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Commitaa203e7

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Don’t push nextid too far forwards in recovery
Doing so allows various crash possibilities. Fix by avoidinghaving PrescanPreparedTransactions() incrementShmemVariableCache->nextXid when it has no 2PC filesBug found by Jeff Janes, diagnosis and patch by Pavan Deolasee,then patch re-designed for clarity and full accuracy byMichael Paquier.Reported-by: Jeff JanesAuthor: Pavan Deolasee, Michael PaquierDiscussion:https://postgr.es/m/CAMkU=1zMLnH_i1-PVQ-biZzvNx7VcuatriquEnh7HNk6K8Ss3Q@mail.gmail.com
1 parent51175f3 commitaa203e7

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1 file changed

+36
-41
lines changed

1 file changed

+36
-41
lines changed

‎src/backend/access/transam/twophase.c‎

Lines changed: 36 additions & 41 deletions
Original file line numberDiff line numberDiff line change
@@ -222,7 +222,7 @@ static void XlogReadTwoPhaseData(XLogRecPtr lsn, char **buf, int *len);
222222
staticchar*ProcessTwoPhaseBuffer(TransactionIdxid,
223223
XLogRecPtrprepare_start_lsn,
224224
boolfromdisk,booloverwriteOK,boolsetParent,
225-
TransactionId*result,TransactionId*maxsubxid);
225+
boolsetNextXid);
226226
staticvoidMarkAsPreparingGuts(GlobalTransactiongxact,TransactionIdxid,
227227
constchar*gid,TimestampTzprepared_at,Oidowner,
228228
Oiddatabaseid);
@@ -1744,7 +1744,7 @@ restoreTwoPhaseData(void)
17441744

17451745
buf=ProcessTwoPhaseBuffer(xid,InvalidXLogRecPtr,
17461746
true, false, false,
1747-
NULL,NULL);
1747+
false);
17481748
if (buf==NULL)
17491749
continue;
17501750

@@ -1786,7 +1786,6 @@ PrescanPreparedTransactions(TransactionId **xids_p, int *nxids_p)
17861786
{
17871787
TransactionIdorigNextXid=ShmemVariableCache->nextXid;
17881788
TransactionIdresult=origNextXid;
1789-
TransactionIdmaxsubxid=origNextXid;
17901789
TransactionId*xids=NULL;
17911790
intnxids=0;
17921791
intallocsize=0;
@@ -1806,11 +1805,18 @@ PrescanPreparedTransactions(TransactionId **xids_p, int *nxids_p)
18061805
buf=ProcessTwoPhaseBuffer(xid,
18071806
gxact->prepare_start_lsn,
18081807
gxact->ondisk, false, false,
1809-
&result,&maxsubxid);
1808+
true);
18101809

18111810
if (buf==NULL)
18121811
continue;
18131812

1813+
/*
1814+
* OK, we think this file is valid. Incorporate xid into the
1815+
* running-minimum result.
1816+
*/
1817+
if (TransactionIdPrecedes(xid,result))
1818+
result=xid;
1819+
18141820
if (xids_p)
18151821
{
18161822
if (nxids==allocsize)
@@ -1839,15 +1845,6 @@ PrescanPreparedTransactions(TransactionId **xids_p, int *nxids_p)
18391845
*nxids_p=nxids;
18401846
}
18411847

1842-
/* update nextXid if needed */
1843-
if (TransactionIdFollowsOrEquals(maxsubxid,ShmemVariableCache->nextXid))
1844-
{
1845-
LWLockAcquire(XidGenLock,LW_EXCLUSIVE);
1846-
ShmemVariableCache->nextXid=maxsubxid;
1847-
TransactionIdAdvance(ShmemVariableCache->nextXid);
1848-
LWLockRelease(XidGenLock);
1849-
}
1850-
18511848
returnresult;
18521849
}
18531850

@@ -1884,7 +1881,7 @@ StandbyRecoverPreparedTransactions(bool overwriteOK)
18841881
buf=ProcessTwoPhaseBuffer(xid,
18851882
gxact->prepare_start_lsn,
18861883
gxact->ondisk,overwriteOK, true,
1887-
NULL,NULL);
1884+
false);
18881885
if (buf!=NULL)
18891886
pfree(buf);
18901887
}
@@ -1924,7 +1921,7 @@ RecoverPreparedTransactions(void)
19241921
buf=ProcessTwoPhaseBuffer(xid,
19251922
gxact->prepare_start_lsn,
19261923
gxact->ondisk, false, false,
1927-
NULL,NULL);
1924+
false);
19281925
if (buf==NULL)
19291926
continue;
19301927

@@ -2012,20 +2009,16 @@ RecoverPreparedTransactions(void)
20122009
* If setParent is true, then use the overwriteOK parameter to set up
20132010
* subtransaction parent linkages.
20142011
*
2015-
* If result and maxsubxid are not NULL, fill them up with smallest
2016-
* running transaction id (lesser than ShmemVariableCache->nextXid)
2017-
* and largest subtransaction id for this transaction respectively.
2012+
* If setNextXid is true, set ShmemVariableCache->nextXid to the newest
2013+
* value scanned.
20182014
*/
20192015
staticchar*
20202016
ProcessTwoPhaseBuffer(TransactionIdxid,
20212017
XLogRecPtrprepare_start_lsn,
20222018
boolfromdisk,booloverwriteOK,
2023-
boolsetParent,TransactionId*result,
2024-
TransactionId*maxsubxid)
2019+
boolsetParent,boolsetNextXid)
20252020
{
20262021
TransactionIdorigNextXid=ShmemVariableCache->nextXid;
2027-
TransactionIdres=InvalidTransactionId;
2028-
TransactionIdmaxsub=InvalidTransactionId;
20292022
TransactionId*subxids;
20302023
char*buf;
20312024
TwoPhaseFileHeader*hdr;
@@ -2034,11 +2027,6 @@ ProcessTwoPhaseBuffer(TransactionId xid,
20342027
if (!fromdisk)
20352028
Assert(prepare_start_lsn!=InvalidXLogRecPtr);
20362029

2037-
if (result)
2038-
res=*result;
2039-
if (maxsubxid)
2040-
maxsub=*maxsubxid;
2041-
20422030
/* Already processed? */
20432031
if (TransactionIdDidCommit(xid)||TransactionIdDidAbort(xid))
20442032
{
@@ -2120,13 +2108,6 @@ ProcessTwoPhaseBuffer(TransactionId xid,
21202108
returnNULL;
21212109
}
21222110

2123-
/*
2124-
* OK, we think this buffer is valid. Incorporate xid into the
2125-
* running-minimum result.
2126-
*/
2127-
if (TransactionIdPrecedes(xid,res))
2128-
res=xid;
2129-
21302111
/*
21312112
* Examine subtransaction XIDs ... they should all follow main
21322113
* XID, and they may force us to advance nextXid.
@@ -2139,17 +2120,31 @@ ProcessTwoPhaseBuffer(TransactionId xid,
21392120
TransactionIdsubxid=subxids[i];
21402121

21412122
Assert(TransactionIdFollows(subxid,xid));
2142-
if (TransactionIdFollowsOrEquals(subxid,maxsub))
2143-
maxsub=subxid;
2123+
2124+
/* update nextXid if needed */
2125+
if (setNextXid&&
2126+
TransactionIdFollowsOrEquals(subxid,
2127+
ShmemVariableCache->nextXid))
2128+
{
2129+
/*
2130+
* We don't expect anyone else to modify nextXid, hence we don't
2131+
* need to hold a lock while examining it. We still acquire the
2132+
* lock to modify it, though, so we recheck.
2133+
*/
2134+
LWLockAcquire(XidGenLock,LW_EXCLUSIVE);
2135+
if (TransactionIdFollowsOrEquals(subxid,
2136+
ShmemVariableCache->nextXid))
2137+
{
2138+
ShmemVariableCache->nextXid=subxid;
2139+
TransactionIdAdvance(ShmemVariableCache->nextXid);
2140+
}
2141+
LWLockRelease(XidGenLock);
2142+
}
2143+
21442144
if (setParent)
21452145
SubTransSetParent(xid,subxid,overwriteOK);
21462146
}
21472147

2148-
if (result)
2149-
*result=res;
2150-
if (maxsubxid)
2151-
*maxsubxid=maxsub;
2152-
21532148
returnbuf;
21542149
}
21552150

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