Fix UPDATE SET ... RETURNING processing for encrypted tuples

If `get_heap_tuple` is NULL, the core uses `copy_heap_tuple` instead. The former returns a pointer to a tuple in the slot and the latter makes a copy of such a tuple. For UPDATE SET, the core uses the slot for INSERT and later for RETURNING processing. If we copy the tuple the next happens:
1. The core creates a slot with the generic tuple.
2. It passed to `pg_tdeam_tuple_update()` and it gets a copy of the tuple here [https://github.com/Percona-Lab/pg_tde/blob/6d4f7e5b7bd2507ce65deb315ea8d5647f474cf9/src17/access/pg_tdeam_handler.c#L336].
3. This generic tuple is filled with the proper data and used for the update here [https://github.com/Percona-Lab/pg_tde/blob/6d4f7e5b7bd2507ce65deb315ea8d5647f474cf9/src17/access/pg_tdeam_handler.c#L343].
4. Later on, RETURNING processing uses the slot's tuple but is still a  generic unmodified one because of the copy.
5. That results in wrong RETURNING data.

To avoid this, we should return a pointer to the slot's tuple instead of copying it.

Fixes PG-1056

expected/update.out

@@ -15,11 +15,11 @@ CREATE TABLE update_test (
     a   INT DEFAULT 10,
     b   INT,
     c   TEXT
-);
+) USING tde_heap_basic;
 CREATE TABLE upsert_test (
     a   INT PRIMARY KEY,
     b   TEXT
-);
+) USING tde_heap_basic;
 INSERT INTO update_test VALUES (5, 10, 'foo');
 INSERT INTO update_test(b, a) VALUES (15, 10);
 INSERT INTO upsert_test VALUES (2, 'Beeble') ON CONFLICT(a)

meson.build

@@ -93,6 +93,7 @@ tests += {
       'move_large_tuples',
       'non_sorted_off_compact',
       'update_compare_indexes',
+      'update',
       'pg_tde_is_encrypted',
       'test_issue_153_fix',
       'multi_insert',

src/access/pg_tde_slot.c

@@ -241,6 +241,17 @@ tdeheap_tts_buffer_heap_copyslot(TupleTableSlot *dstslot, TupleTableSlot *srcslo
 	}
 }
 
+static HeapTuple
+tdeheap_tts_buffer_heap_get_heap_tuple(TupleTableSlot *slot)
+{
+	BufferHeapTupleTableSlot *bslot = (BufferHeapTupleTableSlot *) slot;
+	Assert(!TTS_EMPTY(slot));
+
+	if (!bslot->base.tuple)
+		tdeheap_tts_buffer_heap_materialize(slot);
+	return bslot->base.tuple;
+}
+
 static HeapTuple
 tdeheap_tts_buffer_heap_copy_heap_tuple(TupleTableSlot *slot)
 {
@@ -462,7 +473,7 @@ const TupleTableSlotOps TTSOpsTDEBufferHeapTuple = {
 	.is_current_xact_tuple = tdeheap_buffer_is_current_xact_tuple,
 #endif
 	.copyslot = tdeheap_tts_buffer_heap_copyslot,
-	.get_heap_tuple = NULL,
+	.get_heap_tuple = tdeheap_tts_buffer_heap_get_heap_tuple,
 
 	/* A buffer heap tuple table slot can not "own" a minimal tuple. */
 	.get_minimal_tuple = NULL,