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Merge copies of converting an XID to a FullTransactionId.

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Assume twophase.c is the performance-sensitive caller, and preserve its
choice of unlikely() branch hint.  Add some retrospective rationale for
that choice.  Back-patch to v17, for the next commit to use it.

Reviewed (in earlier versions) by Michael Paquier.

Discussion: https://postgr.es/m/17821-dd8c334263399284@postgresql.org
Discussion: https://postgr.es/m/20250116010051.f3.nmisch@google.com
This commit is contained in:
Noah Misch
2025-01-25 11:28:14 -08:00
parent 4f6ec3831d
commit 81772a495e
5 changed files with 77 additions and 93 deletions
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26
src/backend/access/transam/twophase.c
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@@ -929,32 +929,16 @@ TwoPhaseGetDummyProc(TransactionId xid, bool lock_held)
/*
* Compute the FullTransactionId for the given TransactionId.
*
* The wrap logic is safe here because the span of active xids cannot exceed one
* epoch at any given time.
* This is safe if the xid has not yet reached COMMIT PREPARED or ROLLBACK
* PREPARED. After those commands, concurrent vac_truncate_clog() may make
* the xid cease to qualify as allowable. XXX Not all callers limit their
* calls accordingly.
*/
static inline FullTransactionId
AdjustToFullTransactionId(TransactionId xid)
{
FullTransactionId nextFullXid;
TransactionId nextXid;
uint32 epoch;
Assert(TransactionIdIsValid(xid));
LWLockAcquire(XidGenLock, LW_SHARED);
nextFullXid = TransamVariables->nextXid;
LWLockRelease(XidGenLock);
nextXid = XidFromFullTransactionId(nextFullXid);
epoch = EpochFromFullTransactionId(nextFullXid);
if (unlikely(xid > nextXid))
{
/* Wraparound occurred, must be from a prev epoch. */
Assert(epoch > 0);
epoch--;
}
return FullTransactionIdFromEpochAndXid(epoch, xid);
return FullTransactionIdFromAllowableAt(ReadNextFullTransactionId(), xid);
}
static inline int