BUG #19508: pg_buffercache_pages() crashes the backend with an incompatible caller-supplied record definition

The following bug has been logged on the website:

Bug reference: 19508
Logged by: Nikita Kalinin
Email address: n(dot)kalinin(at)postgrespro(dot)ru
PostgreSQL version: 19beta1
Operating system: Fedora 44
Description:

Hello,

It appears that pg_buffercache_pages() trusts a caller-supplied record
descriptor without verifying that the declared column types match the actual
values returned by the function.

The crash is reproducible on the current master branch with a fresh cluster
after installing the extension:

CREATE EXTENSION pg_buffercache;

SELECT *
FROM pg_buffercache_pages() AS p(
bufferid integer,
relfilenode oid,
reltablespace oid,
reldatabase oid,
relforknumber smallint,
relblocknumber bigint,
isdirty text,
usagecount smallint
)
LIMIT 1;

postgres=# select version();
version
-------------------------------------------------------------------------------------------------------------
PostgreSQL 19beta1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 16.1.1
20260515 (Red Hat 16.1.1-2), 64-bit
(1 row)

The only difference from the types documented for pg_buffercache_pages() is
that isdirty is declared as text instead of boolean.

git blame points to the following commit:

commit 257c8231bf97a77378f6fedb826b1243f0a41612 (HEAD)
Author: Heikki Linnakangas <heikki(dot)linnakangas(at)iki(dot)fi>
Date: Tue Apr 7 16:04:48 2026 +0300

Modernize and optimize pg_buffercache_pages()

Backtrace:

#0 0x00000000004b2565 in VARATT_CAN_MAKE_SHORT (PTR=<optimized out>)
at ../../../../src/include/varatt.h:419
#1 heap_compute_data_size (tupleDesc=tupleDesc(at)entry=0x3e5ba110,
values=values(at)entry=0x7ffd0dc219c0, isnull=isnull(at)entry=0x7ffd0dc219b4)
at heaptuple.c:239
#2 0x00000000004b3bff in heap_form_minimal_tuple
(tupleDescriptor=0x3e5ba110,
values=values(at)entry=0x7ffd0dc219c0, isnull=isnull(at)entry=0x7ffd0dc219b4,
extra=extra(at)entry=0)
at heaptuple.c:1434
#3 0x0000000000a6fa09 in tuplestore_putvalues (state=0x3e5cc0d8,
tdesc=<optimized out>,
values=values(at)entry=0x7ffd0dc219c0, isnull=isnull(at)entry=0x7ffd0dc219b4)
at tuplestore.c:791
#4 0x00007f180fa0447e in pg_buffercache_pages (fcinfo=<optimized out>)
at pg_buffercache_pages.c:202
#5 0x00000000006b7e35 in ExecMakeTableFunctionResult (setexpr=0x3e5b9e98,
econtext=0x3e5b9d38,
argContext=<optimized out>, expectedDesc=0x3e5ba110, randomAccess=false)
at execSRF.c:235
#6 0x00000000006ccc57 in FunctionNext (node=0x3e5b9b28) at
nodeFunctionscan.c:95
#7 0x00000000006daf22 in ExecProcNode (node=0x3e5b9b28)
at ../../../src/include/executor/executor.h:327
#8 ExecLimit (pstate=0x3e5b97b8) at nodeLimit.c:95
#9 0x00000000006ac39a in ExecProcNode (node=0x3e5b97b8)
at ../../../src/include/executor/executor.h:327
#10 ExecutePlan (queryDesc=0x3e5d7d80, operation=CMD_SELECT,
sendTuples=true, numberTuples=0,
direction=<optimized out>, dest=0x3e5e11f8) at execMain.c:1736
#11 standard_ExecutorRun (queryDesc=0x3e5d7d80, direction=<optimized out>,
count=0)
at execMain.c:377
#12 0x00000000008c61d8 in PortalRunSelect (portal=portal(at)entry=0x3e52f130,
forward=forward(at)entry=true, count=0, count(at)entry=9223372036854775807,
dest=dest(at)entry=0x3e5e11f8) at pquery.c:917
#13 0x00000000008c78be in PortalRun (portal=portal(at)entry=0x3e52f130,
count=count(at)entry=9223372036854775807, isTopLevel=isTopLevel(at)entry=true,
dest=dest(at)entry=0x3e5e11f8, altdest=altdest(at)entry=0x3e5e11f8,
qc=qc(at)entry=0x7ffd0dc21e20)
at pquery.c:761
#14 0x00000000008c3548 in exec_simple_query (
query_string=0x3e48b800 "SELECT *\nFROM pg_buffercache_pages() AS p(\n
bufferid integer,\n relfilenode oid,\n reltablespace oid,\n reldatabase
oid,\n relforknumber smallint,\n relblocknumber bigint,\n isdirty text,\n
usagecoun"...) at postgres.c:1290
#15 0x00000000008c5021 in PostgresMain (dbname=<optimized out>,
username=<optimized out>)
at postgres.c:4856
#16 0x00000000008bf01d in BackendMain (startup_data=<optimized out>,
startup_data_len=<optimized out>) at backend_startup.c:124
#17 0x00000000007fefae in postmaster_child_launch (child_type=<optimized
out>, child_slot=1,
startup_data=startup_data(at)entry=0x7ffd0dc22270,
startup_data_len=startup_data_len(at)entry=24,
client_sock=client_sock(at)entry=0x7ffd0dc22290) at launch_backend.c:268
#18 0x00000000008029b6 in BackendStartup (client_sock=0x7ffd0dc22290) at
postmaster.c:3627
#19 ServerLoop () at postmaster.c:1728
#20 0x0000000000804479 in PostmasterMain (argc=argc(at)entry=3,
argv=argv(at)entry=0x3e434fe0)
at postmaster.c:1415
#21 0x00000000004a1c18 in main (argc=3, argv=0x3e434fe0) at main.c:231

For comparison, the same query executed on REL_18_STABLE is rejected with a
regular error:

ERROR: function return row and query-specified return row do not match
DETAIL: Returned type boolean at ordinal position 7, but query expects
text.