"ERROR: cache lookup failed for function %d" while dropping function from another session

Hi,
I am able to hit "ERROR: cache lookup failed for function %d" while I am
dropping function from another session.

*Reproducible steps are as(I tried on current head d8a75b1308b133502ae3):*

*Step 1) Fire below query from session-1 to create a function:*
create function fun1(int) returns record as ' DECLARE retval RECORD;
BEGIN SELECT
INTO retval 5, 10, 15; RETURN retval;
END;' language plpgsql;

*Step 2)* Attach PID of session-1 into gdb and set a breakpoint into
internal_get_result_type and then press c.
(gdb) b internal_get_result_type Breakpoint 1 at 0x5565dd4f3ce4: file
funcapi.c, line 325. (gdb) c
Continuing.

*Step 3)* Fire below select command from session-1:
SELECT * FROM fun1(1500) AS (a int, b int, c int);

*Step 3) Open session-2 and drop the function:*
psql (15devel) Type "help" for help. postgres=# drop function fun1; DROP
FUNCTION
postgres=#

*Step 4)* Press 'c' in gdb for 2 times and we will get an error in the 1st
session.
(gdb) c Continuing. Breakpoint 1, internal_get_result_type (funcid=16384,
call_expr=0x5565de885af0, rsinfo=0x0, resultTypeId=0x7ffc94572608,
resultTupleDesc=0x7ffc94572628) at funcapi.c:325 (gdb) c
Continuing.

postgres=# SELECT * FROM fun1(1500) AS (a int, b int, c int);
ERROR: cache lookup failed for function 16384
*Call stack of error:*

> Breakpoint 2, errfinish (filename=0x55e0b8907c6f "funcapi.c", lineno=336,
> funcname=0x55e0b89083a0 <__func__.23015> "internal_get_result_type") at
> elog.c:515
> 515 ErrorData *edata = &errordata[errordata_stack_depth];
> (gdb) bt
> #0 errfinish (filename=0x55e0b8907c6f "funcapi.c", lineno=336,
> funcname=0x55e0b89083a0 <__func__.23015> "internal_get_result_type") at
> elog.c:515
> #1 0x000055e0b86abdaf in internal_get_result_type (funcid=16384,
> call_expr=0x55e0ba9e2ad0, rsinfo=0x0, resultTypeId=0x7ffcb7ca4918,
> resultTupleDesc=0x7ffcb7ca4938)
> at funcapi.c:336
> #2 0x000055e0b86ab6d0 in get_expr_result_type (expr=0x55e0ba9e2ad0,
> resultTypeId=0x7ffcb7ca4918, resultTupleDesc=0x7ffcb7ca4938) at
> funcapi.c:230
> #3 0x000055e0b7fef027 in ExecInitFunctionScan (node=0x55e0ba9e3cd0,
> estate=0x55e0ba9d2fe0, eflags=16) at nodeFunctionscan.c:370
> #4 0x000055e0b7fbb408 in ExecInitNode (node=0x55e0ba9e3cd0,
> estate=0x55e0ba9d2fe0, eflags=16) at execProcnode.c:254
> #5 0x000055e0b7faa1d5 in InitPlan (queryDesc=0x55e0ba926f50, eflags=16)
> at execMain.c:936
> #6 0x000055e0b7fa84b6 in standard_ExecutorStart
> (queryDesc=0x55e0ba926f50, eflags=16) at execMain.c:263
> #7 0x000055e0b7fa7fc7 in ExecutorStart (queryDesc=0x55e0ba926f50,
> eflags=0) at execMain.c:143
> #8 0x000055e0b83ba8de in PortalStart (portal=0x55e0ba968870, params=0x0,
> eflags=0, snapshot=0x0) at pquery.c:512
> #9 0x000055e0b83ae112 in exec_simple_query (query_string=0x55e0ba906760
> "SELECT * FROM fun1(1500) AS (a int, b int, c int);") at postgres.c:1175
> #10 0x000055e0b83b7a86 in PostgresMain (argc=1, argv=0x7ffcb7ca4f70,
> dbname=0x55e0ba932770 "postgres", username=0x55e0ba932748 "mahendrathalor")
> at postgres.c:4488
> #11 0x000055e0b825161b in BackendRun (port=0x55e0ba929730) at
> postmaster.c:4519
> #12 0x000055e0b8250971 in BackendStartup (port=0x55e0ba929730) at
> postmaster.c:4241
> #13 0x000055e0b8248a1b in ServerLoop () at postmaster.c:1758
> #14 0x000055e0b8247c18 in PostmasterMain (argc=3, argv=0x55e0ba8fee70) at
> postmaster.c:1430
> #15 0x000055e0b808dbe9 in main (argc=3, argv=0x55e0ba8fee70) at main.c:199
>

cache lookup failed errors are never an expected behavior(Thanks Robert
Hass for your opinion) so I think we should fix this error.

I haven't debugged it so I will debug it and will post my findings in the
coming days.

--
Thanks and Regards
Mahendra Singh Thalor
EnterpriseDB: http://www.enterprisedb.com