database/postgres
database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-05-28 05:21:27 +03:00
Code

At update of non-LP_NORMAL TID, fail instead of corrupting page header.

Browse Source 
The right mix of DDL and VACUUM could corrupt a catalog page header such
that PageIsVerified() durably fails, requiring a restore from backup.
This affects only catalogs that both have a syscache and have DDL code
that uses syscache tuples to construct updates.  One of the test
permutations shows a variant not yet fixed.

This makes !TransactionIdIsValid(TM_FailureData.xmax) possible with
TM_Deleted.  I think core and PGXN are indifferent to that.

Per bug #17821 from Alexander Lakhin.  Back-patch to v13 (all supported
versions).  The test case is v17+, since it uses INJECTION_POINT.

Discussion: https://postgr.es/m/17821-dd8c334263399284@postgresql.org
This commit is contained in:
Noah Misch 2025-01-25 11:28:14 -08:00
parent 1587f7b9fc
commit f4af4515bb
10 changed files with 493 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

49
src/backend/access/heap/heapam.c
View File

@ -65,10 +65,12 @@
#include "storage/procarray.h"
#include "storage/standby.h"
#include "utils/datum.h"
#include "utils/injection_point.h"
#include "utils/inval.h"
#include "utils/relcache.h"
#include "utils/snapmgr.h"
#include "utils/spccache.h"
#include "utils/syscache.h"
static HeapTuple heap_prepare_insert(Relation relation, HeapTuple tup,
@ -3251,6 +3253,7 @@ heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
interesting_attrs = bms_add_members(interesting_attrs, id_attrs);
block = ItemPointerGetBlockNumber(otid);
INJECTION_POINT("heap_update-before-pin");
buffer = ReadBuffer(relation, block);
page = BufferGetPage(buffer);
@ -3266,7 +3269,51 @@ heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
lp = PageGetItemId(page, ItemPointerGetOffsetNumber(otid));
Assert(ItemIdIsNormal(lp));
/*
* Usually, a buffer pin and/or snapshot blocks pruning of otid, ensuring
* we see LP_NORMAL here. When the otid origin is a syscache, we may have
* neither a pin nor a snapshot. Hence, we may see other LP_ states, each
* of which indicates concurrent pruning.
*
* Failing with TM_Updated would be most accurate. However, unlike other
* TM_Updated scenarios, we don't know the successor ctid in LP_UNUSED and
* LP_DEAD cases. While the distinction between TM_Updated and TM_Deleted
* does matter to SQL statements UPDATE and MERGE, those SQL statements
* hold a snapshot that ensures LP_NORMAL. Hence, the choice between
* TM_Updated and TM_Deleted affects only the wording of error messages.
* Settle on TM_Deleted, for two reasons. First, it avoids complicating
* the specification of when tmfd->ctid is valid. Second, it creates
* error log evidence that we took this branch.
*
* Since it's possible to see LP_UNUSED at otid, it's also possible to see
* LP_NORMAL for a tuple that replaced LP_UNUSED. If it's a tuple for an
* unrelated row, we'll fail with "duplicate key value violates unique".
* XXX if otid is the live, newer version of the newtup row, we'll discard
* changes originating in versions of this catalog row after the version
* the caller got from syscache. See syscache-update-pruned.spec.
*/
if (!ItemIdIsNormal(lp))
{
Assert(RelationSupportsSysCache(RelationGetRelid(relation)));
UnlockReleaseBuffer(buffer);
Assert(!have_tuple_lock);
if (vmbuffer != InvalidBuffer)
ReleaseBuffer(vmbuffer);
tmfd->ctid = *otid;
tmfd->xmax = InvalidTransactionId;
tmfd->cmax = InvalidCommandId;
*update_indexes = TU_None;
bms_free(hot_attrs);
bms_free(sum_attrs);
bms_free(key_attrs);
bms_free(id_attrs);
/* modified_attrs not yet initialized */
bms_free(interesting_attrs);
return TM_Deleted;
}
/*
* Fill in enough data in oldtup for HeapDetermineColumnsInfo to work

3
src/backend/utils/cache/inval.c vendored
View File

@ -119,6 +119,7 @@
#include "storage/sinval.h"
#include "storage/smgr.h"
#include "utils/catcache.h"
#include "utils/injection_point.h"
#include "utils/inval.h"
#include "utils/memdebug.h"
#include "utils/memutils.h"
@ -1031,6 +1032,8 @@ AtEOXact_Inval(bool isCommit)
/* Must be at top of stack */
Assert(transInvalInfo->my_level == 1 && transInvalInfo->parent == NULL);
INJECTION_POINT("AtEOXact_Inval-with-transInvalInfo");
if (isCommit)
{
/*

3
src/include/access/tableam.h
View File

@ -137,7 +137,8 @@ typedef enum TU_UpdateIndexes
*
* xmax is the outdating transaction's XID. If the caller wants to visit the
* replacement tuple, it must check that this matches before believing the
* replacement is really a match.
* replacement is really a match. This is InvalidTransactionId if the target
* was !LP_NORMAL (expected only for a TID retrieved from syscache).
*
* cmax is the outdating command's CID, but only when the failure code is
* TM_SelfModified (i.e., something in the current transaction outdated the

9
src/test/modules/injection_points/Makefile
View File

@ -1,7 +1,10 @@
# src/test/modules/injection_points/Makefile
MODULES = injection_points
MODULE_big = injection_points
OBJS = \
$(WIN32RES) \
injection_points.o \
regress_injection.o
EXTENSION = injection_points
DATA = injection_points--1.0.sql
PGFILEDESC = "injection_points - facility for injection points"
@ -9,7 +12,7 @@ PGFILEDESC = "injection_points - facility for injection points"
REGRESS = injection_points reindex_conc
REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
ISOLATION = inplace
ISOLATION = inplace syscache-update-pruned
# The injection points are cluster-wide, so disable installcheck
NO_INSTALLCHECK = 1

87
src/test/modules/injection_points/expected/syscache-update-pruned.out Normal file
View File

@ -0,0 +1,87 @@
Parsed test spec with 4 sessions
starting permutation: cachefill1 at2 waitprunable4 vac4 grant1 wakeinval4 wakegrant4
step cachefill1: SELECT FROM vactest.reloid_catcache_set('vactest.orig50');
step at2:
CREATE TRIGGER to_set_relhastriggers BEFORE UPDATE ON vactest.orig50
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
<waiting ...>
step waitprunable4: CALL vactest.wait_prunable();
step vac4: VACUUM pg_class;
step grant1: GRANT SELECT ON vactest.orig50 TO PUBLIC; <waiting ...>
step wakeinval4:
SELECT FROM injection_points_detach('AtEOXact_Inval-with-transInvalInfo');
SELECT FROM injection_points_wakeup('AtEOXact_Inval-with-transInvalInfo');
<waiting ...>
step at2: <... completed>
step wakeinval4: <... completed>
step wakegrant4:
SELECT FROM injection_points_detach('heap_update-before-pin');
SELECT FROM injection_points_wakeup('heap_update-before-pin');
<waiting ...>
step grant1: <... completed>
ERROR: tuple concurrently deleted
step wakegrant4: <... completed>
starting permutation: cachefill1 at2 waitprunable4 vac4 grant1 wakeinval4 mkrels4 wakegrant4
step cachefill1: SELECT FROM vactest.reloid_catcache_set('vactest.orig50');
step at2:
CREATE TRIGGER to_set_relhastriggers BEFORE UPDATE ON vactest.orig50
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
<waiting ...>
step waitprunable4: CALL vactest.wait_prunable();
step vac4: VACUUM pg_class;
step grant1: GRANT SELECT ON vactest.orig50 TO PUBLIC; <waiting ...>
step wakeinval4:
SELECT FROM injection_points_detach('AtEOXact_Inval-with-transInvalInfo');
SELECT FROM injection_points_wakeup('AtEOXact_Inval-with-transInvalInfo');
<waiting ...>
step at2: <... completed>
step wakeinval4: <... completed>
step mkrels4:
SELECT FROM vactest.mkrels('intruder', 1, 100); -- repopulate LP_UNUSED
step wakegrant4:
SELECT FROM injection_points_detach('heap_update-before-pin');
SELECT FROM injection_points_wakeup('heap_update-before-pin');
<waiting ...>
step grant1: <... completed>
ERROR: duplicate key value violates unique constraint "pg_class_oid_index"
step wakegrant4: <... completed>
starting permutation: snap3 cachefill1 at2 mkrels4 r3 waitprunable4 vac4 grant1 wakeinval4 at4 wakegrant4 inspect4
step snap3: BEGIN ISOLATION LEVEL REPEATABLE READ; SELECT;
step cachefill1: SELECT FROM vactest.reloid_catcache_set('vactest.orig50');
step at2:
CREATE TRIGGER to_set_relhastriggers BEFORE UPDATE ON vactest.orig50
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
<waiting ...>
step mkrels4:
SELECT FROM vactest.mkrels('intruder', 1, 100); -- repopulate LP_UNUSED
step r3: ROLLBACK;
step waitprunable4: CALL vactest.wait_prunable();
step vac4: VACUUM pg_class;
step grant1: GRANT SELECT ON vactest.orig50 TO PUBLIC; <waiting ...>
step wakeinval4:
SELECT FROM injection_points_detach('AtEOXact_Inval-with-transInvalInfo');
SELECT FROM injection_points_wakeup('AtEOXact_Inval-with-transInvalInfo');
<waiting ...>
step at2: <... completed>
step wakeinval4: <... completed>
step at4: ALTER TABLE vactest.child50 INHERIT vactest.orig50;
step wakegrant4:
SELECT FROM injection_points_detach('heap_update-before-pin');
SELECT FROM injection_points_wakeup('heap_update-before-pin');
<waiting ...>
step grant1: <... completed>
step wakegrant4: <... completed>
step inspect4:
SELECT relhastriggers, relhassubclass FROM pg_class
WHERE oid = 'vactest.orig50'::regclass;
relhastriggers|relhassubclass
--------------+--------------
f |f
(1 row)

86
src/test/modules/injection_points/expected/syscache-update-pruned_1.out Normal file
View File

@ -0,0 +1,86 @@
Parsed test spec with 4 sessions
starting permutation: cachefill1 at2 waitprunable4 vac4 grant1 wakeinval4 wakegrant4
step cachefill1: SELECT FROM vactest.reloid_catcache_set('vactest.orig50');
step at2:
CREATE TRIGGER to_set_relhastriggers BEFORE UPDATE ON vactest.orig50
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
<waiting ...>
step waitprunable4: CALL vactest.wait_prunable();
step vac4: VACUUM pg_class;
step grant1: GRANT SELECT ON vactest.orig50 TO PUBLIC; <waiting ...>
step wakeinval4:
SELECT FROM injection_points_detach('AtEOXact_Inval-with-transInvalInfo');
SELECT FROM injection_points_wakeup('AtEOXact_Inval-with-transInvalInfo');
<waiting ...>
step at2: <... completed>
step wakeinval4: <... completed>
step wakegrant4:
SELECT FROM injection_points_detach('heap_update-before-pin');
SELECT FROM injection_points_wakeup('heap_update-before-pin');
<waiting ...>
step grant1: <... completed>
step wakegrant4: <... completed>
starting permutation: cachefill1 at2 waitprunable4 vac4 grant1 wakeinval4 mkrels4 wakegrant4
step cachefill1: SELECT FROM vactest.reloid_catcache_set('vactest.orig50');
step at2:
CREATE TRIGGER to_set_relhastriggers BEFORE UPDATE ON vactest.orig50
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
<waiting ...>
step waitprunable4: CALL vactest.wait_prunable();
step vac4: VACUUM pg_class;
step grant1: GRANT SELECT ON vactest.orig50 TO PUBLIC; <waiting ...>
step wakeinval4:
SELECT FROM injection_points_detach('AtEOXact_Inval-with-transInvalInfo');
SELECT FROM injection_points_wakeup('AtEOXact_Inval-with-transInvalInfo');
<waiting ...>
step at2: <... completed>
step wakeinval4: <... completed>
step mkrels4:
SELECT FROM vactest.mkrels('intruder', 1, 100); -- repopulate LP_UNUSED
step wakegrant4:
SELECT FROM injection_points_detach('heap_update-before-pin');
SELECT FROM injection_points_wakeup('heap_update-before-pin');
<waiting ...>
step grant1: <... completed>
step wakegrant4: <... completed>
starting permutation: snap3 cachefill1 at2 mkrels4 r3 waitprunable4 vac4 grant1 wakeinval4 at4 wakegrant4 inspect4
step snap3: BEGIN ISOLATION LEVEL REPEATABLE READ; SELECT;
step cachefill1: SELECT FROM vactest.reloid_catcache_set('vactest.orig50');
step at2:
CREATE TRIGGER to_set_relhastriggers BEFORE UPDATE ON vactest.orig50
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
<waiting ...>
step mkrels4:
SELECT FROM vactest.mkrels('intruder', 1, 100); -- repopulate LP_UNUSED
step r3: ROLLBACK;
step waitprunable4: CALL vactest.wait_prunable();
step vac4: VACUUM pg_class;
step grant1: GRANT SELECT ON vactest.orig50 TO PUBLIC; <waiting ...>
step wakeinval4:
SELECT FROM injection_points_detach('AtEOXact_Inval-with-transInvalInfo');
SELECT FROM injection_points_wakeup('AtEOXact_Inval-with-transInvalInfo');
<waiting ...>
step at2: <... completed>
step wakeinval4: <... completed>
step at4: ALTER TABLE vactest.child50 INHERIT vactest.orig50;
step wakegrant4:
SELECT FROM injection_points_detach('heap_update-before-pin');
SELECT FROM injection_points_wakeup('heap_update-before-pin');
<waiting ...>
step grant1: <... completed>
ERROR: tuple concurrently updated
step wakegrant4: <... completed>
step inspect4:
SELECT relhastriggers, relhassubclass FROM pg_class
WHERE oid = 'vactest.orig50'::regclass;
relhastriggers|relhassubclass
--------------+--------------
t |t
(1 row)

8
src/test/modules/injection_points/injection_points--1.0.sql
View File

@ -54,3 +54,11 @@ CREATE FUNCTION injection_points_detach(IN point_name TEXT)
RETURNS void
AS 'MODULE_PATHNAME', 'injection_points_detach'
LANGUAGE C STRICT PARALLEL UNSAFE;
--
-- regress_injection.c functions
--
CREATE FUNCTION removable_cutoff(rel regclass)
RETURNS xid8
AS 'MODULE_PATHNAME'
LANGUAGE C CALLED ON NULL INPUT;

4
src/test/modules/injection_points/meson.build
View File

@ -6,6 +6,7 @@ endif
injection_points_sources = files(
'injection_points.c',
'regress_injection.c',
)
if host_system == 'windows'
@ -41,7 +42,8 @@ tests += {
'isolation': {
'specs': [
'inplace',
'syscache-update-pruned',
],
'runningcheck': false, # align with GNU make build system
'runningcheck': false, # see syscache-update-pruned
},
}

71
src/test/modules/injection_points/regress_injection.c Normal file
View File

@ -0,0 +1,71 @@
/*--------------------------------------------------------------------------
*
* regress_injection.c
* Functions supporting test-specific subject matter.
*
* Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* src/test/modules/injection_points/regress_injection.c
*
* -------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/table.h"
#include "fmgr.h"
#include "miscadmin.h"
#include "storage/procarray.h"
#include "utils/xid8.h"
/*
* removable_cutoff - for syscache-update-pruned.spec
*
* Wrapper around GetOldestNonRemovableTransactionId(). In general, this can
* move backward. runningcheck=false isolation tests can reasonably prevent
* that. For the causes of backward movement, see
* postgr.es/m/CAEze2Wj%2BV0kTx86xB_YbyaqTr5hnE_igdWAwuhSyjXBYscf5-Q%40mail.gmail.com
* and the header comment for ComputeXidHorizons(). One can assume this
* doesn't move backward if one arranges for concurrent activity not to reach
* AbortTransaction() and not to allocate an XID while connected to another
* database. Non-runningcheck tests can control most concurrent activity,
* except autovacuum and the isolationtester control connection. Neither
* allocates XIDs, and AbortTransaction() in those would justify test failure.
*/
PG_FUNCTION_INFO_V1(removable_cutoff);
Datum
removable_cutoff(PG_FUNCTION_ARGS)
{
Relation rel = NULL;
TransactionId xid;
FullTransactionId next_fxid_before,
next_fxid;
/* could take other relkinds callee takes, but we've not yet needed it */
if (!PG_ARGISNULL(0))
rel = table_open(PG_GETARG_OID(0), AccessShareLock);
/*
* No lock or snapshot necessarily prevents oldestXid from advancing past
* "xid" while this function runs. That concerns us only in that we must
* not ascribe "xid" to the wrong epoch. (That may never arise in
* isolation testing, but let's set a good example.) As a crude solution,
* retry until nextXid doesn't change.
*/
next_fxid = ReadNextFullTransactionId();
do
{
CHECK_FOR_INTERRUPTS();
next_fxid_before = next_fxid;
xid = GetOldestNonRemovableTransactionId(rel);
next_fxid = ReadNextFullTransactionId();
} while (!FullTransactionIdEquals(next_fxid, next_fxid_before));
if (rel)
table_close(rel, AccessShareLock);
PG_RETURN_FULLTRANSACTIONID(FullTransactionIdFromAllowableAt(next_fxid,
xid));
}

179
src/test/modules/injection_points/specs/syscache-update-pruned.spec Normal file
View File

@ -0,0 +1,179 @@
# Test race conditions involving:
# - s1: heap_update($FROM_SYSCACHE), without a snapshot or pin
# - s2: ALTER TABLE making $FROM_SYSCACHE a dead tuple
# - s3: "VACUUM pg_class" making $FROM_SYSCACHE become LP_UNUSED
# This is a derivative work of inplace.spec, which exercises the corresponding
# race condition for inplace updates.
# Despite local injection points, this is incompatible with runningcheck.
# First, removable_cutoff() could move backward, per its header comment.
# Second, other activity could trigger sinval queue overflow, negating our
# efforts to delay inval. Third, this deadlock emerges:
#
# - step at2 waits at an injection point, with interrupts held
# - an unrelated backend waits for at2 to do PROCSIGNAL_BARRIER_SMGRRELEASE
# - step waitprunable4 waits for the unrelated backend to release its xmin
# The alternative expected output is for -DCATCACHE_FORCE_RELEASE, a setting
# that thwarts testing the race conditions this spec seeks.
# Need s2 to make a non-HOT update. Otherwise, "VACUUM pg_class" would leave
# an LP_REDIRECT that persists. To get non-HOT, make rels so the pg_class row
# for vactest.orig50 is on a filled page (assuming BLCKSZ=8192). Just to save
# on filesystem syscalls, use relkind=c for every other rel.
setup
{
CREATE EXTENSION injection_points;
CREATE SCHEMA vactest;
-- Ensure a leader RELOID catcache entry. PARALLEL RESTRICTED since a
-- parallel worker running pg_relation_filenode() would lack that effect.
CREATE FUNCTION vactest.reloid_catcache_set(regclass) RETURNS int
LANGUAGE sql PARALLEL RESTRICTED
AS 'SELECT 0 FROM pg_relation_filenode($1)';
CREATE FUNCTION vactest.mkrels(text, int, int) RETURNS void
LANGUAGE plpgsql SET search_path = vactest AS $$
DECLARE
tname text;
BEGIN
FOR i in $2 .. $3 LOOP
tname := $1 || i;
EXECUTE FORMAT('CREATE TYPE ' || tname || ' AS ()');
RAISE DEBUG '% at %', tname, ctid
FROM pg_class WHERE oid = tname::regclass;
END LOOP;
END
$$;
CREATE PROCEDURE vactest.wait_prunable() LANGUAGE plpgsql AS $$
DECLARE
barrier xid8;
cutoff xid8;
BEGIN
barrier := pg_current_xact_id();
-- autovacuum worker RelationCacheInitializePhase3() or the
-- isolationtester control connection might hold a snapshot that
-- limits pruning. Sleep until that clears.
LOOP
ROLLBACK; -- release MyProc->xmin, which could be the oldest
cutoff := removable_cutoff('pg_class');
EXIT WHEN cutoff >= barrier;
RAISE LOG 'removable cutoff %; waiting for %', cutoff, barrier;
PERFORM pg_sleep(.1);
END LOOP;
END
$$;
}
setup { CALL vactest.wait_prunable(); -- maximize next two VACUUMs }
setup { VACUUM FULL pg_class; -- reduce free space }
setup { VACUUM FREEZE pg_class; -- populate fsm etc. }
setup
{
SELECT FROM vactest.mkrels('orig', 1, 49);
CREATE TABLE vactest.orig50 (c int) WITH (autovacuum_enabled = off);
CREATE TABLE vactest.child50 (c int) WITH (autovacuum_enabled = off);
SELECT FROM vactest.mkrels('orig', 51, 100);
}
teardown
{
DROP SCHEMA vactest CASCADE;
DROP EXTENSION injection_points;
}
# Wait during GRANT. Disable debug_discard_caches, since we're here to
# exercise an outcome that happens under permissible cache staleness.
session s1
setup {
SET debug_discard_caches = 0;
SELECT FROM injection_points_set_local();
SELECT FROM injection_points_attach('heap_update-before-pin', 'wait');
}
step cachefill1 { SELECT FROM vactest.reloid_catcache_set('vactest.orig50'); }
step grant1 { GRANT SELECT ON vactest.orig50 TO PUBLIC; }
# Update of the tuple that grant1 will update. Wait before sending invals, so
# s1 will not get a cache miss. Choose the commands for making such updates
# from among those whose heavyweight locking does not conflict with GRANT's
# heavyweight locking. (GRANT will see our XID as committed, so observing
# that XID in the tuple xmax also won't block GRANT.)
session s2
setup {
SELECT FROM injection_points_set_local();
SELECT FROM
injection_points_attach('AtEOXact_Inval-with-transInvalInfo', 'wait');
}
step at2 {
CREATE TRIGGER to_set_relhastriggers BEFORE UPDATE ON vactest.orig50
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
}
# Hold snapshot to block pruning.
session s3
step snap3 { BEGIN ISOLATION LEVEL REPEATABLE READ; SELECT; }
step r3 { ROLLBACK; }
# Non-blocking actions.
session s4
step waitprunable4 { CALL vactest.wait_prunable(); }
step vac4 { VACUUM pg_class; }
# Reuse the lp that s1 is waiting to change. I've observed reuse at the 1st
# or 18th CREATE, so create excess.
step mkrels4 {
SELECT FROM vactest.mkrels('intruder', 1, 100); -- repopulate LP_UNUSED
}
step wakegrant4 {
SELECT FROM injection_points_detach('heap_update-before-pin');
SELECT FROM injection_points_wakeup('heap_update-before-pin');
}
step at4 { ALTER TABLE vactest.child50 INHERIT vactest.orig50; }
step wakeinval4 {
SELECT FROM injection_points_detach('AtEOXact_Inval-with-transInvalInfo');
SELECT FROM injection_points_wakeup('AtEOXact_Inval-with-transInvalInfo');
}
# Witness effects of steps at2 and/or at4.
step inspect4 {
SELECT relhastriggers, relhassubclass FROM pg_class
WHERE oid = 'vactest.orig50'::regclass;
}
# TID from syscache becomes LP_UNUSED. Before the bug fix, this permutation
# made s1 fail with "attempted to update invisible tuple" or an assert.
# However, suppose a pd_lsn value such that (pd_lsn.xlogid, pd_lsn.xrecoff)
# passed for (xmin, xmax) with xmin known-committed and xmax known-aborted.
# Persistent page header corruption ensued. For example, s1 overwrote
# pd_lower, pd_upper, and pd_special as though they were t_ctid.
permutation
cachefill1 # reads pg_class tuple T0, xmax invalid
at2 # T0 dead, T1 live
waitprunable4 # T0 prunable
vac4 # T0 becomes LP_UNUSED
grant1 # pauses at heap_update(T0)
wakeinval4(at2) # at2 sends inval message
wakegrant4(grant1) # s1 wakes: "tuple concurrently deleted"
# add mkrels4: LP_UNUSED becomes a different rel's row
permutation
cachefill1 # reads pg_class tuple T0, xmax invalid
at2 # T0 dead, T1 live
waitprunable4 # T0 prunable
vac4 # T0 becomes LP_UNUSED
grant1 # pauses at heap_update(T0)
wakeinval4(at2) # at2 sends inval message
mkrels4 # T0 becomes a new rel
wakegrant4(grant1) # s1 wakes: "duplicate key value violates unique"
# TID from syscache becomes LP_UNUSED, then becomes a newer version of the
# original rel's row.
permutation
snap3 # sets MyProc->xmin
cachefill1 # reads pg_class tuple T0, xmax invalid
at2 # T0 dead, T1 live
mkrels4 # T1's page becomes full
r3 # clears MyProc->xmin
waitprunable4 # T0 prunable
vac4 # T0 becomes LP_UNUSED
grant1 # pauses at heap_update(T0)
wakeinval4(at2) # at2 sends inval message
at4 # T1 dead, T0 live
wakegrant4(grant1) # s1 wakes: T0 dead, T2 live
inspect4 # observe loss of at2+at4 changes XXX is an extant bug