Fix confusion about event trigger vs. plain function in plpgsql.

The function hash table keys made by compute_function_hashkey() failed
to distinguish event-trigger call context from regular call context.
This meant that once we'd successfully made a hash entry for an event
trigger (either by validation, or by normal use as an event trigger),
an attempt to call the trigger function as a plain function would
find this hash entry and thereby bypass the you-can't-do-that check in
do_compile().  Thus we'd attempt to execute the function, leading to
strange errors or even crashes, depending on function contents and
server version.

To fix, add an isEventTrigger field to PLpgSQL_func_hashkey,
paralleling the longstanding infrastructure for regular triggers.
This fits into what had been pad space, so there's no risk of an ABI
break, even assuming that any third-party code is looking at these
hash keys.  (I considered replacing isTrigger with a PLpgSQL_trigtype
enum field, but felt that that carried some API/ABI risk.  Maybe we
should change it in HEAD though.)

Per bug #16266 from Alexander Lakhin.  This has been broken since
event triggers were invented, so back-patch to all supported branches.

Discussion: https://postgr.es/m/16266-fcd7f838e97ba5d4@postgresql.org

src/pl/plpgsql/src/pl_comp.c

@@ -2448,11 +2448,16 @@ compute_function_hashkey(FunctionCallInfo fcinfo,
 
 	/* get call context */
 	hashkey->isTrigger = CALLED_AS_TRIGGER(fcinfo);
+	hashkey->isEventTrigger = CALLED_AS_EVENT_TRIGGER(fcinfo);
 
 	/*
 	 * if trigger, get relation OID.  In validation mode we do not know what
 	 * relation is intended to be used, so we leave trigrelOid zero; the hash
 	 * entry built in this case will never really be used.
+	 *
+	 * We don't currently need to distinguish different event trigger usages
+	 * in the same way, since the special parameter variables don't vary in
+	 * type in that case.
 	 */
 	if (hashkey->isTrigger && !forValidator)
 	{

src/pl/plpgsql/src/plpgsql.h

@@ -684,7 +684,8 @@ typedef struct PLpgSQL_func_hashkey
 {								/* Hash lookup key for functions */
 	Oid			funcOid;
 
-	bool		isTrigger;		/* true if called as a trigger */
+	bool		isTrigger;		/* true if called as a DML trigger */
+	bool		isEventTrigger; /* true if called as an event trigger */
 
 	/* be careful that pad bytes in this struct get zeroed! */
 
@@ -692,7 +693,7 @@ typedef struct PLpgSQL_func_hashkey
 	 * For a trigger function, the OID of the relation triggered on is part of
 	 * the hash key --- we want to compile the trigger separately for each
 	 * relation it is used with, in case the rowtype is different.  Zero if
-	 * not called as a trigger.
+	 * not called as a DML trigger.
 	 */
 	Oid			trigrelOid;
 

src/test/regress/expected/event_trigger.out

@@ -9,6 +9,10 @@ BEGIN
     RAISE NOTICE 'test_event_trigger: % %', tg_event, tg_tag;
 END
 $$ language plpgsql;
+-- should fail, can't call it as a plain function
+SELECT test_event_trigger();
+ERROR:  trigger functions can only be called as triggers
+CONTEXT:  compilation of PL/pgSQL function "test_event_trigger" near line 1
 -- should fail, event triggers cannot have declared arguments
 create function test_event_trigger_arg(name text)
 returns event_trigger as $$ BEGIN RETURN 1; END $$ language plpgsql;

src/test/regress/sql/event_trigger.sql

@@ -10,6 +10,9 @@ BEGIN
 END
 $$ language plpgsql;
 
+-- should fail, can't call it as a plain function
+SELECT test_event_trigger();
+
 -- should fail, event triggers cannot have declared arguments
 create function test_event_trigger_arg(name text)
 returns event_trigger as $$ BEGIN RETURN 1; END $$ language plpgsql;