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Fix TwoPhaseGetDummyBackendId().
This was broken in commit ed0b409, which revised the GlobalTransactionData struct to not include the associated PGPROC as its first member, but overlooked one place where a cast was used in reliance on that equivalence. The most effective way of fixing this seems to be to create a new function that looks up the GlobalTransactionData struct given the XID, and make both TwoPhaseGetDummyBackendId and TwoPhaseGetDummyProc rely on that. Per report from Robert Ross.
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‎src/backend/access/transam/twophase.c

Lines changed: 46 additions & 30 deletions
Original file line numberDiff line numberDiff line change
@@ -108,8 +108,8 @@ int max_prepared_xacts = 0;
108108

109109
typedef struct GlobalTransactionData
110110
{
111-
GlobalTransaction next;
112-
int pgprocno; /* dummy proc */
111+
GlobalTransaction next; /* list link for free list */
112+
int pgprocno; /* ID of associated dummy PGPROC */
113113
BackendId dummyBackendId; /* similar to backend id for backends */
114114
TimestampTz prepared_at; /* time of preparation */
115115
XLogRecPtr prepare_lsn; /* XLOG offset of prepare record */
@@ -203,10 +203,13 @@ TwoPhaseShmemInit(void)
203203
sizeof(GlobalTransaction) * max_prepared_xacts));
204204
for (i = 0; i < max_prepared_xacts; i++)
205205
{
206-
gxacts[i].pgprocno = PreparedXactProcs[i].pgprocno;
206+
/* insert into linked list */
207207
gxacts[i].next = TwoPhaseState->freeGXacts;
208208
TwoPhaseState->freeGXacts = &gxacts[i];
209209

210+
/* associate it with a PGPROC assigned by InitProcGlobal */
211+
gxacts[i].pgprocno = PreparedXactProcs[i].pgprocno;
212+
210213
/*
211214
* Assign a unique ID for each dummy proc, so that the range of
212215
* dummy backend IDs immediately follows the range of normal
@@ -301,7 +304,7 @@ MarkAsPreparing(TransactionId xid, const char *gid,
301304
errhint("Increase max_prepared_transactions (currently %d).",
302305
max_prepared_xacts)));
303306
gxact = TwoPhaseState->freeGXacts;
304-
TwoPhaseState->freeGXacts = (GlobalTransaction) gxact->next;
307+
TwoPhaseState->freeGXacts = gxact->next;
305308

306309
proc = &ProcGlobal->allProcs[gxact->pgprocno];
307310
pgxact = &ProcGlobal->allPgXact[gxact->pgprocno];
@@ -680,40 +683,25 @@ pg_prepared_xact(PG_FUNCTION_ARGS)
680683
}
681684

682685
/*
683-
* TwoPhaseGetDummyProc
684-
* Get the dummy backend ID for prepared transaction specified by XID
685-
*
686-
* Dummy backend IDs are similar to real backend IDs of real backends.
687-
* They start at MaxBackends + 1, and are unique across all currently active
688-
* real backends and prepared transactions.
686+
* TwoPhaseGetGXact
687+
* Get the GlobalTransaction struct for a prepared transaction
688+
* specified by XID
689689
*/
690-
BackendId
691-
TwoPhaseGetDummyBackendId(TransactionId xid)
692-
{
693-
PGPROC *proc = TwoPhaseGetDummyProc(xid);
694-
695-
return ((GlobalTransaction) proc)->dummyBackendId;
696-
}
697-
698-
/*
699-
* TwoPhaseGetDummyProc
700-
* Get the PGPROC that represents a prepared transaction specified by XID
701-
*/
702-
PGPROC *
703-
TwoPhaseGetDummyProc(TransactionId xid)
690+
static GlobalTransaction
691+
TwoPhaseGetGXact(TransactionId xid)
704692
{
705-
PGPROC *result = NULL;
693+
GlobalTransaction result = NULL;
706694
int i;
707695

708696
static TransactionId cached_xid = InvalidTransactionId;
709-
static PGPROC *cached_proc = NULL;
697+
static GlobalTransaction cached_gxact = NULL;
710698

711699
/*
712700
* During a recovery, COMMIT PREPARED, or ABORT PREPARED, we'll be called
713701
* repeatedly for the same XID. We can save work with a simple cache.
714702
*/
715703
if (xid == cached_xid)
716-
return cached_proc;
704+
return cached_gxact;
717705

718706
LWLockAcquire(TwoPhaseStateLock, LW_SHARED);
719707

@@ -724,22 +712,50 @@ TwoPhaseGetDummyProc(TransactionId xid)
724712

725713
if (pgxact->xid == xid)
726714
{
727-
result = &ProcGlobal->allProcs[gxact->pgprocno];
715+
result = gxact;
728716
break;
729717
}
730718
}
731719

732720
LWLockRelease(TwoPhaseStateLock);
733721

734722
if (result == NULL) /* should not happen */
735-
elog(ERROR, "failed to find dummy PGPROC for xid %u", xid);
723+
elog(ERROR, "failed to find GlobalTransaction for xid %u", xid);
736724

737725
cached_xid = xid;
738-
cached_proc = result;
726+
cached_gxact = result;
739727

740728
return result;
741729
}
742730

731+
/*
732+
* TwoPhaseGetDummyProc
733+
* Get the dummy backend ID for prepared transaction specified by XID
734+
*
735+
* Dummy backend IDs are similar to real backend IDs of real backends.
736+
* They start at MaxBackends + 1, and are unique across all currently active
737+
* real backends and prepared transactions.
738+
*/
739+
BackendId
740+
TwoPhaseGetDummyBackendId(TransactionId xid)
741+
{
742+
GlobalTransaction gxact = TwoPhaseGetGXact(xid);
743+
744+
return gxact->dummyBackendId;
745+
}
746+
747+
/*
748+
* TwoPhaseGetDummyProc
749+
* Get the PGPROC that represents a prepared transaction specified by XID
750+
*/
751+
PGPROC *
752+
TwoPhaseGetDummyProc(TransactionId xid)
753+
{
754+
GlobalTransaction gxact = TwoPhaseGetGXact(xid);
755+
756+
return &ProcGlobal->allProcs[gxact->pgprocno];
757+
}
758+
743759
/************************************************************************/
744760
/* State file support */
745761
/************************************************************************/

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