Date: Mon, 21 Oct 2024 20:52:15 -0700
From: Noah Misch <noah@leadboat.com>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Nitin Motiani <nitinmotiani@google.com>,
	Heikki Linnakangas <hlinnaka@iki.fi>,
	Robert Haas <robertmhaas@gmail.com>, pgsql-hackers@postgresql.org,
	Tom Lane <tgl@sss.pgh.pa.us>, Smolkin Grigory <smallkeen@gmail.com>,
	Michael Paquier <michael@paquier.xyz>
Subject: Re: race condition in pg_class
Message-ID: <20241022035215.3e.nmisch@google.com>
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On Mon, Oct 21, 2024 at 10:00:00PM +0300, Alexander Lakhin wrote:
> Please look at an anomaly introduced with a07e03fd8.
> With the attached modification for intra-grant-inplace.spec, running this
> test triggers a Valgrind-detected error for me:
> ==00:00:00:09.624 319033== Conditional jump or move depends on uninitialised value(s)
> ==00:00:00:09.624 319033==    at 0x25D120: DoesMultiXactIdConflict (heapam.c:7373)
> ==00:00:00:09.624 319033==    by 0x25B45A: heap_inplace_lock (heapam.c:6265)
> ==00:00:00:09.624 319033==    by 0x27D8CB: systable_inplace_update_begin (genam.c:867)
> ==00:00:00:09.624 319033==    by 0x33F717: index_update_stats (index.c:2856)
> ==00:00:00:09.624 319033==    by 0x33FEE2: index_build (index.c:3106)
> ==00:00:00:09.625 319033==    by 0x33C7D3: index_create (index.c:1276)
> ==00:00:00:09.625 319033==    by 0x451000: DefineIndex (indexcmds.c:1216)
> ==00:00:00:09.625 319033==    by 0x48D091: ATExecAddIndex (tablecmds.c:9156)
> ==00:00:00:09.625 319033==    by 0x483F8E: ATExecCmd (tablecmds.c:5302)
> ==00:00:00:09.625 319033==    by 0x483877: ATRewriteCatalogs (tablecmds.c:5186)
> ==00:00:00:09.625 319033==    by 0x482B9A: ATController (tablecmds.c:4741)
> ==00:00:00:09.625 319033==    by 0x4827A1: AlterTable (tablecmds.c:4387)
> ==00:00:00:09.625 319033==

Thanks.

> Perhaps current_is_member in heap_inplace_lock() should be initialized
> before the DoesMultiXactIdConflict() call as in other places...

heap_inplace_lock() ignores current_is_member after computing it, so let's
just pass NULL, as attached.

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	filename="inplace129-DoesMultiXactIdConflict-v1.patch"

Author:     Noah Misch <noah@leadboat.com>
Commit:     Noah Misch <noah@leadboat.com>

    Stop reading uninitialized memory in heap_inplace_lock().
    
    Stop computing a never-used value.  This removes the read; the read had
    no functional implications.  Back-patch to v12, like commit
    a07e03fd8fa7daf4d1356f7cb501ffe784ea6257.
    
    Reported by Alexander Lakhin.  Reviewed by FIXME.
    
    Discussion: https://postgr.es/m/6c92f59b-f5bc-e58c-9bdd-d1f21c17c786@gmail.com

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index da5e656..82a0492 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -6260,10 +6260,9 @@ heap_inplace_lock(Relation relation,
 			LockTupleMode lockmode = LockTupleNoKeyExclusive;
 			MultiXactStatus mxact_status = MultiXactStatusNoKeyUpdate;
 			int			remain;
-			bool		current_is_member;
 
 			if (DoesMultiXactIdConflict((MultiXactId) xwait, infomask,
-										lockmode, &current_is_member))
+										lockmode, NULL))
 			{
 				LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
 				ret = false;
diff --git a/src/test/isolation/expected/intra-grant-inplace.out b/src/test/isolation/expected/intra-grant-inplace.out
index b5fe8b0..4e9695a 100644
--- a/src/test/isolation/expected/intra-grant-inplace.out
+++ b/src/test/isolation/expected/intra-grant-inplace.out
@@ -63,6 +63,30 @@ step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c); <waiting ...>
 step r3: ROLLBACK;
 step addk2: <... completed>
 
+starting permutation: b3 sfnku3 keyshr5 addk2 r3
+step b3: BEGIN ISOLATION LEVEL READ COMMITTED;
+step sfnku3: 
+	SELECT relhasindex FROM pg_class
+	WHERE oid = 'intra_grant_inplace'::regclass FOR NO KEY UPDATE;
+
+relhasindex
+-----------
+f          
+(1 row)
+
+step keyshr5: 
+	SELECT relhasindex FROM pg_class
+	WHERE oid = 'intra_grant_inplace'::regclass FOR KEY SHARE;
+
+relhasindex
+-----------
+f          
+(1 row)
+
+step addk2: ALTER TABLE intra_grant_inplace ADD PRIMARY KEY (c); <waiting ...>
+step r3: ROLLBACK;
+step addk2: <... completed>
+
 starting permutation: b2 sfnku2 addk2 c2
 step b2: BEGIN;
 step sfnku2: 
diff --git a/src/test/isolation/specs/intra-grant-inplace.spec b/src/test/isolation/specs/intra-grant-inplace.spec
index 2992c85..9936d38 100644
--- a/src/test/isolation/specs/intra-grant-inplace.spec
+++ b/src/test/isolation/specs/intra-grant-inplace.spec
@@ -96,6 +96,14 @@ permutation
 	addk2(r3)
 	r3
 
+# reproduce bug in DoesMultiXactIdConflict() call
+permutation
+	b3
+	sfnku3
+	keyshr5
+	addk2(r3)
+	r3
+
 # same-xact rowmark
 permutation
 	b2

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