Dear hackers,
I found $SUBJECT when I'm playing with the single user mode.
How to reproduce
===========
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
PostgreSQL stand-alone backend 18devel
backend> SELECT pg_create_physical_replication_slot(slot_name := 'physical_slot', immediately_reserve := true);
...
backend> SELECT pg_replication_slot_advance('physical_slot', pg_current_wal_lsn());
1: pg_replication_slot_advance (typeid = 2249, len = -1, typmod = -1, byval = f)
----
TRAP: failed Assert("slot != NULL && (slot->active_pid != 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
=====
We trapped at below assertion in ReplicationSlotRelease(). IIUC, `slot->active_pid` is set
only when the process is under the postmaster, but ReplicationSlotRelease() always requires it.
```
Assert(slot != NULL && slot->active_pid != 0);
```
Possible fix
=======
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 not 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 = MyProcPid;
+ s->active_pid = active_pid = MyProcPid;
LWLockRelease(ReplicationSlotControlLock);
/*
```
[2]:
```
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -671,7 +671,8 @@ ReplicationSlotRelease(void)
bool is_logical = false; /* keep compiler quiet */
TimestampTz now = 0;
- Assert(slot != NULL && slot->active_pid != 0);
+ Assert(slot != NULL &&
+ (slot->active_pid != 0 || !IsUnderPostmaster));
if (am_walsender)
{
```
Best regards,
Hayato Kuroda
FUJITSU LIMITED
