pg_dump (COPY) hanging intermittently

Hi,

I am running into a strange issue with Postgres 10 when using pg_dump with the directory format and jobs parameter set it intermittently hangs.  Seems to occur less frequently the lower I set the jobs parameter but does happen eventually even when set to 2 (could not reproduce when jobs=1).  I've tested with Postgres 11 and no matter how high I set the jobs parameter it never hangs (tested all the way up to jobs=8).  Also with Postgres 10 and other formats I don't run into the issue.  Here are my server specs and software versions (OS and Postgres)

Windows Server 2016 DataCenter  10.0.14393
2 cores  4 logical processors
26GB memory

Postgres 11.4 (port 5433)
Postgres 10.9 (port 5432)

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And here are the psql statements and powershell script to recreate the issue.

CREATE DATABASE pg_dump_hang;
\c pg_dump_hang
CREATE TABLE series1 AS SELECT i FROM generate_series(1,1000000) i;
CREATE TABLE series2 AS SELECT i FROM generate_series(1,1000000) i;
CREATE TABLE series3 AS SELECT i FROM generate_series(1,1000000) i;
CREATE TABLE series4 AS SELECT i FROM generate_series(1,1000000) i;
CREATE TABLE series5 AS SELECT i FROM generate_series(1,1000000) i;
CREATE TABLE series6 AS SELECT i FROM generate_series(1,1000000) i;
CREATE TABLE series7 AS SELECT i FROM generate_series(1,1000000) i;
CREATE TABLE series8 AS SELECT i FROM generate_series(1,1000000) i;


$env:PGPASSWORD='#######'
$env:PGUSER='postgres'
$env:PGPORT=5433
set-location "C:\program files\postgresql\11\bin"
$a = 1
DO
{
   remove-item -Recurse D:\dumphangtest
   .\pg_dump.exe --format=directory --jobs=4 --file=D:\dumphangtest pg_dump_hang
   $a++
   "Completed loop $a at -" + (date)
} While ($a -le 25)

$env:PGPASSWORD='#######'
$env:PGUSER='postgres'
$env:PGPORT=5432
set-location "c:\program files\postgresql\10\bin"
$a = 1
DO
{
   remove-item -Recurse D:\dumphangtest
   .\pg_dump.exe --format=directory --jobs=4 --file=D:\dumphangtest pg_dump_hang
   $a++
   "Completed loop $a at -" + (date)
} While ($a -le 25)

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Here is the output from pg_stat_activity and the filesystem listing when the hang occurs.

--jobs=4
postgres=# select pid,state,state_change,query_start,wait_event,wait_event_type,substring(query,1,35) from pg_stat_activity where datname like 'pg_dump%';
 pid  |        state        |         state_change          |          query_start          | wait_event  | wait_event_type |              substring
------+---------------------+-------------------------------+-------------------------------+-------------+-----------------+-------------------------------------
 5324 | idle in transaction | 2019-06-27 15:29:13.03416+01  | 2019-06-27 15:29:13.03154+01  | ClientRead  | Client          | SELECT at.attname, (SELECT pg_catal
 4144 | idle in transaction | 2019-06-27 15:29:16.222315+01 | 2019-06-27 15:29:15.070106+01 | ClientRead  | Client          | COPY public.series6 (i) TO stdout;
 5576 | active              | 2019-06-27 15:29:13.642069+01 | 2019-06-27 15:29:13.642068+01 | ClientWrite | Client          | COPY public.series2 (i) TO stdout;
 3860 | idle in transaction | 2019-06-27 15:29:17.109549+01 | 2019-06-27 15:29:16.198842+01 | ClientRead  | Client          | COPY public.series8 (i) TO stdout;
 3120 | idle in transaction | 2019-06-27 15:29:16.350121+01 | 2019-06-27 15:29:15.089817+01 | ClientRead  | Client          | COPY public.series7 (i) TO stdout;
(5 rows)

D:\>dir dumphangtest
 Volume in drive D has no label.
 Volume Serial Number is C635-CDEF

 Directory of D:\dumphangtest

06/27/2019  10:29 AM    <DIR>          .
06/27/2019  10:29 AM    <DIR>          ..
06/27/2019  10:29 AM         2,114,899 2863.dat.gz
06/27/2019  10:29 AM           376,832 2864.dat.gz
06/27/2019  10:29 AM         2,114,899 2865.dat.gz
06/27/2019  10:29 AM         2,114,899 2866.dat.gz
06/27/2019  10:29 AM         2,114,899 2867.dat.gz
06/27/2019  10:29 AM         2,114,899 2868.dat.gz
06/27/2019  10:29 AM         2,114,899 2869.dat.gz
06/27/2019  10:29 AM         2,114,899 2870.dat.gz
06/27/2019  10:29 AM             8,452 toc.dat
               9 File(s)     15,189,577 bytes
               2 Dir(s)  102,021,439,488 bytes free

--jobs=2
postgres=# select pid,state,state_change,query_start,wait_event,wait_event_type,substring(query,1,35) from pg_stat_activity where datname like 'pg_dump%';
 pid  |        state        |         state_change          |          query_start          | wait_event  | wait_event_type |              substring
------+---------------------+-------------------------------+-------------------------------+-------------+-----------------+-------------------------------------
 3420 | idle in transaction | 2019-06-27 14:37:55.067559+01 | 2019-06-27 14:37:54.448198+01 | ClientRead  | Client          | COPY public.series8 (i) TO stdout;
 3180 | idle in transaction | 2019-06-27 14:37:49.537396+01 | 2019-06-27 14:37:49.53608+01  | ClientRead  | Client          | SELECT at.attname, (SELECT pg_catal
 5364 | active              | 2019-06-27 14:37:50.151002+01 | 2019-06-27 14:37:50.151001+01 | ClientWrite | Client          | COPY public.series2 (i) TO stdout;
(3 rows)

D:\>dir dumphangtest
 Volume in drive D has no label.
 Volume Serial Number is C635-CDEF

 Directory of D:\dumphangtest

06/27/2019  09:37 AM    <DIR>          .
06/27/2019  09:37 AM    <DIR>          ..
06/27/2019  09:37 AM         2,114,899 2863.dat.gz
06/27/2019  09:37 AM            81,920 2864.dat.gz
06/27/2019  09:37 AM         2,114,899 2865.dat.gz
06/27/2019  09:37 AM         2,114,899 2866.dat.gz
06/27/2019  09:37 AM         2,114,899 2867.dat.gz
06/27/2019  09:37 AM         2,114,899 2868.dat.gz
06/27/2019  09:37 AM         2,114,899 2869.dat.gz
06/27/2019  09:37 AM         2,114,899 2870.dat.gz
06/27/2019  09:37 AM             8,452 toc.dat
               9 File(s)     14,894,665 bytes
               2 Dir(s)  102,056,497,152 bytes free

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