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Commit165d921

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Fix wrong logic in TransactionIdInRecentPast()
The TransactionIdInRecentPast() should return false for all the transactionsolder than TransamVariables->oldestClogXid. However, the function containsa bug in comparison FullTransactionId to TransactionID allowing fulltransactions between nextXid - 2^32 and oldestClogXid - 2^31.This commit fixes TransactionIdInRecentPast() by turning the oldestClogXid intoFullTransactionId first, then performing the comparison.Backpatch to all supported versions.Reported-by: Egor ChindyaskinBug: 18212Discussion:https://postgr.es/m/18212-547307f8adf57262%40postgresql.orgAuthor: Karina LitskevichReviewed-by: Kyotaro HoriguchiBackpatch-through: 12
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‎src/backend/utils/adt/xid8funcs.c

Lines changed: 19 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -98,11 +98,12 @@ StaticAssertDecl(MAX_BACKENDS * 2 <= PG_SNAPSHOT_MAX_NXIP,
9898
staticbool
9999
TransactionIdInRecentPast(FullTransactionIdfxid,TransactionId*extracted_xid)
100100
{
101-
uint32xid_epoch=EpochFromFullTransactionId(fxid);
102101
TransactionIdxid=XidFromFullTransactionId(fxid);
103102
uint32now_epoch;
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TransactionIdnow_epoch_next_xid;
105104
FullTransactionIdnow_fullxid;
105+
TransactionIdoldest_xid;
106+
FullTransactionIdoldest_fxid;
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107108
now_fullxid=ReadNextFullTransactionId();
108109
now_epoch_next_xid=XidFromFullTransactionId(now_fullxid);
@@ -135,17 +136,24 @@ TransactionIdInRecentPast(FullTransactionId fxid, TransactionId *extracted_xid)
135136
Assert(LWLockHeldByMe(XactTruncationLock));
136137

137138
/*
138-
* If the transaction ID has wrapped around, it's definitely too old to
139-
* determine the commit status. Otherwise, we can compare it to
140-
* TransamVariables->oldestClogXid to determine whether the relevant CLOG
141-
* entry is guaranteed to still exist.
139+
* If fxid is not older than TransamVariables->oldestClogXid, the relevant
140+
* CLOG entry is guaranteed to still exist. Convert
141+
* TransamVariables->oldestClogXid into a FullTransactionId to compare it
142+
* with fxid. Determine the right epoch knowing that oldest_fxid
143+
* shouldn't be more than 2^31 older than now_fullxid.
142144
*/
143-
if (xid_epoch+1<now_epoch
144-
|| (xid_epoch+1==now_epoch&&xid<now_epoch_next_xid)
145-
||TransactionIdPrecedes(xid,TransamVariables->oldestClogXid))
146-
return false;
147-
148-
return true;
145+
oldest_xid=TransamVariables->oldestClogXid;
146+
Assert(TransactionIdPrecedesOrEquals(oldest_xid,now_epoch_next_xid));
147+
if (oldest_xid <=now_epoch_next_xid)
148+
{
149+
oldest_fxid=FullTransactionIdFromEpochAndXid(now_epoch,oldest_xid);
150+
}
151+
else
152+
{
153+
Assert(now_epoch>0);
154+
oldest_fxid=FullTransactionIdFromEpochAndXid(now_epoch-1,oldest_xid);
155+
}
156+
return !FullTransactionIdPrecedes(fxid,oldest_fxid);
149157
}
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/*

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