Tom Lane wrote:
> BTW, I did a quick look at all the uses of RecentGlobalXmin in the back
> branches, and I think we might be all right before 8.2. The older
> branches do in fact init RecentGlobalXmin to InvalidTransactionId,
> and the only thing they use it for is "is this tuple dead to everyone"
> tests. Since InvalidTransactionId compares older than anything else,
> the only consequence of not having set it is overly-conservative
> decisions not to mark tuples killed. So unless you see a problem I
> missed, I think we only have an issue-worth-fixing in 8.2 and 8.3.
Actually, 8.2 initializes it to InvalidTransactionId too, so it doesn't
seem like there's a problem there either. Since the problem stems from
using it as initializer for the Min() calculation, there's no actual bug
on 8.2 either. The bug only appeared on 8.3 when the initializer was
changed. And given that there's no HOT in 8.2, then there's no danger
of misusing it in page pruning either.
Still, I produced a patch to 8.2, but given that I'm not sure if it's
worth applying.
--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.
Index: src/backend/commands/vacuum.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/commands/vacuum.c,v
retrieving revision 1.342.2.5
diff -c -p -r1.342.2.5 vacuum.c
*** src/backend/commands/vacuum.c 11 Feb 2008 19:14:38 -0000 1.342.2.5
--- src/backend/commands/vacuum.c 11 Sep 2008 01:08:10 -0000
*************** vac_update_datfrozenxid(void)
*** 764,777 ****
bool dirty = false;
/*
! * Initialize the "min" calculation with RecentGlobalXmin. Any
! * not-yet-committed pg_class entries for new tables must have
! * relfrozenxid at least this high, because any other open xact must have
! * RecentXmin >= its PGPROC.xmin >= our RecentGlobalXmin; see
! * AddNewRelationTuple(). So we cannot produce a wrong minimum by
! * starting with this.
*/
! newFrozenXid = RecentGlobalXmin;
/*
* We must seqscan pg_class to find the minimum Xid, because there is no
--- 764,775 ----
bool dirty = false;
/*
! * Initialize the "min" calculation with GetOldestXmin, which is a
! * reasonable approximation to the minimum relfrozenxid for not-yet-
! * committed pg_class entries for new tables; see AddNewRelationTuple().
! * Se we cannot produce a wrong minimum by starting with this.
*/
! newFrozenXid = GetOldestXmin(true, true);
/*
* We must seqscan pg_class to find the minimum Xid, because there is no
*************** vacuum_rel(Oid relid, VacuumStmt *vacstm
*** 965,982 ****
/* Begin a transaction for vacuuming this relation */
StartTransactionCommand();
! if (vacstmt->full)
! {
! /* functions in indexes may want a snapshot set */
! ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
! }
! else
{
/*
! * During a lazy VACUUM we do not run any user-supplied functions, and
! * so it should be safe to not create a transaction snapshot.
! *
! * We can furthermore set the inVacuum flag, which lets other
* concurrent VACUUMs know that they can ignore this one while
* determining their OldestXmin. (The reason we don't set inVacuum
* during a full VACUUM is exactly that we may have to run user-
--- 963,978 ----
/* Begin a transaction for vacuuming this relation */
StartTransactionCommand();
! /*
! * Functions in indexes may want a snapshot set. Also, setting
! * a snapshot ensures that RecentGlobalXmin is kept truly recent.
! */
! ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
!
! if (!vacstmt->full)
{
/*
! * During a lazy VACUUM we can set the inVacuum flag, which lets other
* concurrent VACUUMs know that they can ignore this one while
* determining their OldestXmin. (The reason we don't set inVacuum
* during a full VACUUM is exactly that we may have to run user-
Index: src/backend/utils/init/postinit.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/utils/init/postinit.c,v
retrieving revision 1.172
diff -c -p -r1.172 postinit.c
*** src/backend/utils/init/postinit.c 5 Nov 2006 22:42:09 -0000 1.172
--- src/backend/utils/init/postinit.c 10 Sep 2008 23:43:45 -0000
***************
*** 42,47 ****
--- 42,48 ----
#include "utils/guc.h"
#include "utils/portal.h"
#include "utils/relcache.h"
+ #include "utils/tqual.h"
#include "utils/syscache.h"
#include "pgstat.h"
*************** InitPostgres(const char *dbname, const c
*** 380,389 ****
on_shmem_exit(ShutdownPostgres, 0);
/*
! * Start a new transaction here before first access to db
*/
if (!bootstrap)
StartTransactionCommand();
/*
* Now that we have a transaction, we can take locks. Take a writer's
--- 381,395 ----
on_shmem_exit(ShutdownPostgres, 0);
/*
! * Start a new transaction here before first access to db, and get a
! * snapshot. We don't have a use for the snapshot itself, but we're
! * interested in the secondary effect that it sets RecentGlobalXmin.
*/
if (!bootstrap)
+ {
StartTransactionCommand();
+ (void) GetTransactionSnapshot();
+ }
/*
* Now that we have a transaction, we can take locks. Take a writer's
Index: src/backend/access/heap/heapam.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/access/heap/heapam.c,v
retrieving revision 1.249.2.2
diff -c -p -r1.249.2.2 heapam.c
*** src/backend/access/heap/heapam.c 8 Mar 2008 21:58:07 -0000 1.249.2.2
--- src/backend/access/heap/heapam.c 10 Sep 2008 23:18:00 -0000
*************** heapgetpage(HeapScanDesc scan, BlockNumb
*** 212,217 ****
--- 212,218 ----
/*
* Prune and repair fragmentation for the whole page, if possible.
*/
+ Assert(TransactionIdIsValid(RecentGlobalXmin));
heap_page_prune_opt(scan->rs_rd, buffer, RecentGlobalXmin);
/*
*************** heap_hot_search_buffer(ItemPointer tid,
*** 1488,1493 ****
--- 1489,1496 ----
if (all_dead)
*all_dead = true;
+ Assert(TransactionIdIsValid(RecentGlobalXmin));
+
Assert(ItemPointerGetBlockNumber(tid) == BufferGetBlockNumber(buffer));
offnum = ItemPointerGetOffsetNumber(tid);
at_chain_start = true;
Index: src/backend/access/index/indexam.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/access/index/indexam.c,v
retrieving revision 1.101
diff -c -p -r1.101 indexam.c
*** src/backend/access/index/indexam.c 1 Jan 2008 19:45:46 -0000 1.101
--- src/backend/access/index/indexam.c 10 Sep 2008 23:18:00 -0000
*************** index_getnext(IndexScanDesc scan, ScanDi
*** 419,424 ****
--- 419,426 ----
SCAN_CHECKS;
GET_SCAN_PROCEDURE(amgettuple);
+ Assert(TransactionIdIsValid(RecentGlobalXmin));
+
/*
* We always reset xs_hot_dead; if we are here then either we are just
* starting the scan, or we previously returned a visible tuple, and in
Index: src/backend/commands/vacuum.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/commands/vacuum.c,v
retrieving revision 1.364.2.1
diff -c -p -r1.364.2.1 vacuum.c
*** src/backend/commands/vacuum.c 14 Mar 2008 17:26:00 -0000 1.364.2.1
--- src/backend/commands/vacuum.c 10 Sep 2008 23:21:34 -0000
*************** vac_update_datfrozenxid(void)
*** 781,794 ****
bool dirty = false;
/*
! * Initialize the "min" calculation with RecentGlobalXmin. Any
! * not-yet-committed pg_class entries for new tables must have
! * relfrozenxid at least this high, because any other open xact must have
! * RecentXmin >= its PGPROC.xmin >= our RecentGlobalXmin; see
! * AddNewRelationTuple(). So we cannot produce a wrong minimum by
! * starting with this.
*/
! newFrozenXid = RecentGlobalXmin;
/*
* We must seqscan pg_class to find the minimum Xid, because there is no
--- 781,792 ----
bool dirty = false;
/*
! * Initialize the "min" calculation with GetOldestXmin, which is a
! * reasonable approximation to the minimum relfrozenxid for not-yet-
! * committed pg_class entries for new tables; see AddNewRelationTuple().
! * Se we cannot produce a wrong minimum by starting with this.
*/
! newFrozenXid = GetOldestXmin(true, true);
/*
* We must seqscan pg_class to find the minimum Xid, because there is no
*************** vacuum_rel(Oid relid, VacuumStmt *vacstm
*** 982,999 ****
/* Begin a transaction for vacuuming this relation */
StartTransactionCommand();
! if (vacstmt->full)
! {
! /* functions in indexes may want a snapshot set */
! ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
! }
! else
{
/*
! * During a lazy VACUUM we do not run any user-supplied functions, and
! * so it should be safe to not create a transaction snapshot.
! *
! * We can furthermore set the PROC_IN_VACUUM flag, which lets other
* concurrent VACUUMs know that they can ignore this one while
* determining their OldestXmin. (The reason we don't set it during a
* full VACUUM is exactly that we may have to run user- defined
--- 980,995 ----
/* Begin a transaction for vacuuming this relation */
StartTransactionCommand();
! /*
! * Functions in indexes may want a snapshot set. Also, setting
! * a snapshot ensures that RecentGlobalXmin is kept truly recent.
! */
! ActiveSnapshot = CopySnapshot(GetTransactionSnapshot());
!
! if (!vacstmt->full)
{
/*
! * During a lazy VACUUM we can set the PROC_IN_VACUUM flag, which lets other
* concurrent VACUUMs know that they can ignore this one while
* determining their OldestXmin. (The reason we don't set it during a
* full VACUUM is exactly that we may have to run user- defined
Index: src/backend/executor/nodeBitmapHeapscan.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/executor/nodeBitmapHeapscan.c,v
retrieving revision 1.22
diff -c -p -r1.22 nodeBitmapHeapscan.c
*** src/backend/executor/nodeBitmapHeapscan.c 1 Jan 2008 19:45:49 -0000 1.22
--- src/backend/executor/nodeBitmapHeapscan.c 10 Sep 2008 23:22:26 -0000
***************
*** 36,41 ****
--- 36,42 ----
#include "postgres.h"
#include "access/heapam.h"
+ #include "access/transam.h"
#include "executor/execdebug.h"
#include "executor/nodeBitmapHeapscan.h"
#include "pgstat.h"
*************** bitgetpage(HeapScanDesc scan, TBMIterate
*** 258,263 ****
--- 259,265 ----
/*
* Prune and repair fragmentation for the whole page, if possible.
*/
+ Assert(TransactionIdIsValid(RecentGlobalXmin));
heap_page_prune_opt(scan->rs_rd, buffer, RecentGlobalXmin);
/*
Index: src/backend/utils/init/postinit.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/utils/init/postinit.c,v
retrieving revision 1.180
diff -c -p -r1.180 postinit.c
*** src/backend/utils/init/postinit.c 1 Jan 2008 19:45:53 -0000 1.180
--- src/backend/utils/init/postinit.c 10 Sep 2008 23:25:26 -0000
***************
*** 44,49 ****
--- 44,50 ----
#include "utils/plancache.h"
#include "utils/portal.h"
#include "utils/relcache.h"
+ #include "utils/tqual.h"
#include "utils/syscache.h"
*************** InitPostgres(const char *in_dbname, Oid
*** 457,466 ****
on_shmem_exit(ShutdownPostgres, 0);
/*
! * Start a new transaction here before first access to db
*/
if (!bootstrap)
StartTransactionCommand();
/*
* Now that we have a transaction, we can take locks. Take a writer's
--- 458,472 ----
on_shmem_exit(ShutdownPostgres, 0);
/*
! * Start a new transaction here before first access to db, and get a
! * snapshot. We don't have a use for the snapshot itself, but we're
! * interested in the secondary effect that it sets RecentGlobalXmin.
*/
if (!bootstrap)
+ {
StartTransactionCommand();
+ (void) GetTransactionSnapshot();
+ }
/*
* Now that we have a transaction, we can take locks. Take a writer's
Index: src/backend/utils/time/tqual.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/utils/time/tqual.c,v
retrieving revision 1.109
diff -c -p -r1.109 tqual.c
*** src/backend/utils/time/tqual.c 1 Jan 2008 19:45:55 -0000 1.109
--- src/backend/utils/time/tqual.c 10 Sep 2008 23:22:59 -0000
*************** Snapshot ActiveSnapshot = NULL;
*** 75,84 ****
* These are updated by GetSnapshotData. We initialize them this way
* for the convenience of TransactionIdIsInProgress: even in bootstrap
* mode, we don't want it to say that BootstrapTransactionId is in progress.
*/
TransactionId TransactionXmin = FirstNormalTransactionId;
TransactionId RecentXmin = FirstNormalTransactionId;
! TransactionId RecentGlobalXmin = FirstNormalTransactionId;
/* local functions */
static bool XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot);
--- 75,88 ----
* These are updated by GetSnapshotData. We initialize them this way
* for the convenience of TransactionIdIsInProgress: even in bootstrap
* mode, we don't want it to say that BootstrapTransactionId is in progress.
+ *
+ * RecentGlobalXmin is initialized to InvalidTransactionId, to ensure that no
+ * one tries to use a stale value. Readers should ensure that it has been set
+ * to something else before using it.
*/
TransactionId TransactionXmin = FirstNormalTransactionId;
TransactionId RecentXmin = FirstNormalTransactionId;
! TransactionId RecentGlobalXmin = InvalidTransactionId;
/* local functions */
static bool XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot);
Index: src/backend/access/heap/heapam.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/access/heap/heapam.c,v
retrieving revision 1.262
diff -c -p -r1.262 heapam.c
*** src/backend/access/heap/heapam.c 11 Aug 2008 11:05:10 -0000 1.262
--- src/backend/access/heap/heapam.c 10 Sep 2008 19:50:36 -0000
*************** heapgetpage(HeapScanDesc scan, BlockNumb
*** 219,224 ****
--- 219,225 ----
/*
* Prune and repair fragmentation for the whole page, if possible.
*/
+ Assert(TransactionIdIsValid(RecentGlobalXmin));
heap_page_prune_opt(scan->rs_rd, buffer, RecentGlobalXmin);
/*
*************** heap_hot_search_buffer(ItemPointer tid,
*** 1469,1474 ****
--- 1470,1477 ----
if (all_dead)
*all_dead = true;
+ Assert(TransactionIdIsValid(RecentGlobalXmin));
+
Assert(ItemPointerGetBlockNumber(tid) == BufferGetBlockNumber(buffer));
offnum = ItemPointerGetOffsetNumber(tid);
at_chain_start = true;
Index: src/backend/access/index/indexam.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/access/index/indexam.c,v
retrieving revision 1.109
diff -c -p -r1.109 indexam.c
*** src/backend/access/index/indexam.c 19 Jun 2008 00:46:03 -0000 1.109
--- src/backend/access/index/indexam.c 10 Sep 2008 19:51:38 -0000
*************** index_getnext(IndexScanDesc scan, ScanDi
*** 419,424 ****
--- 419,426 ----
SCAN_CHECKS;
GET_SCAN_PROCEDURE(amgettuple);
+ Assert(TransactionIdIsValid(RecentGlobalXmin));
+
/*
* We always reset xs_hot_dead; if we are here then either we are just
* starting the scan, or we previously returned a visible tuple, and in
Index: src/backend/commands/vacuum.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/commands/vacuum.c,v
retrieving revision 1.376
diff -c -p -r1.376 vacuum.c
*** src/backend/commands/vacuum.c 13 Aug 2008 00:07:50 -0000 1.376
--- src/backend/commands/vacuum.c 10 Sep 2008 20:17:32 -0000
*************** vac_update_datfrozenxid(void)
*** 790,803 ****
bool dirty = false;
/*
! * Initialize the "min" calculation with RecentGlobalXmin. Any
! * not-yet-committed pg_class entries for new tables must have
! * relfrozenxid at least this high, because any other open xact must have
! * RecentXmin >= its PGPROC.xmin >= our RecentGlobalXmin; see
! * AddNewRelationTuple(). So we cannot produce a wrong minimum by
! * starting with this.
*/
! newFrozenXid = RecentGlobalXmin;
/*
* We must seqscan pg_class to find the minimum Xid, because there is no
--- 790,801 ----
bool dirty = false;
/*
! * Initialize the "min" calculation with GetOldestXmin, which is a
! * reasonable approximation to the minimum relfrozenxid for not-yet-
! * committed pg_class entries for new tables; see AddNewRelationTuple().
! * Se we cannot produce a wrong minimum by starting with this.
*/
! newFrozenXid = GetOldestXmin(true, true);
/*
* We must seqscan pg_class to find the minimum Xid, because there is no
*************** vacuum_rel(Oid relid, VacuumStmt *vacstm
*** 990,1007 ****
/* Begin a transaction for vacuuming this relation */
StartTransactionCommand();
! if (vacstmt->full)
! {
! /* functions in indexes may want a snapshot set */
! PushActiveSnapshot(GetTransactionSnapshot());
! }
! else
{
/*
! * During a lazy VACUUM we do not run any user-supplied functions, and
! * so it should be safe to not create a transaction snapshot.
! *
! * We can furthermore set the PROC_IN_VACUUM flag, which lets other
* concurrent VACUUMs know that they can ignore this one while
* determining their OldestXmin. (The reason we don't set it during a
* full VACUUM is exactly that we may have to run user- defined
--- 988,1003 ----
/* Begin a transaction for vacuuming this relation */
StartTransactionCommand();
! /*
! * Functions in indexes may want a snapshot set. Also, setting
! * a snapshot ensures that RecentGlobalXmin is kept truly recent.
! */
! PushActiveSnapshot(GetTransactionSnapshot());
!
! if (!vacstmt->full)
{
/*
! * In lazy vacuum, we can set the PROC_IN_VACUUM flag, which lets other
* concurrent VACUUMs know that they can ignore this one while
* determining their OldestXmin. (The reason we don't set it during a
* full VACUUM is exactly that we may have to run user- defined
*************** vacuum_rel(Oid relid, VacuumStmt *vacstm
*** 1050,1057 ****
if (!onerel)
{
! if (vacstmt->full)
! PopActiveSnapshot();
CommitTransactionCommand();
return;
}
--- 1046,1052 ----
if (!onerel)
{
! PopActiveSnapshot();
CommitTransactionCommand();
return;
}
*************** vacuum_rel(Oid relid, VacuumStmt *vacstm
*** 1082,1089 ****
(errmsg("skipping \"%s\" --- only table or database owner can vacuum it",
RelationGetRelationName(onerel))));
relation_close(onerel, lmode);
! if (vacstmt->full)
! PopActiveSnapshot();
CommitTransactionCommand();
return;
}
--- 1077,1083 ----
(errmsg("skipping \"%s\" --- only table or database owner can vacuum it",
RelationGetRelationName(onerel))));
relation_close(onerel, lmode);
! PopActiveSnapshot();
CommitTransactionCommand();
return;
}
*************** vacuum_rel(Oid relid, VacuumStmt *vacstm
*** 1099,1106 ****
(errmsg("skipping \"%s\" --- cannot vacuum indexes, views, or special system tables",
RelationGetRelationName(onerel))));
relation_close(onerel, lmode);
! if (vacstmt->full)
! PopActiveSnapshot();
CommitTransactionCommand();
return;
}
--- 1093,1099 ----
(errmsg("skipping \"%s\" --- cannot vacuum indexes, views, or special system tables",
RelationGetRelationName(onerel))));
relation_close(onerel, lmode);
! PopActiveSnapshot();
CommitTransactionCommand();
return;
}
*************** vacuum_rel(Oid relid, VacuumStmt *vacstm
*** 1115,1122 ****
if (isOtherTempNamespace(RelationGetNamespace(onerel)))
{
relation_close(onerel, lmode);
! if (vacstmt->full)
! PopActiveSnapshot();
CommitTransactionCommand();
return;
}
--- 1108,1114 ----
if (isOtherTempNamespace(RelationGetNamespace(onerel)))
{
relation_close(onerel, lmode);
! PopActiveSnapshot();
CommitTransactionCommand();
return;
}
*************** vacuum_rel(Oid relid, VacuumStmt *vacstm
*** 1168,1175 ****
/*
* Complete the transaction and free all temporary memory used.
*/
! if (vacstmt->full)
! PopActiveSnapshot();
CommitTransactionCommand();
/*
--- 1160,1166 ----
/*
* Complete the transaction and free all temporary memory used.
*/
! PopActiveSnapshot();
CommitTransactionCommand();
/*
Index: src/backend/executor/nodeBitmapHeapscan.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/executor/nodeBitmapHeapscan.c,v
retrieving revision 1.29
diff -c -p -r1.29 nodeBitmapHeapscan.c
*** src/backend/executor/nodeBitmapHeapscan.c 19 Jun 2008 00:46:04 -0000 1.29
--- src/backend/executor/nodeBitmapHeapscan.c 10 Sep 2008 20:18:52 -0000
***************
*** 37,42 ****
--- 37,43 ----
#include "access/heapam.h"
#include "access/relscan.h"
+ #include "access/transam.h"
#include "executor/execdebug.h"
#include "executor/nodeBitmapHeapscan.h"
#include "pgstat.h"
*************** bitgetpage(HeapScanDesc scan, TBMIterate
*** 262,267 ****
--- 263,269 ----
/*
* Prune and repair fragmentation for the whole page, if possible.
*/
+ Assert(TransactionIdIsValid(RecentGlobalXmin));
heap_page_prune_opt(scan->rs_rd, buffer, RecentGlobalXmin);
/*
Index: src/backend/utils/init/postinit.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/utils/init/postinit.c,v
retrieving revision 1.184
diff -c -p -r1.184 postinit.c
*** src/backend/utils/init/postinit.c 12 May 2008 00:00:52 -0000 1.184
--- src/backend/utils/init/postinit.c 10 Sep 2008 20:19:56 -0000
***************
*** 47,52 ****
--- 47,53 ----
#include "utils/plancache.h"
#include "utils/portal.h"
#include "utils/relcache.h"
+ #include "utils/snapmgr.h"
#include "utils/syscache.h"
#include "utils/tqual.h"
*************** InitPostgres(const char *in_dbname, Oid
*** 461,470 ****
on_shmem_exit(ShutdownPostgres, 0);
/*
! * Start a new transaction here before first access to db
*/
if (!bootstrap)
StartTransactionCommand();
/*
* Now that we have a transaction, we can take locks. Take a writer's
--- 462,476 ----
on_shmem_exit(ShutdownPostgres, 0);
/*
! * Start a new transaction here before first access to db, and get a
! * snapshot. We don't have a use for the snapshot itself, but we're
! * interested in the secondary effect that it sets RecentGlobalXmin.
*/
if (!bootstrap)
+ {
StartTransactionCommand();
+ (void) GetTransactionSnapshot();
+ }
/*
* Now that we have a transaction, we can take locks. Take a writer's
Index: src/backend/utils/time/snapmgr.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/utils/time/snapmgr.c,v
retrieving revision 1.4
diff -c -p -r1.4 snapmgr.c
*** src/backend/utils/time/snapmgr.c 11 Jul 2008 02:10:14 -0000 1.4
--- src/backend/utils/time/snapmgr.c 10 Sep 2008 19:49:09 -0000
*************** static Snapshot SecondarySnapshot = NULL
*** 59,68 ****
* These are updated by GetSnapshotData. We initialize them this way
* for the convenience of TransactionIdIsInProgress: even in bootstrap
* mode, we don't want it to say that BootstrapTransactionId is in progress.
*/
TransactionId TransactionXmin = FirstNormalTransactionId;
TransactionId RecentXmin = FirstNormalTransactionId;
! TransactionId RecentGlobalXmin = FirstNormalTransactionId;
/*
* Elements of the list of registered snapshots.
--- 59,72 ----
* These are updated by GetSnapshotData. We initialize them this way
* for the convenience of TransactionIdIsInProgress: even in bootstrap
* mode, we don't want it to say that BootstrapTransactionId is in progress.
+ *
+ * RecentGlobalXmin is initialized to InvalidTransactionId, to ensure that no
+ * one tries to use a stale value. Readers should ensure that it has been set
+ * to something else before using it.
*/
TransactionId TransactionXmin = FirstNormalTransactionId;
TransactionId RecentXmin = FirstNormalTransactionId;
! TransactionId RecentGlobalXmin = InvalidTransactionId;
/*
* Elements of the list of registered snapshots.
--
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