From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Ranier Vilela <ranier.vf@gmail.com>,
        Melanie Plageman <melanieplageman@gmail.com>,
        Thomas Munro <thomas.munro@gmail.com>,
        Masahiko Sawada <sawada.mshk@gmail.com>,
        Tomas Vondra <tomas@vondra.me>, Noah Misch <noah@leadboat.com>,
        vignesh C <vignesh21@gmail.com>,
        Pg Hackers <pgsql-hackers@postgresql.org>,
        Heikki Linnakangas <hlinnaka@iki.fi>,
        Nazir Bilal Yavuz <byavuz81@gmail.com>,
        Robert Haas <robertmhaas@gmail.com>,
        "Andrey M. Borodin" <x4mmm@yandex-team.ru>
Subject: Re: Confine vacuum skip logic to lazy_scan_skip
In-reply-to: <qphdlszwk3z2w4w6chs4tfheujtpanyayljdijd4fvsdsiq6sl@menvperc55q6>
References: 
 <CA+hUKG+g6aXpi2FEHqeLOzE+xYw=OV+-N5jhOEnnV+F0USM9xA@mail.gmail.com>
 <CA+hUKGJ84L0yFD2S05WeCOXmBgBcYb2S5wMmzyohcMJeCwgksw@mail.gmail.com>
 <3915749.1739574230@sss.pgh.pa.us>
 <CA+hUKGKgHs9xjOZrjjuaEoYSxBvQiYOgDDQiQ6yTSzqFwx6d2Q@mail.gmail.com>
 <CAAKRu_Z35kymRA5ru1TeAZtnOd46o7iDCyJgF9P8TH5geKy46A@mail.gmail.com>
 <CA+hUKGL8bAsQNj-bB_qxFFzoQW+RMZ3P3zK2SVa6b08G55BdRg@mail.gmail.com>
 <626104.1739729538@sss.pgh.pa.us>
 <CAAKRu_ZE21Q5ic03bHgAAz5YV6gm7Yu3V9eKnLiDtkh5=0porw@mail.gmail.com>
 <CAEudQAr_QXGB2DaK+BhTHuzw76FZCFSp54mzT3MSDn0E5XbcTA@mail.gmail.com>
 <l4xcxe5ekugabzcgwqocg6v7qgbzanxh7pstmgddm6y6i4honk@7lpjn3bf6ytz>
 <qphdlszwk3z2w4w6chs4tfheujtpanyayljdijd4fvsdsiq6sl@menvperc55q6>
Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Thu, 27 Feb 2025 12:49:34 -0500"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <2600077.1740679727.1@sss.pgh.pa.us>
Date: Thu, 27 Feb 2025 13:08:47 -0500
Message-ID: <2600078.1740679727@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/2600078.1740679727%40sss.pgh.pa.us>
Precedence: bulk

Andres Freund <andres@anarazel.de> writes:
> Ah, no, it isn't. But I still think the coverity alert and the patch don't
> make sense, as per the below:

Coverity's alert makes perfect sense if you posit that Coverity
doesn't assume that this read_stream_next_buffer call will
only be applied to a stream that has per_buffer_data_size > 0.
(Even if it did understand that, I wouldn't assume that it's
smart enough to see that the fast path will never be taken.)

I wonder if it'd be a good idea to add something like

		Assert(stream->distance == 1);
		Assert(stream->pending_read_nblocks == 0);
		Assert(stream->per_buffer_data_size == 0);
+		Assert(per_buffer_data == NULL);

in read_stream_next_buffer.  I doubt that this will shut Coverity
up, but it would help to catch caller coding errors, i.e. passing
a per_buffer_data pointer when there's no per-buffer data.

On the whole I doubt we can get rid of this warning without some
significant redesign of the read_stream API, and I don't think
it's worth the trouble.  Coverity is a tool not a requirement.
I'm content to just dismiss the warning.

			regards, tom lane