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postgres_fdw: Inherit the local transaction's access/deferrable modes.

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Previously, postgres_fdw always 1) opened a remote transaction in READ
WRITE mode even when the local transaction was READ ONLY, causing a READ
ONLY transaction using it that references a foreign table mapped to a
remote view executing a volatile function to write in the remote side,
and 2) opened the remote transaction in NOT DEFERRABLE mode even when
the local transaction was DEFERRABLE, causing a SERIALIZABLE READ ONLY
DEFERRABLE transaction using it to abort due to a serialization failure
in the remote side.

To avoid these, modify postgres_fdw to open a remote transaction in the
same access/deferrable modes as the local transaction.  This commit also
modifies it to open a remote subtransaction in the same access mode as
the local subtransaction.

Although these issues exist since the introduction of postgres_fdw,
there have been no reports from the field.  So it seems fine to just fix
them in master only.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAPmGK16n_hcUUWuOdmeUS%2Bw4Q6dZvTEDHb%3DOP%3D5JBzo-M3QmpQ%40mail.gmail.com
This commit is contained in:
Etsuro Fujita
2025-06-01 17:30:00 +09:00
parent b006bcd531
commit e5a3c9d9b5
6 changed files with 347 additions and 8 deletions
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28
src/backend/access/transam/xact.c
View File

@ -1044,6 +1044,34 @@ TransactionStartedDuringRecovery(void)
return CurrentTransactionState->startedInRecovery;
}
/*
* GetTopReadOnlyTransactionNestLevel
*
* Note: this will return zero when not inside any transaction or when neither
* a top-level transaction nor subtransactions are read-only, one when the
* top-level transaction is read-only, two when one level of subtransaction is
* read-only, etc.
*
* Note: subtransactions of the topmost read-only transaction are also
* read-only, because they inherit read-only mode from the transaction, and
* thus can't change to read-write mode. See check_transaction_read_only().
*/
int
GetTopReadOnlyTransactionNestLevel(void)
{
TransactionState s = CurrentTransactionState;
if (!XactReadOnly)
return 0;
while (s->nestingLevel > 1)
{
if (!s->prevXactReadOnly)
return s->nestingLevel;
s = s->parent;
}
return s->nestingLevel;
}
/*
* EnterParallelMode
*/

1
src/include/access/xact.h
View File

@ -458,6 +458,7 @@ extern TimestampTz GetCurrentTransactionStopTimestamp(void);
extern void SetCurrentStatementStartTimestamp(void);
extern int GetCurrentTransactionNestLevel(void);
extern bool TransactionIdIsCurrentTransactionId(TransactionId xid);
extern int GetTopReadOnlyTransactionNestLevel(void);
extern void CommandCounterIncrement(void);
extern void ForceSyncCommit(void);
extern void StartTransactionCommand(void);