Add sql_drop event for event triggers

This event takes place just before ddl_command_end, and is fired if and
only if at least one object has been dropped by the command.  (For
instance, DROP TABLE IF EXISTS of a table that does not in fact exist
will not lead to such a trigger firing).  Commands that drop multiple
objects (such as DROP SCHEMA or DROP OWNED BY) will cause a single event
to fire.  Some firings might be surprising, such as
ALTER TABLE DROP COLUMN.

The trigger is fired after the drop has taken place, because that has
been deemed the safest design, to avoid exposing possibly-inconsistent
internal state (system catalogs as well as current transaction) to the
user function code.  This means that careful tracking of object
identification is required during the object removal phase.

Like other currently existing events, there is support for tag
filtering.

To support the new event, add a new pg_event_trigger_dropped_objects()
set-returning function, which returns a set of rows comprising the
objects affected by the command.  This is to be used within the user
function code, and is mostly modelled after the recently introduced
pg_identify_object() function.

Catalog version bumped due to the new function.

Dimitri Fontaine and Álvaro Herrera
Review by Robert Haas, Tom Lane

5 files changed, 17 insertions, 3 deletions

diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
index cbc4673d1b4..0eb8eefb8ff 100644
--- a/src/include/catalog/catversion.h
+++ b/src/include/catalog/catversion.h
@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201303201
+#define CATALOG_VERSION_NO	201303271
 
 #endif
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 4aee00233a9..151987ec1d3 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -4693,6 +4693,9 @@ DATA(insert OID = 3473 (  spg_range_quad_leaf_consistent	PGNSP PGUID 12 1 0 0 0
 DESCR("SP-GiST support for quad tree over range");
 
 
+/* event triggers */
+DATA(insert OID = 3566 (  pg_event_trigger_dropped_objects		PGNSP PGUID 12 10 100 0 0 f f f f t t s 0 0 2249 "" "{26,26,23,25,25,25,25}" "{o,o,o,o,o,o,o}" "{classid, objid, objsubid, object_type, schema_name, object_name, object_identity}" _null_ pg_event_trigger_dropped_objects _null_ _null_ _null_ ));
+DESCR("list objects dropped by the current command");
 /*
  * Symbolic values for provolatile column: these indicate whether the result
  * of a function is dependent *only* on the values of its explicit arguments,
diff --git a/src/include/commands/event_trigger.h b/src/include/commands/event_trigger.h
index 74c150bd084..8ea99c19c99 100644
--- a/src/include/commands/event_trigger.h
+++ b/src/include/commands/event_trigger.h
@@ -13,13 +13,14 @@
 #ifndef EVENT_TRIGGER_H
 #define EVENT_TRIGGER_H
 
+#include "catalog/objectaddress.h"
 #include "catalog/pg_event_trigger.h"
 #include "nodes/parsenodes.h"
 
 typedef struct EventTriggerData
 {
 	NodeTag		type;
-	char	   *event;				/* event name */
+	const char *event;				/* event name */
 	Node	   *parsetree;			/* parse tree */
 	const char *tag;				/* command tag */
 } EventTriggerData;
@@ -42,5 +43,11 @@ extern void AlterEventTriggerOwner_oid(Oid, Oid newOwnerId);
 extern bool EventTriggerSupportsObjectType(ObjectType obtype);
 extern void EventTriggerDDLCommandStart(Node *parsetree);
 extern void EventTriggerDDLCommandEnd(Node *parsetree);
+extern void EventTriggerSQLDrop(Node *parsetree);
+
+extern bool EventTriggerBeginCompleteQuery(void);
+extern void EventTriggerEndCompleteQuery(void);
+extern bool trackDroppedObjectsNeeded(void);
+extern void EventTriggerSQLDropAddObject(ObjectAddress *object);
 
 #endif   /* EVENT_TRIGGER_H */
diff --git a/src/include/utils/builtins.h b/src/include/utils/builtins.h
index cd8ac9462b5..e71876502e1 100644
--- a/src/include/utils/builtins.h
+++ b/src/include/utils/builtins.h
@@ -1151,6 +1151,9 @@ extern Datum pg_identify_object(PG_FUNCTION_ARGS);
 /* commands/constraint.c */
 extern Datum unique_key_recheck(PG_FUNCTION_ARGS);
 
+/* commands/event_trigger.c */
+extern Datum pg_event_trigger_dropped_objects(PG_FUNCTION_ARGS);
+
 /* commands/extension.c */
 extern Datum pg_available_extensions(PG_FUNCTION_ARGS);
 extern Datum pg_available_extension_versions(PG_FUNCTION_ARGS);
diff --git a/src/include/utils/evtcache.h b/src/include/utils/evtcache.h
index c230995212d..945e5b53cb8 100644
--- a/src/include/utils/evtcache.h
+++ b/src/include/utils/evtcache.h
@@ -19,7 +19,8 @@
 typedef enum
 {
 	EVT_DDLCommandStart,
-	EVT_DDLCommandEnd
+	EVT_DDLCommandEnd,
+	EVT_SQLDrop
 } EventTriggerEvent;
 
 typedef struct