vinyl: fix assertion failure in vy_tx_handle_deferred_delete

vy_tx_handle_deferred_delete() expects (righteously) that in case a
deferred DELETE overwrites a statement in a secondary index write set
and the overwritten statement wasn't skipped on commit (i.e. doesn't
have txv->is_overwritten flag set), both the old and the new statement
must be REPLACEs (see the comment to the corresponding assertions for
more details).

The problem is we don't set is_overwritten flag in case a statement
doesn't have any effect (txv->is_nop is set), even if it was, in fact,
overwritten in the primary index write set (see vy_tx_prepare). As
a result, we get an assertion failure when we delete such statement
in the same transaction, e.g.

  s = box.schema.space.create('test', {engine = 'vinyl'})
  s:create_index('pk', {parts = {1, 'unsigned'}})
  s:create_index('sk', {parts = {2, 'unsigned'}})
  s:replace{1, 1, 1}
  box.begin()
  s:update(1, {{'+', 3, 1}}) -- adds no-op REPLACE to sk write set
  s:delete(1)                -- produces deferred DELETE for sk
  box.commit()

results in

  vy_tx_handle_deferred_delete: Assertion `vy_stmt_type(stmt) == IPROTO_REPLACE' failed.

Fix this by making sure we set is_overwritten for all overwritten
statements in a secondary index write set.

Closes #4294

src/box/vy_tx.c

@@ -717,10 +717,6 @@ vy_tx_prepare(struct vy_tx *tx)
 		}
 		assert(lsm->space_id == current_space_id);
 
-		/* Do not save statements that was overwritten by the same tx */
-		if (v->is_overwritten || v->is_nop)
-			continue;
-
 		if (lsm->index_id > 0 && repsert == NULL && delete == NULL) {
 			/*
 			 * This statement is for a secondary index,
@@ -735,9 +731,12 @@ vy_tx_prepare(struct vy_tx *tx)
 			 * skip it for secondary indexes as well.
 			 */
 			v->is_overwritten = true;
-			continue;
 		}
 
+		/* Do not save statements that was overwritten by the same tx */
+		if (v->is_overwritten || v->is_nop)
+			continue;
+
 		enum iproto_type type = vy_stmt_type(v->entry.stmt);
 
 		/* Optimize out INSERT + DELETE for the same key. */

test/vinyl/gh.result

@@ -718,3 +718,40 @@ while finished ~= 2 do fiber.sleep(0.01) end
 s:drop()
 ---
 ...
+--
+-- gh-4294: assertion failure when deleting a no-op statement from
+-- a secondary index write set.
+--
+s = box.schema.space.create('test', {engine = 'vinyl'})
+---
+...
+_ = s:create_index('pk')
+---
+...
+_ = s:create_index('sk', {parts = {2, 'unsigned'}})
+---
+...
+s:replace{1, 1, 1}
+---
+- [1, 1, 1]
+...
+box.begin()
+---
+...
+s:update(1, {{'+', 3, 1}})
+---
+- [1, 1, 2]
+...
+s:delete(1)
+---
+...
+box.commit()
+---
+...
+s:select()
+---
+- []
+...
+s:drop()
+---
+...

test/vinyl/gh.test.lua

@@ -312,3 +312,18 @@ cont = false
 while finished ~= 2 do fiber.sleep(0.01) end
 
 s:drop()
+
+--
+-- gh-4294: assertion failure when deleting a no-op statement from
+-- a secondary index write set.
+--
+s = box.schema.space.create('test', {engine = 'vinyl'})
+_ = s:create_index('pk')
+_ = s:create_index('sk', {parts = {2, 'unsigned'}})
+s:replace{1, 1, 1}
+box.begin()
+s:update(1, {{'+', 3, 1}})
+s:delete(1)
+box.commit()
+s:select()
+s:drop()