Fix copying data into slots with FDW batching

Commit b676ac44 optimized handling of tuple slots with bulk inserts
into foreign tables, so that the slots are initialized only once and
reused for all batches. The data was however copied into the slots only
after the initialization, inserting duplicate values when the slot gets
reused. Fixed by moving the ExecCopySlot outside the init branch.

The existing postgres_fdw tests failed to catch this due to inserting
data into foreign tables without unique indexes, and then checking only
the number of inserted rows. This adds a new test with both a unique
index and a check of inserted values.

Reported-by: Alexander Pyhalov
Discussion: https://postgr.es/m/7a8cf8d56b3d18e5c0bccd6cd42d04ac%40postgrespro.ru

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -9761,7 +9761,87 @@ SELECT tableoid::regclass, * FROM batch_cp_upd_test;
 (2 rows)
 
 -- Clean up
-DROP TABLE batch_table, batch_cp_upd_test CASCADE;
+DROP TABLE batch_table, batch_cp_upd_test, batch_table_p0, batch_table_p1 CASCADE;
+-- Use partitioning
+ALTER SERVER loopback OPTIONS (ADD batch_size '10');
+CREATE TABLE batch_table ( x int, field1 text, field2 text) PARTITION BY HASH (x);
+CREATE TABLE batch_table_p0 (LIKE batch_table);
+ALTER TABLE batch_table_p0 ADD CONSTRAINT p0_pkey PRIMARY KEY (x);
+CREATE FOREIGN TABLE batch_table_p0f
+	PARTITION OF batch_table
+	FOR VALUES WITH (MODULUS 2, REMAINDER 0)
+	SERVER loopback
+	OPTIONS (table_name 'batch_table_p0');
+CREATE TABLE batch_table_p1 (LIKE batch_table);
+ALTER TABLE batch_table_p1 ADD CONSTRAINT p1_pkey PRIMARY KEY (x);
+CREATE FOREIGN TABLE batch_table_p1f
+	PARTITION OF batch_table
+	FOR VALUES WITH (MODULUS 2, REMAINDER 1)
+	SERVER loopback
+	OPTIONS (table_name 'batch_table_p1');
+INSERT INTO batch_table SELECT i, 'test'||i, 'test'|| i FROM generate_series(1, 50) i;
+SELECT COUNT(*) FROM batch_table;
+ count 
+-------
+    50
+(1 row)
+
+SELECT * FROM batch_table ORDER BY x;
+ x  | field1 | field2 
+----+--------+--------
+  1 | test1  | test1
+  2 | test2  | test2
+  3 | test3  | test3
+  4 | test4  | test4
+  5 | test5  | test5
+  6 | test6  | test6
+  7 | test7  | test7
+  8 | test8  | test8
+  9 | test9  | test9
+ 10 | test10 | test10
+ 11 | test11 | test11
+ 12 | test12 | test12
+ 13 | test13 | test13
+ 14 | test14 | test14
+ 15 | test15 | test15
+ 16 | test16 | test16
+ 17 | test17 | test17
+ 18 | test18 | test18
+ 19 | test19 | test19
+ 20 | test20 | test20
+ 21 | test21 | test21
+ 22 | test22 | test22
+ 23 | test23 | test23
+ 24 | test24 | test24
+ 25 | test25 | test25
+ 26 | test26 | test26
+ 27 | test27 | test27
+ 28 | test28 | test28
+ 29 | test29 | test29
+ 30 | test30 | test30
+ 31 | test31 | test31
+ 32 | test32 | test32
+ 33 | test33 | test33
+ 34 | test34 | test34
+ 35 | test35 | test35
+ 36 | test36 | test36
+ 37 | test37 | test37
+ 38 | test38 | test38
+ 39 | test39 | test39
+ 40 | test40 | test40
+ 41 | test41 | test41
+ 42 | test42 | test42
+ 43 | test43 | test43
+ 44 | test44 | test44
+ 45 | test45 | test45
+ 46 | test46 | test46
+ 47 | test47 | test47
+ 48 | test48 | test48
+ 49 | test49 | test49
+ 50 | test50 | test50
+(50 rows)
+
+ALTER SERVER loopback OPTIONS (DROP batch_size);
 -- ===================================================================
 -- test asynchronous execution
 -- ===================================================================

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -3083,7 +3083,34 @@ UPDATE batch_cp_upd_test t SET a = 1 FROM (VALUES (1), (2)) s(a) WHERE t.a = s.a
 SELECT tableoid::regclass, * FROM batch_cp_upd_test;
 
 -- Clean up
-DROP TABLE batch_table, batch_cp_upd_test CASCADE;
+DROP TABLE batch_table, batch_cp_upd_test, batch_table_p0, batch_table_p1 CASCADE;
+
+-- Use partitioning
+ALTER SERVER loopback OPTIONS (ADD batch_size '10');
+
+CREATE TABLE batch_table ( x int, field1 text, field2 text) PARTITION BY HASH (x);
+
+CREATE TABLE batch_table_p0 (LIKE batch_table);
+ALTER TABLE batch_table_p0 ADD CONSTRAINT p0_pkey PRIMARY KEY (x);
+CREATE FOREIGN TABLE batch_table_p0f
+	PARTITION OF batch_table
+	FOR VALUES WITH (MODULUS 2, REMAINDER 0)
+	SERVER loopback
+	OPTIONS (table_name 'batch_table_p0');
+
+CREATE TABLE batch_table_p1 (LIKE batch_table);
+ALTER TABLE batch_table_p1 ADD CONSTRAINT p1_pkey PRIMARY KEY (x);
+CREATE FOREIGN TABLE batch_table_p1f
+	PARTITION OF batch_table
+	FOR VALUES WITH (MODULUS 2, REMAINDER 1)
+	SERVER loopback
+	OPTIONS (table_name 'batch_table_p1');
+
+INSERT INTO batch_table SELECT i, 'test'||i, 'test'|| i FROM generate_series(1, 50) i;
+SELECT COUNT(*) FROM batch_table;
+SELECT * FROM batch_table ORDER BY x;
+
+ALTER SERVER loopback OPTIONS (DROP batch_size);
 
 -- ===================================================================
 -- test asynchronous execution

src/backend/executor/nodeModifyTable.c

@@ -717,18 +717,20 @@ ExecInsert(ModifyTableState *mtstate,
 
 				resultRelInfo->ri_Slots[resultRelInfo->ri_NumSlots] =
 					MakeSingleTupleTableSlot(tdesc, slot->tts_ops);
-				ExecCopySlot(resultRelInfo->ri_Slots[resultRelInfo->ri_NumSlots],
-							 slot);
 
 				resultRelInfo->ri_PlanSlots[resultRelInfo->ri_NumSlots] =
 					MakeSingleTupleTableSlot(tdesc, planSlot->tts_ops);
-				ExecCopySlot(resultRelInfo->ri_PlanSlots[resultRelInfo->ri_NumSlots],
-							 planSlot);
 
 				/* remember how many batch slots we initialized */
 				resultRelInfo->ri_NumSlotsInitialized++;
 			}
 
+			ExecCopySlot(resultRelInfo->ri_Slots[resultRelInfo->ri_NumSlots],
+						 slot);
+
+			ExecCopySlot(resultRelInfo->ri_PlanSlots[resultRelInfo->ri_NumSlots],
+						 planSlot);
+
 			resultRelInfo->ri_NumSlots++;
 
 			MemoryContextSwitchTo(oldContext);