Active connection is closed, when Lua GC is invoked

[Serpentian]

Bug description

The connection is closed, when it's active but it doesn't have any strong references. It's caused by Lua GC. Related: tarantool/vshard#469

• OS: Linux
• OS Version: NixOS
• Architecture: amd64

Tarantool 3.1.0-entrypoint-51-g47940311e
Target: Linux-x86_64-Debug
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/var/empty/local -DENABLE_BACKTRACE=FALSE
Compiler: GNU-12.3.0
C_FLAGS: -fexceptions -funwind-tables -fasynchronous-unwind-tables -fno-common -msse2 -Wformat -Wformat-security -Werror=format-security -fstack-protector-strong -fPIC -fmacro-prefix-map=/home/serpentian/Programming/tnt/tarantool=. -std=c11 -Wall -Wextra -Wno-gnu-alignof-expression -fno-gnu89-inline -Wno-cast-function-type -Werror -g -ggdb -O0 
CXX_FLAGS: -fexceptions -funwind-tables -fasynchronous-unwind-tables -fno-common -msse2 -Wformat -Wformat-security -Werror=format-security -fstack-protector-strong -fPIC -fmacro-prefix-map=/home/serpentian/Programming/tnt/tarantool=. -std=c++11 -Wall -Wextra -Wno-invalid-offsetof -Wno-gnu-alignof-expression -Wno-cast-function-type -Werror -g -ggdb **-O0**

Steps to reproduce

local t = require('luatest')
local server = require('luatest.server')
local nb = require('net.box')
local g = t.group()

g.before_all = function(lg)
    lg.master = server:new({alias = 'master'})
    lg.master:start()
    lg.master:exec(function()
        rawset(_G, 'echo', function(arg)
            return arg
        end)
    end)
end

g.after_all = function(lg)
    lg.master:drop()
end

g.test_conn_not_deleted_without_ref = function(lg)
    local a = string.rep('a', 1024 * 1024 * 500)
    local opts = {is_async = true}
    local f = nb.connect(lg.master.net_box_uri):call('_G.echo', {a}, opts)
    collectgarbage()
    local res, err = f:wait_result(10)
    t.assert_equals(err, nil)
    t.assert_equals(#res[1], #a)
end

Actual behavior

[001] not ok 1	replication-luatest.lua_gc.test_conn_not_deleted_without_ref
[001] #   ...g/tnt/tarantool/test/replication-luatest/lua_gc_test.lua:26: expected: nil, actual: Connection closed
[001] #   stack traceback:
[001] #   	...g/tnt/tarantool/test/replication-luatest/lua_gc_test.lua:26: in function 'replication-luatest.lua_gc.test_conn_not_deleted_without_ref'
[001] #   	...
[001] #   	[C]: in function 'xpcall'
[001] # Ran 1 tests in 1.849 seconds, 0 succeeded, 1 failed

If you'll delete collectgarbage the test will probably pass. If you'll create ref to conn, test also passes e.g:

diff --git a/test/replication-luatest/lua_gc_test.lua b/test/replication-luatest/lua_gc_test.lua
index 77052385c..0fda9f62e 100644
--- a/test/replication-luatest/lua_gc_test.lua
+++ b/test/replication-luatest/lua_gc_test.lua
@@ -20,7 +20,8 @@ end
 g.test_conn_not_deleted_without_ref = function(lg)
     local a = string.rep('a', 1024 * 1024 * 500)
     local opts = {is_async = true}
-    local f = nb.connect(lg.master.net_box_uri):call('_G.echo', {a}, opts)
+    local conn = nb.connect(lg.master.net_box_uri)
+    local f = conn:call('_G.echo', {a}, opts)
     collectgarbage()
     local res, err = f:wait_result(10)
     t.assert_equals(err, nil)

Expected behavior

Connection should be active while request is done.

[sergos]

Apparently future object should keep a reference to the connection.