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From: Dilip Kumar <dilipbalaut@gmail.com>
Date: Wed, 25 Jun 2025 11:47:46 +0530
Message-ID: 
 <CAFiTN-uGkrCDi9kqXJqLB+3ATn_ZULmRER73VhkLSDSAnO8SNg@mail.gmail.com>
Subject: Re: Slot's restart_lsn may point to removed WAL segment after hard
 restart unexpectedly
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Alexander Korotkov <aekorotkov@gmail.com>,
	"Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>,
 Amit Kapila <amit.kapila16@gmail.com>,
	Vitaly Davydov <v.davydov@postgrespro.ru>, Tom Lane <tgl@sss.pgh.pa.us>,
	Alexander Lakhin <exclusion@gmail.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>,
	Masahiko Sawada <sawada.mshk@gmail.com>, "tomas@vondra.me" <tomas@vondra.me>,
 vignesh C <vignesh21@gmail.com>
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On Wed, Jun 25, 2025 at 10:57=E2=80=AFAM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> Hi,
>
> After commit ca307d5, I noticed another crash when testing
> some other logical replication features.
>
> The server with max_replication_slots set to 0 would crash when executing=
 CHECKPOINT.
>
> TRAP: failed Assert("ReplicationSlotCtl !=3D NULL"), File: "slot.c", Line=
: 1162, PID: 577315
> postgres: checkpointer (ExceptionalCondition+0x9e)[0xc046cb]
> postgres: checkpointer (ReplicationSlotsComputeRequiredLSN+0x30)[0x99697f=
]
> postgres: checkpointer (CheckPointReplicationSlots+0x191)[0x997dc1]
> postgres: checkpointer [0x597b1b]
> postgres: checkpointer (CreateCheckPoint+0x6d1)[0x59729e]
> postgres: checkpointer (CheckpointerMain+0x559)[0x93ee79]
> postgres: checkpointer (postmaster_child_launch+0x15f)[0x940311]
> postgres: checkpointer [0x9468b0]
> postgres: checkpointer (PostmasterMain+0x1258)[0x9434f8]
> postgres: checkpointer (main+0x2fe)[0x7f5f9c]
> /lib64/libc.so.6(__libc_start_main+0xe5)[0x7f7585f81d85]
> postgres: checkpointer (_start+0x2e)[0x4958ee]
>
> I think it is trying to access the replication slots when the shared memo=
ry
> for them was not allocated.


I do not understand why CheckPointReplicationSlots() calls
ReplicationSlotsComputeRequiredLSN() unconditionally, shouldn't this
be called under the check[1], If not then instead of asserting
Assert("ReplicationSlotCtl !=3D NULL"), this should just return if
ReplicationSlotCtl is NULL, isn't it, because ReplicationSlotCtl is
not allocated if max_replication_slots is 0.


[1]

--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c

@@ -2131,7 +2131,8 @@ CheckPointReplicationSlots(bool is_shutdown)
         * Recompute the required LSN as SaveSlotToPath() updated
         * last_saved_restart_lsn for slots.
         */
-       ReplicationSlotsComputeRequiredLSN();
+       if (max_replication_slots > 0)
+               ReplicationSlotsComputeRequiredLSN();
 }


--=20
Regards,
Dilip Kumar
Google