Re: Fix race condition in XLogLogicalInfo and ProcSignal initialization.

> On May 1, 2026, at 01:53, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Wed, Apr 29, 2026 at 8:11 PM Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:
>>
>>
>>
>>> On Apr 29, 2026, at 09:28, Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:
>>>
>>>
>>>
>>>> On Apr 29, 2026, at 05:15, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>>>>
>>>> Hi all,
>>>>
>>>> I found a race condition issue between XLogLogicalInfo and ProcSignal
>>>> initialization while reviewing another issue[1]. I'm starting a
>>>> separate thread for the subject as it's not related to the issue
>>>> reported on that thread.
>>>>
>>>> The issue is that child processes could miss the
>>>> PROCSIGNAL_BARRIER_UPDATE_XLOG_LOGICAL_INFO signal during the
>>>> initialization and end up in an inconsistent state because
>>>> InitializeProcessXLogLogicalInfo() is called (in BaseInit()) before
>>>> ProcSignalInit(). If the startup emits the signal to a process who is
>>>> between two steps, the process would not reflect the latest
>>>> XLogLogicalInfo state. I think we should move
>>>> InitializeProcessXLogLogicalInfo() after ProcSignalInit() like we do
>>>> so for InitLocalDataChecksumState().
>>>
>>> I think this is correct.
>>>
>>> After moving InitializeProcessXLogLogicalInfo() out of BaseInit(), background worker processes (BackgroundWorkerMain) will no longer hold a valid value of XLogLogicalInfo, but I guess that is fine as those processes don’t call ProcSignalInit() anyway.
>
> No, even after moving the InitializeProcessXLogLogicalInfo(),
> bgworkers who connected a database will call InitPostgres(),
> initializing the proc signals and XLogLogicalInfo.
>
>>
>> I met Zhijie Hou at HOW 2026 a few days ago. When we talked about a feature requirement I recently heard from a DBA, Zhijie pointed me to 67c20979ce (Toggle logical decoding dynamically based on logical slot presence).
>>
>> The requirement is that storage is expensive today, and users are sensitive to the total size of WAL. In some deployments, users may only want to replicate a small set of tables intermittently, but to enable logical replication, they still have to set wal_level to logical, which significantly increases the total WAL volume. I believe this feature could help address that concern, so I reviewed the code and played a bit with it.
>>
>> I found an issue related to this patch, so I am sharing my findings here, although the problem also exists before this patch.
>>
>> In InitPostgres(), in the standalone backend path, StartupXLOG() is called, but XLogLogicalInfo is not updated. As a result, if we switch to standalone mode for some emergency maintenance, make data changes, and then switch back to normal mode, changes made during standalone mode would not include logical replication metadata, which may potentially break future logical replication.
>>
>> To verify that, I did a test like:
>>
>> * Start a new instance with wal_level = replica
>> * Create a table, insert some data, then create a logical replication slot
>> ```
>> evantest=# CREATE TABLE t1(id int);
>> CREATE TABLE
>> evantest=# INSERT INTO t1 VALUES (1), (2);
>> INSERT 0 2
>> evantest=# SELECT * FROM pg_create_logical_replication_slot('s1', 'test_decoding');
>> slot_name | lsn
>> -----------+------------
>> s1 | 0/01D6E6D0
>> (1 row)
>> ```
>>
>> * Stop the server, and start with standalone mode, and truncate the table:
>> ```
>> % postgres --single -F -D . evantest
>>
>> PostgreSQL stand-alone backend 19devel
>> backend> show effective_wal_level;
>> 1: effective_wal_level (typeid = 25, len = -1, typmod = -1, byval = f)
>> ----
>> 1: effective_wal_level = "replica" (typeid = 25, len = -1, typmod = -1, byval = f)
>> ----
>> backend> truncate t1;
>> backend> 2026-04-29 21:13:24.625 CST [68316] LOG: checkpoint starting: shutdown fast
>> ```
>>
>> * Start the server normally, and real WAL through the logical slot.
>> ```
>> evantest=# SELECT data FROM pg_logical_slot_get_changes('s1', NULL, NULL);
>> data
>> ------------
>> BEGIN 721
>> COMMIT 721
>> (2 rows)
>> ```
>>
>> The TRUNCATE does not appear, which I think is wrong. To fix that, we only need to call InitializeProcessXLogLogicalInfo()after StartupXLOG() in the standalone path. Since the fix is based on this patch, I added it as 0002 in this patch set.
>
> Good catch. I've updated the patch.
>
>>
>> One more thought: I think this feature partially addresses the user requirement I described earlier. When wal_level is replicaand some logical slots are created, the extra WAL data should only be enabled for tables included in those slots. That avoids generating unnecessary WAL data for tables that are not targets of replication, and therefore saves storage. WDYT? Maybe a candidate for v20?
>>
>
> This would require additional functionality to logical replication
> slots so that they include the specific tables, and then when writing
> WAL records each backend process needs to figure out whether the table
> is included in any replication slots. While the idea sounds
> interesting, it also sounds complex and potentially introduces
> overheads.

I have realized that there is no easy or direct way to determine whether a table is included in some replication slot, so we may need to maintain some extra information. But I think this would be a feature that users and DBAs would like very much.

Although PG already has a mechanism to clean up old WAL files, from what I have heard from DBAs, many users store WAL files for years in external storage, so they always try to keep WAL as small as possible. I have heard repeated concerns that storage has become too expensive.

I am currently focusing on testing PG19 new features. After that, I can spend some time studying a possible solution. At that time, your help and support would be greatly appreciated.

>
>> BTW, in 0001, I helped fix the typos.
>
> Thank you!
>
> Regards,
> --
> Masahiko Sawada
> Amazon Web Services: https://aws.amazon.com
> <v3-0001-Fix-race-condition-in-XLogLogicalInfo-and-ProcSig.patch>

V3 LGTM.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/