amcheck: Fix FullTransactionIdFromXidAndCtx() for xids before epoch 0

64bit xids can't represent xids before epoch 0 (see also be504a3e974). When
FullTransactionIdFromXidAndCtx() was passed such an xid, it'd create a 64bit
xid far into the future. Noticed while adding assertions in the course of
investigating be504a3e974, as amcheck's test create such xids.

To fix the issue, just return FirstNormalFullTransactionId in this case. A
freshly initdb'd cluster already has a newer horizon. The most minimal version
of this would make the messages for some detected corruptions differently
inaccurate. To make those cases accurate, switch
FullTransactionIdFromXidAndCtx() to use the 32bit modulo difference between
xid and nextxid to compute the 64bit xid, yielding sensible "in the future" /
"in the past" answers.

Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://postgr.es/m/20230108002923.cyoser3ttmt63bfn@awork3.anarazel.de
Backpatch: 14-, where heapam verification was introduced

1 files changed, 20 insertions, 8 deletions

diff --git a/src/bin/pg_amcheck/t/004_verify_heapam.pl b/src/bin/pg_amcheck/t/004_verify_heapam.pl
index bbada168f09..25f1478c42a 100644
--- a/src/bin/pg_amcheck/t/004_verify_heapam.pl
+++ b/src/bin/pg_amcheck/t/004_verify_heapam.pl
@@ -217,7 +217,7 @@ my $rel = $node->safe_psql('postgres',
 my $relpath = "$pgdata/$rel";
 
 # Insert data and freeze public.test
-use constant ROWCOUNT => 16;
+use constant ROWCOUNT => 17;
 $node->safe_psql(
 	'postgres', qq(
 	INSERT INTO public.test (a, b, c)
@@ -378,23 +378,24 @@ for (my $tupidx = 0; $tupidx < ROWCOUNT; $tupidx++)
 	elsif ($offnum == 3)
 	{
 		# Corruptly set xmin < datfrozenxid, further back, noting circularity
-		# of xid comparison.  For a new cluster with epoch = 0, the corrupt
-		# xmin will be interpreted as in the future
-		$tup->{t_xmin} = 4026531839;
+		# of xid comparison.
+		my $xmin = 4026531839;
+		$tup->{t_xmin} = $xmin;
 		$tup->{t_infomask} &= ~HEAP_XMIN_COMMITTED;
 		$tup->{t_infomask} &= ~HEAP_XMIN_INVALID;
 
 		push @expected,
-		  qr/${$header}xmin 4026531839 equals or exceeds next valid transaction ID 0:\d+/;
+		  qr/${$header}xmin ${xmin} precedes oldest valid transaction ID 0:\d+/;
 	}
 	elsif ($offnum == 4)
 	{
 		# Corruptly set xmax < relminmxid;
-		$tup->{t_xmax} = 4026531839;
+		my $xmax = 4026531839;
+		$tup->{t_xmax} = $xmax;
 		$tup->{t_infomask} &= ~HEAP_XMAX_INVALID;
 
 		push @expected,
-		  qr/${$header}xmax 4026531839 equals or exceeds next valid transaction ID 0:\d+/;
+		  qr/${$header}xmax ${xmax} precedes oldest valid transaction ID 0:\d+/;
 	}
 	elsif ($offnum == 5)
 	{
@@ -502,7 +503,7 @@ for (my $tupidx = 0; $tupidx < ROWCOUNT; $tupidx++)
 		push @expected,
 		  qr/${header}multitransaction ID 4 equals or exceeds next valid multitransaction ID 1/;
 	}
-	elsif ($offnum == 15)    # Last offnum must equal ROWCOUNT
+	elsif ($offnum == 15)
 	{
 		# Set both HEAP_XMAX_COMMITTED and HEAP_XMAX_IS_MULTI
 		$tup->{t_infomask} |= HEAP_XMAX_COMMITTED;
@@ -512,6 +513,17 @@ for (my $tupidx = 0; $tupidx < ROWCOUNT; $tupidx++)
 		push @expected,
 		  qr/${header}multitransaction ID 4000000000 precedes relation minimum multitransaction ID threshold 1/;
 	}
+	elsif ($offnum == 16)    # Last offnum must equal ROWCOUNT
+	{
+		# Corruptly set xmin > next_xid to be in the future.
+		my $xmin = 123456;
+		$tup->{t_xmin} = $xmin;
+		$tup->{t_infomask} &= ~HEAP_XMIN_COMMITTED;
+		$tup->{t_infomask} &= ~HEAP_XMIN_INVALID;
+
+		push @expected,
+          qr/${$header}xmin ${xmin} equals or exceeds next valid transaction ID 0:\d+/;
+	}
 	write_tuple($file, $offset, $tup);
 }
 close($file)