From: Noah Misch <noah@leadboat.com>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: exclusion@gmail.com
Cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #17928: Standby fails to decode WAL on termination of primary
Date: Sat, 12 Aug 2023 20:30:34 -0700
Message-ID: <20230813033034.GD2326466@rfd.leadboat.com> (raw)
In-Reply-To: <CA+hUKGKcSyHRppTyGZy7q29E1JVtQKdKCNzY0QARG3gfqpLaXg@mail.gmail.com>
References: <20230717004905.GE3826297@rfd.leadboat.com>
<ZNSVlR/v8gTDIMsU@paquier.xyz>
<20230811025808.GA2261232@rfd.leadboat.com>
<ZNXQWvnmt1Fpt6vu@paquier.xyz>
<20230811140008.GB2261449@rfd.leadboat.com>
<CA+hUKGLEF7-bK3=KFYfaDMOh8wy2XdMOncbUrn=JrhmjX0_vPQ@mail.gmail.com>
<ZNbQUDcZK0ELn1o9@paquier.xyz>
<351991.1691800320@sss.pgh.pa.us>
<20230812211327.GB2326466@rfd.leadboat.com>
<CA+hUKGKcSyHRppTyGZy7q29E1JVtQKdKCNzY0QARG3gfqpLaXg@mail.gmail.com>
On Sun, Aug 13, 2023 at 03:12:34PM +1200, Thomas Munro wrote:
> On Sun, Aug 13, 2023 at 9:13 AM Noah Misch <noah@leadboat.com> wrote:
> > Any user could call pg_logical_emit_message() to silently terminate the WAL
> > stream, which is far worse than the original bug. So far, I'm seeing one way
> > to clearly fix $SUBJECT without that harm. When a record header spans a page
> > boundary, read the next page to reassemble the header. If
> > !ValidXLogRecordHeader() (invalid xl_rmid or xl_prev), treat as end of WAL.
> > Otherwise, read the whole record in chunks, calculating CRC. If CRC is
> > invalid, treat as end of WAL. Otherwise, ereport(FATAL) for the oversized
> > record. A side benefit would be avoiding useless large allocations (1GB
> > backend, 4GB frontend) as discussed upthread. May as well do the xl_rmid and
> > xl_prev checks in all branches, to avoid needless XLogRecordMaxSize-1
> > allocations. Thoughts? For invalid-length records in v16+, since every such
> > record is end-of-wal or corruption, those versions could skip the CRC.
>
> That sounds quite strong. But... why couldn't the existing
> xlp_rem_len cross-check protect us from this failure mode? If we
> could defer the allocation until after that check (and the usual
> ValidXLogRecordHeader() check), I think we'd know that we're really
> looking at a size that was actually written in both pages (which must
> also have survived xlp_pageaddr check), no?
Hmm, I think that is right. A coincidental match of the 32-bit CRC is more
probable than having all those fields appear valid by coincidence, especially
xlp_pageaddr.
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To: pgsql-bugs@postgresql.org
Cc: noah@leadboat.com, thomas.munro@gmail.com, tgl@sss.pgh.pa.us, michael@paquier.xyz, horikyota.ntt@gmail.com, exclusion@gmail.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #17928: Standby fails to decode WAL on termination of primary
In-Reply-To: <20230813033034.GD2326466@rfd.leadboat.com>
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