From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: pgsql-hackers@lists.postgresql.org, Justin Pryzby <pryzby@telsasoft.com>,
        Dilip Kumar <dilipbalaut@gmail.com>
Subject: Re: longfin and tamandua aren't too happy but I'm not sure why
In-reply-to: <3825454.1664310917@sss.pgh.pa.us>
References: 
 <CA+TgmoYO5ee6EWhbxz+TE9hcSDONCJvczPNv--33Uk8Xd9tMHQ@mail.gmail.com>
 <20220927200712.GH6256@telsasoft.com> <3825454.1664310917@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Tue, 27 Sep 2022 16:35:17 -0400"
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Date: Tue, 27 Sep 2022 16:50:47 -0400
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I wrote:
>   * frame #0: 0x000000010a36af8c postgres`ParseCommitRecord(info=3D'\x80=
', xlrec=3D0x00007fa0678a8090, parsed=3D0x00007ff7b5c50e78) at xactdesc.c:=
102:30

Okay, so the problem is this: by widening RelFileNumber to 64 bits,
you have increased the alignment requirement of struct RelFileLocator,
and thereby also SharedInvalidationMessage, to 8 bytes where it had
been 4.  longfin's alignment check is therefore expecting that
xl_xact_twophase will likewise be 8-byte-aligned, but it isn't:

(lldb) p data
(char *) $0 =3D 0x00007fa06783a0a4 "\U00000001"

I'm not sure whether the code that generates commit WAL records is
breaking a contract it should maintain, or xactdesc.c needs to be
taught to not assume that this data is adequately aligned.

There is a second problem that I am going to hold your feet to the
fire about:

(lldb) p sizeof(SharedInvalidationMessage)
(unsigned long) $1 =3D 24

We have sweated a good deal for a long time to keep that struct
to 16 bytes.  I do not think 50% bloat is acceptable.

			regards, tom lane