Introduce a testing GUC, to burn through XIDs more quickly.

This makes it faster to reach XID wraparound. I haven't written any
automated tests for that, but at least this makes manual testing easier.

src/backend/access/transam/varsup.c

@@ -186,6 +186,28 @@ GetNewTransactionId(bool isSubXact)
 	 */
 	TransactionIdAdvance(ShmemVariableCache->nextXid);
 
+	/*
+	 * To aid testing, you can set the debug_burn_xids GUC, to consume XIDs
+	 * faster. If set, we bump the XID counter to the next value divisible by
+	 * 1024, minus one. The idea is to skip over "boring" XID ranges, but
+	 * still step through XID wraparound, CLOG page boundaries etc. one XID
+	 * at a time.
+	 */
+	if (Debug_burn_xids)
+	{
+		TransactionId xx;
+		uint32		r;
+
+		xx = ShmemVariableCache->nextXid;
+
+		r = xx % 1024;
+		if (r > 1 && r < 1023)
+		{
+			xx += 1024 - r - 1;
+			ShmemVariableCache->nextXid = xx;
+		}
+	}
+
 	/*
 	 * We must store the new XID into the shared ProcArray before releasing
 	 * XidGenLock.	This ensures that every active XID older than

src/backend/cdb/cdbvars.c

@@ -75,6 +75,8 @@ bool		Debug_print_slice_table;	/* Shall we log the slice table? */
 
 bool		Debug_resource_group;	/* Shall we log the resource group? */
 
+bool		Debug_burn_xids;
+
 bool		gp_external_enable_exec = true; /* allow ext tables with EXECUTE */
 
 int			gp_external_max_segs;	/* max segdbs per gpfdist/gpfdists URI */

src/backend/utils/misc/guc_gp.c

@@ -1130,6 +1130,16 @@ struct config_bool ConfigureNamesBool_gp[] =
 		false, NULL, NULL
 	},
 
+	{
+		{"debug_burn_xids", PGC_USERSET, DEVELOPER_OPTIONS,
+			gettext_noop("Consume a lot of XIDs, for testing purposes."),
+			NULL,
+			GUC_NO_SHOW_ALL | GUC_NOT_IN_SAMPLE
+		},
+		&Debug_burn_xids,
+		false, NULL, NULL
+	},
+
 	{
 		{"gp_cost_hashjoin_chainwalk", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Enable the cost for walking the chain in the hash join"),

src/include/cdb/cdbvars.h

@@ -239,6 +239,8 @@ extern bool Debug_print_slice_table;
 
 extern bool Debug_resource_group;
 
+extern bool Debug_burn_xids;
+
 /*
  * gp_external_enable_exec
  *