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Commit0d472b1

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Initialize TransactionState and user ID consistently at transaction start
If a failure happens when a transaction is starting between the momentthe transaction status is changed from TRANS_DEFAULT to TRANS_START andthe moment the current user ID and security context flags are fetchedvia GetUserIdAndSecContext(), or before initializing its basic fields,then those may get reset to incorrect values when the transactionaborts, leaving the session in an inconsistent state.One problem reported is that failing a starting transaction at the firstquery of a session could cause several kinds of system crashes on thefollow-up queries.In order to solve that, move the initialization of the transaction statefields and the call of GetUserIdAndSecContext() in charge of fetchingthe current user ID close to the point where the transaction status isswitched to TRANS_START, where there cannot be any error triggeredin-between, per an idea of Tom Lane. This properly ensures that thecurrent user ID, the security context flags and that the basic fields ofTransactionState remain consistent even if the transaction fails whilestarting.Reported-by: Richard GuoDiagnosed-By: Richard GuoAuthor: Michael PaquierReviewed-by: Tom LaneDiscussion:https://postgr.es/m/CAN_9JTxECSb=pEPcb0a8d+6J+bDcOZ4=DgRo_B7Y5gRHJUM=Rw@mail.gmail.comBackpatch-through: 9.4
1 parente1f2590 commit0d472b1

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lines changed
  • src/backend/access/transam

1 file changed

+26
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‎src/backend/access/transam/xact.c

Lines changed: 26 additions & 22 deletions
Original file line numberDiff line numberDiff line change
@@ -1830,20 +1830,38 @@ StartTransaction(void)
18301830

18311831
Assert(XactTopTransactionId==InvalidTransactionId);
18321832

1833-
/*
1834-
* check the current transaction state
1835-
*/
1836-
if (s->state!=TRANS_DEFAULT)
1837-
elog(WARNING,"StartTransaction while in %s state",
1838-
TransStateAsString(s->state));
1833+
/* check the current transaction state */
1834+
Assert(s->state==TRANS_DEFAULT);
18391835

18401836
/*
1841-
* set the current transaction state information appropriately during
1842-
* start processing
1837+
* Set the current transaction state information appropriately during
1838+
* start processing. Note that once the transaction status is switched
1839+
* this process cannot fail until the user ID and the security context
1840+
* flags are fetched below.
18431841
*/
18441842
s->state=TRANS_START;
18451843
s->transactionId=InvalidTransactionId;/* until assigned */
18461844

1845+
/*
1846+
* initialize current transaction state fields
1847+
*
1848+
* note: prevXactReadOnly is not used at the outermost level
1849+
*/
1850+
s->nestingLevel=1;
1851+
s->gucNestLevel=1;
1852+
s->childXids=NULL;
1853+
s->nChildXids=0;
1854+
s->maxChildXids=0;
1855+
1856+
/*
1857+
* Once the current user ID and the security context flags are fetched,
1858+
* both will be properly reset even if transaction startup fails.
1859+
*/
1860+
GetUserIdAndSecContext(&s->prevUser,&s->prevSecContext);
1861+
1862+
/* SecurityRestrictionContext should never be set outside a transaction */
1863+
Assert(s->prevSecContext==0);
1864+
18471865
/*
18481866
* Make sure we've reset xact state variables
18491867
*
@@ -1924,20 +1942,6 @@ StartTransaction(void)
19241942
xactStopTimestamp=0;
19251943
pgstat_report_xact_timestamp(xactStartTimestamp);
19261944

1927-
/*
1928-
* initialize current transaction state fields
1929-
*
1930-
* note: prevXactReadOnly is not used at the outermost level
1931-
*/
1932-
s->nestingLevel=1;
1933-
s->gucNestLevel=1;
1934-
s->childXids=NULL;
1935-
s->nChildXids=0;
1936-
s->maxChildXids=0;
1937-
GetUserIdAndSecContext(&s->prevUser,&s->prevSecContext);
1938-
/* SecurityRestrictionContext should never be set outside a transaction */
1939-
Assert(s->prevSecContext==0);
1940-
19411945
/*
19421946
* initialize other subsystems for new transaction
19431947
*/

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