BUG #17344: Assert failed on queiring async_capable foreign table with inheritance

The following bug has been logged on the website:

Bug reference:      17344
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 14.1
Operating system:   Ubuntu 20.04
Description:

When executing the following query:
create extension postgres_fdw;
do $d$
    begin
        execute $$create server loopback foreign data wrapper postgres_fdw
            options (dbname '$$||current_database()||$$',
                     port '$$||current_setting('port')||$$',
                     async_capable 'true'
            )$$;
    end;
$d$;
create user mapping for current_user server loopback;

create table tab1 (a int);
insert into tab1 values(1), (2), (3);
create foreign table ftab1 (b int) server loopback options (table_name
'tab1');

create foreign table ftab2 () inherits (ftab1) server loopback;
select a from tab1 where a in (select a from ftab1);

I get an assertion failed with the stacktrace:
Core was generated by `postgres: law regression [local] SELECT
                        '.
Program terminated with signal SIGABRT, Aborted.
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007f2a2f9fa859 in __GI_abort () at abort.c:79
#2  0x0000557dece3c5d8 in ExceptionalCondition
(conditionName=conditionName@entry=0x7f2a225a8da7 "areq != pendingAreq", 
    errorType=errorType@entry=0x7f2a225a76aa "FailedAssertion",
fileName=0x7ffde2262540 "\271\305\343\354}U", 
    fileName@entry=0x7f2a225a88af "postgres_fdw.c",
lineNumber=lineNumber@entry=6974) at assert.c:69
#3  0x00007f2a225a4e34 in produce_tuple_asynchronously (areq=<optimized
out>, fetch=<optimized out>)
    at postgres_fdw.c:6974
#4  0x0000557decb6e152 in ExecAsyncRequest (areq=0x557ded812670) at
execAsync.c:38
#5  0x0000557decb94e4e in ExecAppendAsyncBegin (node=0x557ded7f2040) at
nodeAppend.c:907
#6  ExecAppend (pstate=0x557ded7f2040) at nodeAppend.c:317
#7  0x0000557decbb0183 in ExecProcNode (node=0x557ded7f2040) at
../../../src/include/executor/executor.h:257
#8  ExecResult (pstate=0x557ded7f1e38) at nodeResult.c:115
#9  0x0000557decbb263d in ExecProcNode (node=0x557ded7f1e38) at
../../../src/include/executor/executor.h:257
#10 ExecScanSubPlan (isNull=0x557ded81ba35, econtext=0x557ded81b7e8,
node=0x557ded81c2e0) at nodeSubplan.c:323
#11 ExecSubPlan (node=node@entry=0x557ded81c2e0,
econtext=econtext@entry=0x557ded81b7e8, isNull=0x557ded81ba35)
    at nodeSubplan.c:89
#12 0x0000557decb77beb in ExecEvalSubPlan (econtext=0x557ded81b7e8,
op=<optimized out>, state=0x557ded81ba30)
    at execExprInterp.c:3932
#13 ExecInterpExpr (state=0x557ded81ba30, econtext=0x557ded81b7e8,
isnull=<optimized out>) at execExprInterp.c:1564
#14 0x0000557decb865d5 in ExecEvalExprSwitchContext (isNull=0x7ffde2262ad7,
econtext=0x557ded81b7e8, 
    state=0x557ded81ba30) at ../../../src/include/executor/executor.h:339
#15 ExecQual (econtext=0x557ded81b7e8, state=0x557ded81ba30) at
../../../src/include/executor/executor.h:408
#16 ExecScan (node=0x557ded7f3748, accessMtd=0x557decbb0ad0 <SeqNext>,
recheckMtd=0x557decbb0ac0 <SeqRecheck>)
    at execScan.c:227
#17 0x0000557decb7cc7a in ExecProcNode (node=0x557ded7f3748) at
../../../src/include/executor/executor.h:257
#18 ExecutePlan (execute_once=<optimized out>, dest=0x557ded80a428,
direction=<optimized out>, numberTuples=0, 
    sendTuples=<optimized out>, operation=CMD_SELECT,
use_parallel_mode=<optimized out>, planstate=0x557ded7f3748, 
    estate=0x557ded7f1b90) at execMain.c:1551
#19 standard_ExecutorRun (queryDesc=0x557ded800c20, direction=<optimized
out>, count=0, execute_once=<optimized out>)
    at execMain.c:361
#20 0x0000557decd18b9c in PortalRunSelect (portal=0x557ded756860,
forward=<optimized out>, count=0, 
    dest=<optimized out>) at pquery.c:921
#21 0x0000557decd1a2cb in PortalRun (portal=portal@entry=0x557ded756860,
count=count@entry=9223372036854775807, 
    isTopLevel=isTopLevel@entry=true, run_once=run_once@entry=true,
dest=dest@entry=0x557ded80a428, 
    altdest=altdest@entry=0x557ded80a428, qc=0x7ffde2262d80) at
pquery.c:765
#22 0x0000557decd1614b in exec_simple_query (
    query_string=0x557ded6e7960 "select a from tab1 where a in (select a
from ftab1);") at postgres.c:1214
#23 0x0000557decd17d51 in PostgresMain (argc=argc@entry=1,
argv=argv@entry=0x7ffde22631f0, dbname=<optimized out>, 
    username=<optimized out>) at postgres.c:4486
#24 0x0000557decc83c8d in BackendRun (port=0x557ded70b3d0,
port=0x557ded70b3d0) at postmaster.c:4530
#25 BackendStartup (port=0x557ded70b3d0) at postmaster.c:4252
#26 ServerLoop () at postmaster.c:1745
#27 0x0000557decc84c31 in PostmasterMain (argc=<optimized out>,
argv=<optimized out>) at postmaster.c:1417
#28 0x0000557dec9b24b2 in main (argc=3, argv=0x557ded6e1990) at main.c:209

With async_capable = 'false' I get expected results.
But when ftab1 defined as
create foreign table ftab1 (a int) server loopback options (table_name
'tab1');
The select fails with:
psql:query.sql:19: ERROR:  could not connect to server "loopback"
DETAIL:  connection to server on socket "/tmp/.s.PGSQL.5432" failed: FATAL:
sorry, too many clients already
CONTEXT:  remote SQL command: SELECT a FROM public.ftab2
remote SQL command: SELECT a FROM public.ftab2
remote SQL command: SELECT a FROM public.ftab2
remote SQL command: SELECT a FROM public.ftab2
...
with or without async_capable.