*** pgsql/src/backend/access/transam/twophase.c	2009/09/01 04:15:45	1.55
--- pgsql/src/backend/access/transam/twophase.c	2009/11/23 09:58:36	1.56
***************
*** 7,13 ****
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   * IDENTIFICATION
!  *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.54 2009/06/25 19:05:52 heikki Exp $
   *
   * NOTES
   *		Each global transaction is associated with a global transaction
--- 7,13 ----
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   * IDENTIFICATION
!  *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.55 2009/09/01 04:15:45 tgl Exp $
   *
   * NOTES
   *		Each global transaction is associated with a global transaction
*************** int			max_prepared_xacts = 0;
*** 109,114 ****
--- 109,115 ----
  typedef struct GlobalTransactionData
  {
  	PGPROC		proc;			/* dummy proc */
+ 	BackendId	dummyBackendId;	/* similar to backend id for backends */
  	TimestampTz prepared_at;	/* time of preparation */
  	XLogRecPtr	prepare_lsn;	/* XLOG offset of prepare record */
  	Oid			owner;			/* ID of user that executed the xact */
*************** TwoPhaseShmemInit(void)
*** 200,205 ****
--- 201,220 ----
  		{
  			gxacts[i].proc.links.next = (SHM_QUEUE *) TwoPhaseState->freeGXacts;
  			TwoPhaseState->freeGXacts = &gxacts[i];
+ 
+ 			/*
+ 			 * Assign a unique ID for each dummy proc, so that the range of
+ 			 * dummy backend IDs immediately follows the range of normal
+ 			 * backend IDs. We don't dare to assign a real backend ID to
+ 			 * dummy procs, because prepared transactions don't take part in
+ 			 * cache invalidation like a real backend ID would imply, but
+ 			 * having a unique ID for them is nevertheless handy. This
+ 			 * arrangement allows you to allocate an array of size
+ 			 * (MaxBackends + max_prepared_xacts + 1), and have a slot for
+ 			 * every backend and prepared transaction. Currently multixact.c
+ 			 * uses that technique.
+ 			 */
+ 			gxacts[i].dummyBackendId = MaxBackends + 1 + i;
  		}
  	}
  	else
*************** pg_prepared_xact(PG_FUNCTION_ARGS)
*** 648,653 ****
--- 663,684 ----
  }
  
  /*
+  * TwoPhaseGetDummyProc
+  *		Get the dummy backend ID for prepared transaction specified by XID
+  *
+  * Dummy backend IDs are similar to real backend IDs of real backends.
+  * They start at MaxBackends + 1, and are unique across all currently active
+  * real backends and prepared transactions.
+  */
+ BackendId
+ TwoPhaseGetDummyBackendId(TransactionId xid)
+ {
+ 	PGPROC *proc = TwoPhaseGetDummyProc(xid);
+ 
+ 	return ((GlobalTransaction) proc)->dummyBackendId;
+ }
+ 
+ /*
   * TwoPhaseGetDummyProc
   *		Get the PGPROC that represents a prepared transaction specified by XID
   */