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.kalinin@postgrespro.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.linnakangas@iki.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@entry=0x3e5ba110,
    values=values@entry=0x7ffd0dc219c0, isnull=isnull@entry=0x7ffd0dc219b4)
at heaptuple.c:239
#2  0x00000000004b3bff in heap_form_minimal_tuple
(tupleDescriptor=0x3e5ba110,
    values=values@entry=0x7ffd0dc219c0, isnull=isnull@entry=0x7ffd0dc219b4,
extra=extra@entry=0)
    at heaptuple.c:1434
#3  0x0000000000a6fa09 in tuplestore_putvalues (state=0x3e5cc0d8,
tdesc=<optimized out>,
    values=values@entry=0x7ffd0dc219c0, isnull=isnull@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@entry=0x3e52f130,
    forward=forward@entry=true, count=0, count@entry=9223372036854775807,
    dest=dest@entry=0x3e5e11f8) at pquery.c:917
#13 0x00000000008c78be in PortalRun (portal=portal@entry=0x3e52f130,
    count=count@entry=9223372036854775807, isTopLevel=isTopLevel@entry=true,
    dest=dest@entry=0x3e5e11f8, altdest=altdest@entry=0x3e5e11f8,
qc=qc@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@entry=0x7ffd0dc22270,
startup_data_len=startup_data_len@entry=24,
    client_sock=client_sock@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@entry=3,
argv=argv@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.