Defend against bad trigger definitions in contrib/lo's lo_manage() trigger.

This function formerly crashed if called as a statement-level trigger,
or if a column-name argument wasn't given.

In passing, add the trigger name to all error messages from the function.
(None of them are expected cases, so this shouldn't pose any compatibility
risk.)

Marc Cousin, reviewed by Sawada Masahiko

contrib/lo/lo.c

@@ -40,7 +40,12 @@ lo_manage(PG_FUNCTION_ARGS)
 	HeapTuple	trigtuple;		/* The original value of tuple	*/
 
 	if (!CALLED_AS_TRIGGER(fcinfo))		/* internal error */
-		elog(ERROR, "not fired by trigger manager");
+		elog(ERROR, "%s: not fired by trigger manager",
+			 trigdata->tg_trigger->tgname);
+
+	if (!TRIGGER_FIRED_FOR_ROW(trigdata->tg_event))		/* internal error */
+		elog(ERROR, "%s: must be fired for row",
+			 trigdata->tg_trigger->tgname);
 
 	/*
 	 * Fetch some values from trigdata
@@ -50,6 +55,10 @@ lo_manage(PG_FUNCTION_ARGS)
 	tupdesc = trigdata->tg_relation->rd_att;
 	args = trigdata->tg_trigger->tgargs;
 
+	if (args == NULL)			/* internal error */
+		elog(ERROR, "%s: no column name provided in the trigger definition",
+			 trigdata->tg_trigger->tgname);
+
 	/* tuple to return to Executor */
 	if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
 		rettuple = newtuple;
@@ -63,7 +72,8 @@ lo_manage(PG_FUNCTION_ARGS)
 	attnum = SPI_fnumber(tupdesc, args[0]);
 
 	if (attnum <= 0)
-		elog(ERROR, "column \"%s\" does not exist", args[0]);
+		elog(ERROR, "%s: column \"%s\" does not exist",
+			 trigdata->tg_trigger->tgname, args[0]);
 
 	/*
 	 * Handle updates