Date: Fri, 5 Feb 2021 00:06:24 +0800
From: Julien Rouhaud <rjuju123@gmail.com>
To: Mahendra Singh Thalor <mahi6run@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Faulty HEAP_XMAX_LOCK_ONLY & HEAP_KEYS_UPDATED hintbit
 combination
Message-ID: <20210204160624.GA22612@nol>
References: <20210124061758.GA11756@nol>
 <20210201170048.GA22366@alvherre.pgsql>
 <20210202042133.GA72239@nol>
 <CAKYtNArKa-ZFTmUy+yUdw5Hs+BuhWr+FuXUXbK4=k0FNDZhfzQ@mail.gmail.com>
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On Thu, Feb 04, 2021 at 08:34:13PM +0530, Mahendra Singh Thalor wrote:
> On Tue, 2 Feb 2021 at 09:51, Julien Rouhaud <rjuju123@gmail.com> wrote:
> >
> > On Mon, Feb 01, 2021 at 02:00:48PM -0300, Alvaro Herrera wrote:
> > > So I think that I misspoke earlier in this thread when I said this is a
> > > bug, and that the right fix here is to remove the Assert() and change
> > > amcheck to match.
> >
> > I'm attaching a patch to do so.
> 
> Thanks Julien for the patch.
> 
> Patch looks good to me and it is fixing the problem. I think we can
> register in CF.

Thanks for looking at it!  I just created an entry for the next commitfest.

> >
> > Changing the name may be overkill, but claryfing the hint bit usage in
> > README.tuplock would definitely be useful, especially since the combination
> > isn't always produced.  How about adding something like:
> >
> >  HEAP_KEYS_UPDATED
> >   This bit lives in t_infomask2.  If set, indicates that the XMAX updated
> >   this tuple and changed the key values, or it deleted the tuple.
> > +  It can also be set in combination of HEAP_XMAX_LOCK_ONLY.
> >   It's set regardless of whether the XMAX is a TransactionId or a MultiXactId.
> Make sense. Please can you update this?

Sure, done in attached v2!

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	filename="v2-0001-Allow-HEAP_XMAX_LOCK_ONLY-and-HEAP_KEYS_UPDATED-c.patch"

From d74c216d8817659237f26baec789c6b79d337296 Mon Sep 17 00:00:00 2001
From: Julien Rouhaud <julien.rouhaud@free.fr>
Date: Fri, 22 Jan 2021 00:06:34 +0800
Subject: [PATCH v2] Allow HEAP_XMAX_LOCK_ONLY and HEAP_KEYS_UPDATED
 combination.

This combination of hint bits was previously detected as a form of corruption,
but it can be obtained with some mix of SELECT ... FOR UPDATE and UPDATE
queries.

Discussion: https://postgr.es/m/20210124061758.GA11756@nol
---
 contrib/amcheck/t/001_verify_heapam.pl | 14 +++++++++++++-
 contrib/amcheck/verify_heapam.c        |  7 -------
 src/backend/access/heap/README.tuplock |  5 +++--
 src/backend/access/heap/hio.c          |  8 +++-----
 4 files changed, 19 insertions(+), 15 deletions(-)

diff --git a/contrib/amcheck/t/001_verify_heapam.pl b/contrib/amcheck/t/001_verify_heapam.pl
index a2f65b826d..6050feb712 100644
--- a/contrib/amcheck/t/001_verify_heapam.pl
+++ b/contrib/amcheck/t/001_verify_heapam.pl
@@ -4,7 +4,7 @@ use warnings;
 use PostgresNode;
 use TestLib;
 
-use Test::More tests => 79;
+use Test::More tests => 80;
 
 my ($node, $result);
 
@@ -47,6 +47,9 @@ detects_heap_corruption(
 #
 fresh_test_table('test');
 $node->safe_psql('postgres', q(VACUUM (FREEZE, DISABLE_PAGE_SKIPPING) test));
+detects_no_corruption(
+	"verify_heapam('test')",
+	"all-frozen not corrupted table");
 corrupt_first_page('test');
 detects_heap_corruption("verify_heapam('test')",
 	"all-frozen corrupted table");
@@ -92,6 +95,15 @@ sub fresh_test_table
 		ALTER TABLE $relname ALTER b SET STORAGE external;
 		INSERT INTO $relname (a, b)
 			(SELECT gs, repeat('b',gs*10) FROM generate_series(1,1000) gs);
+		BEGIN;
+		SAVEPOINT s1;
+		SELECT 1 FROM $relname WHERE a = 42 FOR UPDATE;
+		UPDATE $relname SET b = b WHERE a = 42;
+		RELEASE s1;
+		SAVEPOINT s1;
+		SELECT 1 FROM $relname WHERE a = 42 FOR UPDATE;
+		UPDATE $relname SET b = b WHERE a = 42;
+		COMMIT;
 	));
 }
 
diff --git a/contrib/amcheck/verify_heapam.c b/contrib/amcheck/verify_heapam.c
index 88ab32490c..49f5ca0ef2 100644
--- a/contrib/amcheck/verify_heapam.c
+++ b/contrib/amcheck/verify_heapam.c
@@ -608,13 +608,6 @@ check_tuple_header_and_visibilty(HeapTupleHeader tuphdr, HeapCheckContext *ctx)
 								   ctx->tuphdr->t_hoff, ctx->lp_len));
 		header_garbled = true;
 	}
-	if ((ctx->tuphdr->t_infomask & HEAP_XMAX_LOCK_ONLY) &&
-		(ctx->tuphdr->t_infomask2 & HEAP_KEYS_UPDATED))
-	{
-		report_corruption(ctx,
-						  pstrdup("tuple is marked as only locked, but also claims key columns were updated"));
-		header_garbled = true;
-	}
 
 	if ((ctx->tuphdr->t_infomask & HEAP_XMAX_COMMITTED) &&
 		(ctx->tuphdr->t_infomask & HEAP_XMAX_IS_MULTI))
diff --git a/src/backend/access/heap/README.tuplock b/src/backend/access/heap/README.tuplock
index d03ddf6cdc..6d357565a8 100644
--- a/src/backend/access/heap/README.tuplock
+++ b/src/backend/access/heap/README.tuplock
@@ -147,8 +147,9 @@ The following infomask bits are applicable:
 
 - HEAP_KEYS_UPDATED
   This bit lives in t_infomask2.  If set, indicates that the XMAX updated
-  this tuple and changed the key values, or it deleted the tuple.
-  It's set regardless of whether the XMAX is a TransactionId or a MultiXactId.
+  this tuple and changed the key values, or it deleted the tuple.  It can also
+  be set in combination of HEAP_XMAX_LOCK_ONLY.  It's set regardless of whether
+  the XMAX is a TransactionId or a MultiXactId.
 
 We currently never set the HEAP_XMAX_COMMITTED when the HEAP_XMAX_IS_MULTI bit
 is set.
diff --git a/src/backend/access/heap/hio.c b/src/backend/access/heap/hio.c
index fb7ad0bab4..75223c9bea 100644
--- a/src/backend/access/heap/hio.c
+++ b/src/backend/access/heap/hio.c
@@ -49,12 +49,10 @@ RelationPutHeapTuple(Relation relation,
 
 	/*
 	 * Do not allow tuples with invalid combinations of hint bits to be placed
-	 * on a page.  These combinations are detected as corruption by the
-	 * contrib/amcheck logic, so if you disable one or both of these
-	 * assertions, make corresponding changes there.
+	 * on a page.  This combination is detected as corruption by the
+	 * contrib/amcheck logic, so if you disable this assertion, make
+	 * corresponding changes there.
 	 */
-	Assert(!((tuple->t_data->t_infomask & HEAP_XMAX_LOCK_ONLY) &&
-			 (tuple->t_data->t_infomask2 & HEAP_KEYS_UPDATED)));
 	Assert(!((tuple->t_data->t_infomask & HEAP_XMAX_COMMITTED) &&
 			 (tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI)));
 
-- 
2.20.1


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