From: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
To: "'pgsql-hackers@lists.postgresql.org'"
	<pgsql-hackers@lists.postgresql.org>
Subject: ReplicationSlotRelease() crashes when the instance is in the single
 user mode
Thread-Topic: ReplicationSlotRelease() crashes when the instance is in the
 single user mode
Thread-Index: AduBxJdn64KPa5CETf+apI8CWWnvvA==
Date: Tue, 18 Feb 2025 05:33:05 +0000
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Precedence: bulk

Dear hackers,

I found $SUBJECT when I'm playing with the single user mode.

How to reproduce
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
You can reproduce the failure with below steps.

```
# Initialize an instance
$ initdb -D data -U postgres
# Start it as single user mode
$ postgres --single -D data/ postgres=20

PostgreSQL stand-alone backend 18devel
backend> SELECT pg_create_physical_replication_slot(slot_name :=3D 'physica=
l_slot', immediately_reserve :=3D true);
...
backend> SELECT pg_replication_slot_advance('physical_slot', pg_current_wal=
_lsn());
         1: pg_replication_slot_advance (typeid =3D 2249, len =3D -1, typmo=
d =3D -1, byval =3D f)
        ----
TRAP: failed Assert("slot !=3D NULL && (slot->active_pid !=3D 0)"), File: "=
../postgres/src/backend/replication/slot.c", Line: 674, PID: 430860
postgres(ExceptionalCondition+0xab)[0xb86a2a]
postgres(ReplicationSlotRelease+0x5a)[0x8df10b]
postgres(pg_replication_slot_advance+0x330)[0x8e46ed]
...
```

Analysis
=3D=3D=3D=3D=3D
We trapped at below assertion in ReplicationSlotRelease(). IIUC, `slot->act=
ive_pid` is set
only when the process is under the postmaster, but ReplicationSlotRelease()=
 always requires it.

```
	Assert(slot !=3D NULL && slot->active_pid !=3D 0);
```

Possible fix
=3D=3D=3D=3D=3D=3D=3D

Naively considered, there are two approaches to fix this. 1) set active_pid=
 when even in the single
user mode [1], or 2) ease the condition to accept the situation [2]. I'm no=
t familiar with the mode,
but [1] seems better if we want to unify codes.

Thought?

[1]:
```
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -599,7 +599,7 @@ retry:
                SpinLockRelease(&s->mutex);
        }
        else
-               active_pid =3D MyProcPid;
+               s->active_pid =3D active_pid =3D MyProcPid;
        LWLockRelease(ReplicationSlotControlLock);
=20
        /*
```
[2]:
```
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -671,7 +671,8 @@ ReplicationSlotRelease(void)
        bool            is_logical =3D false; /* keep compiler quiet */
        TimestampTz now =3D 0;
=20
-       Assert(slot !=3D NULL && slot->active_pid !=3D 0);
+       Assert(slot !=3D NULL &&
+                  (slot->active_pid !=3D 0 || !IsUnderPostmaster));
=20
        if (am_walsender)
        {
```

Best regards,
Hayato Kuroda
FUJITSU LIMITED