Make new event trigger facility actually do something.

Commit 3855968f32 added syntax, pg_dump,
psql support, and documentation, but the triggers didn't actually fire.
With this commit, they now do.  This is still a pretty basic facility
overall because event triggers do not get a whole lot of information
about what the user is trying to do unless you write them in C; and
there's still no option to fire them anywhere except at the very
beginning of the execution sequence, but it's better than nothing,
and a good building block for future work.

Along the way, add a regression test for ALTER LARGE OBJECT, since
testing of event triggers reveals that we haven't got one.

Dimitri Fontaine and Robert Haas

src/include/commands/event_trigger.h

@@ -16,6 +16,21 @@
 #include "catalog/pg_event_trigger.h"
 #include "nodes/parsenodes.h"
 
+typedef struct EventTriggerData
+{
+	NodeTag		type;
+	char	   *event;				/* event name */
+	Node	   *parsetree;			/* parse tree */
+	const char *tag;				/* command tag */
+} EventTriggerData;
+
+/*
+ * EventTriggerData is the node type that is passed as fmgr "context" info
+ * when a function is called by the event trigger manager.
+ */
+#define CALLED_AS_EVENT_TRIGGER(fcinfo) \
+	((fcinfo)->context != NULL && IsA((fcinfo)->context, EventTriggerData))
+
 extern void CreateEventTrigger(CreateEventTrigStmt *stmt);
 extern void RemoveEventTriggerById(Oid ctrigOid);
 extern Oid	get_event_trigger_oid(const char *trigname, bool missing_ok);
@@ -25,4 +40,7 @@ extern void RenameEventTrigger(const char* trigname, const char *newname);
 extern void AlterEventTriggerOwner(const char *name, Oid newOwnerId);
 extern void AlterEventTriggerOwner_oid(Oid, Oid newOwnerId);
 
+extern bool EventTriggerSupportsObjectType(ObjectType obtype);
+extern void EventTriggerDDLCommandStart(Node *parsetree);
+
 #endif   /* EVENT_TRIGGER_H */