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

2 files changed, 299 insertions, 4 deletions

diff --git a/src/test/regress/expected/event_trigger.out b/src/test/regress/expected/event_trigger.out
index bf020de16f6..060cd722bef 100644
--- a/src/test/regress/expected/event_trigger.out
+++ b/src/test/regress/expected/event_trigger.out
@@ -93,11 +93,204 @@ ERROR:  event trigger "regress_event_trigger" does not exist
 drop role regression_bob;
 ERROR:  role "regression_bob" cannot be dropped because some objects depend on it
 DETAIL:  owner of event trigger regress_event_trigger3
+-- cleanup before next test
 -- these are all OK; the second one should emit a NOTICE
 drop event trigger if exists regress_event_trigger2;
 drop event trigger if exists regress_event_trigger2;
 NOTICE:  event trigger "regress_event_trigger2" does not exist, skipping
 drop event trigger regress_event_trigger3;
 drop event trigger regress_event_trigger_end;
-drop function test_event_trigger();
-drop role regression_bob;
+-- test support for dropped objects
+CREATE SCHEMA schema_one authorization regression_bob;
+CREATE SCHEMA schema_two authorization regression_bob;
+CREATE SCHEMA audit_tbls authorization regression_bob;
+SET SESSION AUTHORIZATION regression_bob;
+CREATE TABLE schema_one.table_one(a int);
+CREATE TABLE schema_one."table two"(a int);
+CREATE TABLE schema_one.table_three(a int);
+CREATE TABLE audit_tbls.schema_one_table_two(the_value text);
+CREATE TABLE schema_two.table_two(a int);
+CREATE TABLE schema_two.table_three(a int, b text);
+CREATE TABLE audit_tbls.schema_two_table_three(the_value text);
+CREATE OR REPLACE FUNCTION schema_two.add(int, int) RETURNS int LANGUAGE plpgsql
+  CALLED ON NULL INPUT
+  AS $$ BEGIN RETURN coalesce($1,0) + coalesce($2,0); END; $$;
+CREATE AGGREGATE schema_two.newton
+  (BASETYPE = int, SFUNC = schema_two.add, STYPE = int);
+RESET SESSION AUTHORIZATION;
+CREATE TABLE undroppable_objs (
+	object_type text,
+	object_identity text
+);
+INSERT INTO undroppable_objs VALUES
+('table', 'schema_one.table_three'),
+('table', 'audit_tbls.schema_two_table_three');
+CREATE TABLE dropped_objects (
+	type text,
+	schema text,
+	object text
+);
+-- This tests errors raised within event triggers; the one in audit_tbls
+-- uses 2nd-level recursive invocation via test_evtrig_dropped_objects().
+CREATE OR REPLACE FUNCTION undroppable() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+DECLARE
+	obj record;
+BEGIN
+	PERFORM 1 FROM pg_tables WHERE tablename = 'undroppable_objs';
+	IF NOT FOUND THEN
+		RAISE NOTICE 'table undroppable_objs not found, skipping';
+		RETURN;
+	END IF;
+	FOR obj IN
+		SELECT * FROM pg_event_trigger_dropped_objects() JOIN
+			undroppable_objs USING (object_type, object_identity)
+	LOOP
+		RAISE EXCEPTION 'object % of type % cannot be dropped',
+			obj.object_identity, obj.object_type;
+	END LOOP;
+END;
+$$;
+CREATE EVENT TRIGGER undroppable ON sql_drop
+	EXECUTE PROCEDURE undroppable();
+CREATE OR REPLACE FUNCTION test_evtrig_dropped_objects() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+DECLARE
+    obj record;
+BEGIN
+    FOR obj IN SELECT * FROM pg_event_trigger_dropped_objects()
+    LOOP
+        IF obj.object_type = 'table' THEN
+                EXECUTE format('DROP TABLE IF EXISTS audit_tbls.%I',
+					format('%s_%s', obj.schema_name, obj.object_name));
+        END IF;
+
+	INSERT INTO dropped_objects
+		(type, schema, object) VALUES
+		(obj.object_type, obj.schema_name, obj.object_identity);
+    END LOOP;
+END
+$$;
+CREATE EVENT TRIGGER regress_event_trigger_drop_objects ON sql_drop
+	WHEN TAG IN ('drop table', 'drop function', 'drop view',
+		'drop owned', 'drop schema', 'alter table')
+	EXECUTE PROCEDURE test_evtrig_dropped_objects();
+ALTER TABLE schema_one.table_one DROP COLUMN a;
+DROP SCHEMA schema_one, schema_two CASCADE;
+NOTICE:  drop cascades to 7 other objects
+DETAIL:  drop cascades to table schema_two.table_two
+drop cascades to table schema_two.table_three
+drop cascades to function schema_two.add(integer,integer)
+drop cascades to function schema_two.newton(integer)
+drop cascades to table schema_one.table_one
+drop cascades to table schema_one."table two"
+drop cascades to table schema_one.table_three
+NOTICE:  table "schema_two_table_two" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_two_table_two"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+NOTICE:  table "audit_tbls_schema_two_table_three" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.audit_tbls_schema_two_table_three"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_two_table_three"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+ERROR:  object audit_tbls.schema_two_table_three of type table cannot be dropped
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_two_table_three"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+DELETE FROM undroppable_objs WHERE object_identity = 'audit_tbls.schema_two_table_three';
+DROP SCHEMA schema_one, schema_two CASCADE;
+NOTICE:  drop cascades to 7 other objects
+DETAIL:  drop cascades to table schema_two.table_two
+drop cascades to table schema_two.table_three
+drop cascades to function schema_two.add(integer,integer)
+drop cascades to function schema_two.newton(integer)
+drop cascades to table schema_one.table_one
+drop cascades to table schema_one."table two"
+drop cascades to table schema_one.table_three
+NOTICE:  table "schema_two_table_two" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_two_table_two"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+NOTICE:  table "audit_tbls_schema_two_table_three" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.audit_tbls_schema_two_table_three"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_two_table_three"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+NOTICE:  table "schema_one_table_one" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_one_table_one"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+NOTICE:  table "schema_one_table two" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls."schema_one_table two""
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+NOTICE:  table "schema_one_table_three" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_one_table_three"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+ERROR:  object schema_one.table_three of type table cannot be dropped
+DELETE FROM undroppable_objs WHERE object_identity = 'schema_one.table_three';
+DROP SCHEMA schema_one, schema_two CASCADE;
+NOTICE:  drop cascades to 7 other objects
+DETAIL:  drop cascades to table schema_two.table_two
+drop cascades to table schema_two.table_three
+drop cascades to function schema_two.add(integer,integer)
+drop cascades to function schema_two.newton(integer)
+drop cascades to table schema_one.table_one
+drop cascades to table schema_one."table two"
+drop cascades to table schema_one.table_three
+NOTICE:  table "schema_two_table_two" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_two_table_two"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+NOTICE:  table "audit_tbls_schema_two_table_three" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.audit_tbls_schema_two_table_three"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_two_table_three"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+NOTICE:  table "schema_one_table_one" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_one_table_one"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+NOTICE:  table "schema_one_table two" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls."schema_one_table two""
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+NOTICE:  table "schema_one_table_three" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.schema_one_table_three"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+SELECT * FROM dropped_objects WHERE schema IS NULL OR schema <> 'pg_toast';
+     type     |   schema   |               object                
+--------------+------------+-------------------------------------
+ table column | schema_one | schema_one.table_one.a
+ schema       |            | schema_two
+ table        | schema_two | schema_two.table_two
+ type         | schema_two | schema_two.table_two
+ type         | schema_two | schema_two.table_two[]
+ table        | audit_tbls | audit_tbls.schema_two_table_three
+ type         | audit_tbls | audit_tbls.schema_two_table_three
+ type         | audit_tbls | audit_tbls.schema_two_table_three[]
+ table        | schema_two | schema_two.table_three
+ type         | schema_two | schema_two.table_three
+ type         | schema_two | schema_two.table_three[]
+ function     | schema_two | schema_two.add(integer,integer)
+ aggregate    | schema_two | schema_two.newton(integer)
+ schema       |            | schema_one
+ table        | schema_one | schema_one.table_one
+ type         | schema_one | schema_one.table_one
+ type         | schema_one | schema_one.table_one[]
+ table        | schema_one | schema_one."table two"
+ type         | schema_one | schema_one."table two"
+ type         | schema_one | schema_one."table two"[]
+ table        | schema_one | schema_one.table_three
+ type         | schema_one | schema_one.table_three
+ type         | schema_one | schema_one.table_three[]
+(23 rows)
+
+DROP OWNED BY regression_bob;
+NOTICE:  table "audit_tbls_schema_one_table_two" does not exist, skipping
+CONTEXT:  SQL statement "DROP TABLE IF EXISTS audit_tbls.audit_tbls_schema_one_table_two"
+PL/pgSQL function test_evtrig_dropped_objects() line 8 at EXECUTE statement
+SELECT * FROM dropped_objects WHERE type = 'schema';
+  type  | schema |   object   
+--------+--------+------------
+ schema |        | schema_two
+ schema |        | schema_one
+ schema |        | audit_tbls
+(3 rows)
+
+DROP ROLE regression_bob;
+DROP EVENT TRIGGER regress_event_trigger_drop_objects;
+DROP EVENT TRIGGER undroppable;
diff --git a/src/test/regress/sql/event_trigger.sql b/src/test/regress/sql/event_trigger.sql
index a07dcd75546..11d2ce5c133 100644
--- a/src/test/regress/sql/event_trigger.sql
+++ b/src/test/regress/sql/event_trigger.sql
@@ -97,10 +97,112 @@ drop event trigger regress_event_trigger;
 -- should fail, regression_bob owns regress_event_trigger2/3
 drop role regression_bob;
 
+-- cleanup before next test
 -- these are all OK; the second one should emit a NOTICE
 drop event trigger if exists regress_event_trigger2;
 drop event trigger if exists regress_event_trigger2;
 drop event trigger regress_event_trigger3;
 drop event trigger regress_event_trigger_end;
-drop function test_event_trigger();
-drop role regression_bob;
+
+-- test support for dropped objects
+CREATE SCHEMA schema_one authorization regression_bob;
+CREATE SCHEMA schema_two authorization regression_bob;
+CREATE SCHEMA audit_tbls authorization regression_bob;
+SET SESSION AUTHORIZATION regression_bob;
+
+CREATE TABLE schema_one.table_one(a int);
+CREATE TABLE schema_one."table two"(a int);
+CREATE TABLE schema_one.table_three(a int);
+CREATE TABLE audit_tbls.schema_one_table_two(the_value text);
+
+CREATE TABLE schema_two.table_two(a int);
+CREATE TABLE schema_two.table_three(a int, b text);
+CREATE TABLE audit_tbls.schema_two_table_three(the_value text);
+
+CREATE OR REPLACE FUNCTION schema_two.add(int, int) RETURNS int LANGUAGE plpgsql
+  CALLED ON NULL INPUT
+  AS $$ BEGIN RETURN coalesce($1,0) + coalesce($2,0); END; $$;
+CREATE AGGREGATE schema_two.newton
+  (BASETYPE = int, SFUNC = schema_two.add, STYPE = int);
+
+RESET SESSION AUTHORIZATION;
+
+CREATE TABLE undroppable_objs (
+	object_type text,
+	object_identity text
+);
+INSERT INTO undroppable_objs VALUES
+('table', 'schema_one.table_three'),
+('table', 'audit_tbls.schema_two_table_three');
+
+CREATE TABLE dropped_objects (
+	type text,
+	schema text,
+	object text
+);
+
+-- This tests errors raised within event triggers; the one in audit_tbls
+-- uses 2nd-level recursive invocation via test_evtrig_dropped_objects().
+CREATE OR REPLACE FUNCTION undroppable() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+DECLARE
+	obj record;
+BEGIN
+	PERFORM 1 FROM pg_tables WHERE tablename = 'undroppable_objs';
+	IF NOT FOUND THEN
+		RAISE NOTICE 'table undroppable_objs not found, skipping';
+		RETURN;
+	END IF;
+	FOR obj IN
+		SELECT * FROM pg_event_trigger_dropped_objects() JOIN
+			undroppable_objs USING (object_type, object_identity)
+	LOOP
+		RAISE EXCEPTION 'object % of type % cannot be dropped',
+			obj.object_identity, obj.object_type;
+	END LOOP;
+END;
+$$;
+
+CREATE EVENT TRIGGER undroppable ON sql_drop
+	EXECUTE PROCEDURE undroppable();
+
+CREATE OR REPLACE FUNCTION test_evtrig_dropped_objects() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+DECLARE
+    obj record;
+BEGIN
+    FOR obj IN SELECT * FROM pg_event_trigger_dropped_objects()
+    LOOP
+        IF obj.object_type = 'table' THEN
+                EXECUTE format('DROP TABLE IF EXISTS audit_tbls.%I',
+					format('%s_%s', obj.schema_name, obj.object_name));
+        END IF;
+
+	INSERT INTO dropped_objects
+		(type, schema, object) VALUES
+		(obj.object_type, obj.schema_name, obj.object_identity);
+    END LOOP;
+END
+$$;
+
+CREATE EVENT TRIGGER regress_event_trigger_drop_objects ON sql_drop
+	WHEN TAG IN ('drop table', 'drop function', 'drop view',
+		'drop owned', 'drop schema', 'alter table')
+	EXECUTE PROCEDURE test_evtrig_dropped_objects();
+
+ALTER TABLE schema_one.table_one DROP COLUMN a;
+DROP SCHEMA schema_one, schema_two CASCADE;
+DELETE FROM undroppable_objs WHERE object_identity = 'audit_tbls.schema_two_table_three';
+DROP SCHEMA schema_one, schema_two CASCADE;
+DELETE FROM undroppable_objs WHERE object_identity = 'schema_one.table_three';
+DROP SCHEMA schema_one, schema_two CASCADE;
+
+SELECT * FROM dropped_objects WHERE schema IS NULL OR schema <> 'pg_toast';
+
+DROP OWNED BY regression_bob;
+SELECT * FROM dropped_objects WHERE type = 'schema';
+
+DROP ROLE regression_bob;
+
+DROP EVENT TRIGGER regress_event_trigger_drop_objects;
+DROP EVENT TRIGGER undroppable;