in exec_stmt (stmt=0x1e67470, 
estate=0x7ffd7bfc18e0) at pl_exec.c:1986
#21 exec_stmts (estate=0x7ffd7bfc18e0, stmts=<optimized out>) at 
pl_exec.c:1877
#22 0x00007fe80c6b4f42 in exec_stmt_block 
(estate=estate@entry=0x7ffd7bfc18e0, block=0x1e70e20) at pl_exec.c:1818
#23 0x00007fe80c6b5690 in plpgsql_exec_trigger 
(func=func@entry=0x1cb2b98, trigdata=0x7ffd7bfc20f0) at pl_exec.c:958
#24 0x00007fe80c6a9382 in plpgsql_call_handler (fcinfo=0x7ffd7bfc1c70) 
at pl_handler.c:254
#25 0x00000000005d28c8 in ExecCallTriggerFunc 
(trigdata=trigdata@entry=0x7ffd7bfc20f0, tgindx=tgindx@entry=1, 
finfo=0x2bac7c0, finfo@entry=0x2bac790,
     instr=instr@entry=0x0, 
per_tuple_context=per_tuple_context@entry=0x27e73a0) at trigger.c:2412
#26 0x00000000005d3f1d in AfterTriggerExecute (trigdesc=0x2bac5d0, 
trigdesc=0x2bac5d0, trig_tuple_slot2=0x0, trig_tuple_slot1=0x0, 
per_tuple_context=0x27e73a0, instr=0x0,
     finfo=0x2bac790, rel=0x1d1f5f8, event=0x2a6127c) at trigger.c:4359
#27 afterTriggerInvokeEvents (events=events@entry=0x2c28790, 
firing_id=1, estate=estate@entry=0x2bac280, 
delete_ok=delete_ok@entry=false) at trigger.c:4550
#28 0x00000000005d9397 in AfterTriggerEndQuery 
(estate=estate@entry=0x2bac280) at trigger.c:4860
#29 0x00000000005f6609 in standard_ExecutorFinish (queryDesc=0x2c2f6a0)
at execMain.c:439
#30 0x00007fe89b23fe95 in pgss_ExecutorFinish (queryDesc=0x2c2f6a0) at 
pg_stat_statements.c:915
#31 0x000000000072e6e8 in ProcessQuery (plan=<optimized out>,
     sourceText=0x2c2f610 "UPDATE importer.config SET value='f' WHERE 
param='active' AND hostname = 'dc-int' AND filetype='ssa5'", params=0x0, 
queryEnv=0x0,
     dest=0xccd120 <donothingDR>, completionTag=0x7ffd7bfc2600 "UPDATE 
1") at pquery.c:205
#32 0x000000000072e8c2 in PortalRunMulti (portal=portal@entry=0x1d0f6a0, 
isTopLevel=isTopLevel@entry=true, 
setHoldSnapshot=setHoldSnapshot@entry=false,
     dest=0xccd120 <donothingDR>, dest@entry=0x1c88a70, altdest=0xccd120 
<donothingDR>, altdest@entry=0x1c88a70,
     completionTag=completionTag@entry=0x7ffd7bfc2600 "UPDATE 1") at 
pquery.c:1286
#33 0x000000000072f38d in PortalRun (portal=portal@entry=0x1d0f6a0, 
count=count@entry=1, isTopLevel=isTopLevel@entry=true, 
run_once=<optimized out>,
     dest=dest@entry=0x1c88a70, altdest=altdest@entry=0x1c88a70, 
completionTag=completionTag@entry=0x7ffd7bfc2600 "UPDATE 1") at pquery.c:799
#34 0x000000000072ce8d in exec_execute_message (max_rows=1, 
portal_name=0x1c88660 "") at postgres.c:2036
#35 PostgresMain (argc=<optimized out>, argv=argv@entry=0x1cb1358, 
dbname=0x1cb1148 "voip", username=<optimized out>) at postgres.c:4245
#36 0x000000000047e313 in BackendRun (port=0x1caedd0) at postmaster.c:4358
#37 BackendStartup (port=0x1caedd0) at postmaster.c:4030
#38 ServerLoop () at postmaster.c:1707

*** Remote server ***


postgres=# select 

pid,wait_event_type,wait_event,datid,datname,backend_start,xact_start,query_start,state_change,backend_xid,backend_xmin,backend_type,substring(query,1,50)

from pg_stat_activity where state <> 'idle';
   pid  | wait_event_type |  wait_event   | datid |    datname |         
backend_start         |          xact_start |          
query_start          |         s
tate_change          | backend_xid | backend_xmin |  backend_type 
|                     substring

-------+-----------------+---------------+-------+---------------+-------------------------------+-------------------------------+-------------------------------+----------

---------------------+-------------+--------------+----------------+----------------------------------------------------
  25536 | Lock            | transactionid | 16401 | datacollector | 
2019-06-11 12:24:46.567576+02 | 2019-06-11 12:24:46.607314+02 | 
2019-06-11 12:24:46.615009+02 | 2019-06-11 12:24:46.61501+02 |     
4292464 |      4292463 | client backend | DELETE FROM importer.config
  10183 | Client          | ClientRead    | 16401 | datacollector | 
2019-06-11 11:58:09.340121+02 | 2019-06-11 12:24:45.870476+02 | 
2019-06-11 12:24:46.711237+02 | 2019-06-11 12:24:46.711292+02 |     
4292463 |      4292463 | client backend | DEALLOCATE pgsql_fdw_prep_178
(3 rows)

postgres=# select * from pg_locks where pid=25536 and granted=false;
    locktype    | database | relation | page | tuple | virtualxid | 
transactionid | classid | objid | objsubid | virtualtransaction | pid  
|   mode    | granted | fastpath

---------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-------+-----------+---------+----------
  transactionid |          |          |      |       | |       4292463 
|         |       |          | 52/155384 | 25536 | ShareLock | f       | f
(1 row)

postgres=# select * from pg_locks where transactionid=4292463;
    locktype    | database | relation | page | tuple | virtualxid | 
transactionid | classid | objid | objsubid | virtualtransaction | pid  
|     mode      | granted | fastp
ath

---------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-------+---------------+---------+------
----
  transactionid |          |          |      |       | |       4292463 
|         |       |          | 52/155384 | 25536 | ShareLock     | 
f       | f
  transactionid |          |          |      |       | |       4292463 
|         |       |          | 56/28816 | 10183 | ExclusiveLock | 
t       | f
(2 rows)


postgres=# select * from pg_locks where pid=10183;
    locktype    | database | relation | page | tuple | virtualxid | 
transactionid | classid | objid | objsubid | virtualtransaction | pid  
|       mode       | granted | fa
stpath

---------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-------+------------------+---------+---
-------
  relation      |    16401 |    16874 |      |       | |               
|         |       |          | 56/28816 | 10183 | RowExclusiveLock | 
t       | t
  virtualxid    |          |          |      |       | 56/28816 
|               |         |       |          | 56/28816 | 10183 | 
ExclusiveLock    | t       | t
  transactionid |          |          |      |       | |       4292463 
|         |       |          | 56/28816 | 10183 | ExclusiveLock    | 
t       | f
  relation      |    16401 |    16867 |      |       | |               
|         |       |          | 56/28816 | 10183 | RowExclusiveLock | 
t       | f
(4 rows)



(gdb) bt
#0  0x00007f71bb99d463 in __epoll_wait_nocancel () from /lib64/libc.so.6
#1  0x000000000070c81e in WaitEventSetWaitBlock (nevents=1, 
occurred_events=0x7ffdde17a910, cur_timeout=-1, set=0x22b50e8) at 
latch.c:1048
#2  WaitEventSetWait (set=0x22b50e8, timeout=timeout@entry=-1, 
occurred_events=occurred_events@entry=0x7ffdde17a910, 
nevents=nevents@entry=1,
     wait_event_info=wait_event_info@entry=100663296) at latch.c:1000
#3  0x000000000062d4d3 in secure_read (port=0x22b2ed0, ptr=0xcda640 
<PqRecvBuffer>, len=8192) at be-secure.c:173
#4  0x00000000006364b8 in pq_recvbuf () at pqcomm.c:963
#5  0x0000000000637085 in pq_getbyte () at pqcomm.c:1006
#6  0x000000000072c4e2 in SocketBackend (inBuf=0x7ffdde17aa80) at 
postgres.c:341
#7  ReadCommand (inBuf=0x7ffdde17aa80) at postgres.c:514
#8  PostgresMain (argc=<optimized out>, argv=argv@entry=0x22b5348, 
dbname=0x22b5218 "datacollector", username=<optimized out>) at 
postgres.c:4124
#9  0x000000000047e313 in BackendRun (port=0x22b2ed0) at postmaster.c:4358
#10 BackendStartup (port=0x22b2ed0) at postmaster.c:4030
#11 ServerLoop () at postmaster.c:1707
#12 0x00000000006c3719 in PostmasterMain (argc=argc@entry=5, 
argv=argv@entry=0x22842d0) at postmaster.c:1380
#13 0x000000000047ed8b in main (argc=5, argv=0x22842d0) at main.c:228

Any advice on how to debug this further ?

Regards,
Tobias