Allow to mix blackhole statements in other engines' transactions

Blackhole doesn't need transaction control as it doesn't actually store
anything so we can mark it with ENGINE_BYPASS_TX.

src/box/blackhole.c

@@ -206,5 +206,6 @@ blackhole_engine_new(void)
 
 	engine->vtab = &blackhole_engine_vtab;
 	engine->name = "blackhole";
+	engine->flags = ENGINE_BYPASS_TX;
 	return engine;
 }

test/box/blackhole.result

@@ -95,6 +95,58 @@ s:on_replace(nil, f1)
 s:before_replace(nil, f2)
 ---
 ...
+-- Blackhole statements can be mixed in other engines' transactions.
+memtx = box.schema.space.create('memtx', {engine = 'memtx'})
+---
+...
+_ = memtx:create_index('pk')
+---
+...
+test_run:cmd("setopt delimiter ';'")
+---
+- true
+...
+box.begin()
+s:replace{1}
+memtx:replace{1}
+s:replace{2}
+memtx:replace{2}
+box.commit();
+---
+...
+test_run:cmd("setopt delimiter ''");
+---
+- true
+...
+memtx:select()
+---
+- - [1]
+  - [2]
+...
+f = s:on_replace(function(old, new) memtx:replace(new) end)
+---
+...
+s:replace{3}
+---
+- [3]
+...
+s:replace{4}
+---
+- [4]
+...
+memtx:select()
+---
+- - [1]
+  - [2]
+  - [3]
+  - [4]
+...
+s:on_replace(nil, f)
+---
+...
+memtx:drop()
+---
+...
 -- Test recovery.
 test_run:cmd('restart server default')
 s = box.space.test

test/box/blackhole.test.lua

@@ -33,6 +33,25 @@ s_old, s_new
 s:on_replace(nil, f1)
 s:before_replace(nil, f2)
 
+-- Blackhole statements can be mixed in other engines' transactions.
+memtx = box.schema.space.create('memtx', {engine = 'memtx'})
+_ = memtx:create_index('pk')
+test_run:cmd("setopt delimiter ';'")
+box.begin()
+s:replace{1}
+memtx:replace{1}
+s:replace{2}
+memtx:replace{2}
+box.commit();
+test_run:cmd("setopt delimiter ''");
+memtx:select()
+f = s:on_replace(function(old, new) memtx:replace(new) end)
+s:replace{3}
+s:replace{4}
+memtx:select()
+s:on_replace(nil, f)
+memtx:drop()
+
 -- Test recovery.
 test_run:cmd('restart server default')
 s = box.space.test