Fix cross-leftover pollution in FOR PORTION OF insert triggers

When we insert temporal leftovers after an UPDATE FOR PORTION OF, we
must make a new copy of the tuple before each insert.  Otherwise, if
an insert trigger assigns to attributes of NEW, the second leftover
sees those changes.

Author: Sergei Patiakin <sergei.patiakin@enterprisedb.com>
Reviewed-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/flat/CANE55rCqcse_pwXBMWhbj3_7XROb8Dks6%3DOLFmKy3bO3zDsCsg%40mail.gmail.com

src/backend/executor/nodeModifyTable.c

@@ -1601,6 +1601,18 @@ ExecForPortionOfLeftovers(ModifyTableContext *context,
 
 			didInit = true;
 		}
+		else
+		{
+			/*
+			 * Re-copy the original row into leftoverSlot because ExecInsert
+			 * might pass leftoverSlot to BEFORE ROW INSERT triggers, which can
+			 * modify the slot contents.
+			 */
+			if (map != NULL)
+				execute_attr_map_slot(map->attrMap, oldtupleSlot, leftoverSlot);
+			else
+				ExecForceStoreHeapTuple(oldtuple, leftoverSlot, false);
+		}
 
 		leftoverSlot->tts_values[forPortionOf->rangeVar->varattno - 1] = leftover;
 		leftoverSlot->tts_isnull[forPortionOf->rangeVar->varattno - 1] = false;

src/test/regress/expected/for_portion_of.out

@@ -1793,6 +1793,44 @@ SELECT * FROM for_portion_of_test WHERE id = '[4,5)' ORDER BY id, valid_at;
 (3 rows)
 
 DROP TRIGGER fpo_after_delete_row ON for_portion_of_test;
+-- Test that a tuple-modifying BEFORE INSERT ROW trigger acts
+-- consistently on both temporal leftovers.
+-- When FOR PORTION OF splits a row into two leftovers, both triggers
+-- should get the original row's values.
+DROP TABLE for_portion_of_test;
+CREATE TABLE for_portion_of_test (
+  id int,
+  valid_at daterange,
+  name text
+);
+CREATE FUNCTION fpo_append_name_suffix()
+RETURNS TRIGGER LANGUAGE plpgsql AS
+$$
+BEGIN
+  NEW.name := NEW.name || '+insert';
+  RETURN NEW;
+END;
+$$;
+CREATE TRIGGER fpo_before_insert_row
+  BEFORE INSERT ON for_portion_of_test
+  FOR EACH ROW EXECUTE PROCEDURE fpo_append_name_suffix();
+INSERT INTO for_portion_of_test VALUES (1, '[2020-01-01,2020-12-31)', 'foo');
+UPDATE for_portion_of_test
+  FOR PORTION OF valid_at FROM '2020-04-01' TO '2020-08-01'
+  SET name = 'bar'
+  WHERE id = 1;
+-- Both leftovers should have the same name: 'foo+insert+insert'.
+SELECT * FROM for_portion_of_test ORDER BY valid_at;
+ id |        valid_at         |       name        
+----+-------------------------+-------------------
+  1 | [2020-01-01,2020-04-01) | foo+insert+insert
+  1 | [2020-04-01,2020-08-01) | bar
+  1 | [2020-08-01,2020-12-31) | foo+insert+insert
+(3 rows)
+
+DROP FUNCTION fpo_append_name_suffix CASCADE;
+NOTICE:  drop cascades to trigger fpo_before_insert_row on table for_portion_of_test
+DROP TABLE for_portion_of_test;
 -- Test with multiranges
 CREATE TABLE for_portion_of_test2 (
   id int4range NOT NULL,

src/test/regress/sql/for_portion_of.sql

@@ -1169,6 +1169,44 @@ SELECT * FROM for_portion_of_test WHERE id = '[4,5)' ORDER BY id, valid_at;
 
 DROP TRIGGER fpo_after_delete_row ON for_portion_of_test;
 
+-- Test that a tuple-modifying BEFORE INSERT ROW trigger acts
+-- consistently on both temporal leftovers.
+-- When FOR PORTION OF splits a row into two leftovers, both triggers
+-- should get the original row's values.
+
+DROP TABLE for_portion_of_test;
+CREATE TABLE for_portion_of_test (
+  id int,
+  valid_at daterange,
+  name text
+);
+
+CREATE FUNCTION fpo_append_name_suffix()
+RETURNS TRIGGER LANGUAGE plpgsql AS
+$$
+BEGIN
+  NEW.name := NEW.name || '+insert';
+  RETURN NEW;
+END;
+$$;
+
+CREATE TRIGGER fpo_before_insert_row
+  BEFORE INSERT ON for_portion_of_test
+  FOR EACH ROW EXECUTE PROCEDURE fpo_append_name_suffix();
+
+INSERT INTO for_portion_of_test VALUES (1, '[2020-01-01,2020-12-31)', 'foo');
+
+UPDATE for_portion_of_test
+  FOR PORTION OF valid_at FROM '2020-04-01' TO '2020-08-01'
+  SET name = 'bar'
+  WHERE id = 1;
+
+-- Both leftovers should have the same name: 'foo+insert+insert'.
+SELECT * FROM for_portion_of_test ORDER BY valid_at;
+
+DROP FUNCTION fpo_append_name_suffix CASCADE;
+DROP TABLE for_portion_of_test;
+
 -- Test with multiranges
 
 CREATE TABLE for_portion_of_test2 (