Avoid passing NULL to memcmp() in lookups of zero-argument functions.

A few places assumed they could pass NULL for the argtypes array when
looking up functions known to have zero arguments.  At first glance
it seems that this should be safe enough, since memcmp() is surely not
allowed to fetch any bytes if its count argument is zero.  However,
close reading of the C standard says that such calls have undefined
behavior, so we'd probably best avoid it.

Since the number of places doing this is quite small, and some other
places looking up zero-argument functions were already passing dummy
arrays, let's standardize on the latter solution rather than hacking
the function lookup code to avoid calling memcmp() in these cases.
I also added Asserts to catch any future violations of the new rule.

Given the utter lack of any evidence that this actually causes any
problems in the field, I don't feel a need to back-patch this change.

Per report from Piotr Stefaniak, though this is not his patch.

src/backend/commands/event_trigger.c

@@ -165,6 +165,7 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 	HeapTuple	tuple;
 	Oid			funcoid;
 	Oid			funcrettype;
+	Oid			fargtypes[1];	/* dummy */
 	Oid			evtowner = GetUserId();
 	ListCell   *lc;
 	List	   *tags = NULL;
@@ -230,7 +231,7 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 						stmt->trigname)));
 
 	/* Find and validate the trigger function. */
-	funcoid = LookupFuncName(stmt->funcname, 0, NULL, false);
+	funcoid = LookupFuncName(stmt->funcname, 0, fargtypes, false);
 	funcrettype = get_func_rettype(funcoid);
 	if (funcrettype != EVTTRIGGEROID)
 		ereport(ERROR,

src/backend/commands/foreigncmds.c

@@ -474,12 +474,13 @@ static Oid
 lookup_fdw_handler_func(DefElem *handler)
 {
 	Oid			handlerOid;
+	Oid			funcargtypes[1];	/* dummy */
 
 	if (handler == NULL || handler->arg == NULL)
 		return InvalidOid;
 
 	/* handlers have no arguments */
-	handlerOid = LookupFuncName((List *) handler->arg, 0, NULL, false);
+	handlerOid = LookupFuncName((List *) handler->arg, 0, funcargtypes, false);
 
 	/* check that handler has correct return type */
 	if (get_func_rettype(handlerOid) != FDW_HANDLEROID)

src/backend/parser/parse_func.c

@@ -1415,6 +1415,9 @@ func_get_detail(List *funcname,
 	FuncCandidateList raw_candidates;
 	FuncCandidateList best_candidate;
 
+	/* Passing NULL for argtypes is no longer allowed */
+	Assert(argtypes);
+
 	/* initialize output arguments to silence compiler warnings */
 	*funcid = InvalidOid;
 	*rettype = InvalidOid;
@@ -2043,6 +2046,9 @@ LookupFuncName(List *funcname, int nargs, const Oid *argtypes, bool noError)
 {
 	FuncCandidateList clist;
 
+	/* Passing NULL for argtypes is no longer allowed */
+	Assert(argtypes);
+
 	clist = FuncnameGetCandidates(funcname, nargs, NIL, false, false, noError);
 
 	while (clist)

src/backend/utils/adt/ruleutils.c

@@ -718,6 +718,7 @@ pg_get_triggerdef_worker(Oid trigid, bool pretty)
 	SysScanDesc tgscan;
 	int			findx = 0;
 	char	   *tgname;
+	Oid			argtypes[1];	/* dummy */
 	Datum		value;
 	bool		isnull;
 
@@ -893,7 +894,7 @@ pg_get_triggerdef_worker(Oid trigid, bool pretty)
 
 	appendStringInfo(&buf, "EXECUTE PROCEDURE %s(",
 					 generate_function_name(trigrec->tgfoid, 0,
-											NIL, NULL,
+											NIL, argtypes,
 											false, NULL, EXPR_KIND_NONE));
 
 	if (trigrec->tgnargs > 0)