Minimal config to reproduce lua pending timer bug

nginx.conf

error_log  logs/error.log debug;
events {
    worker_connections  1024;
}

http {
    default_type  application/octet-stream;

    lua_shared_dict test_dict 1m;

    keepalive_timeout  65;

    server {
        listen 80;

        location / {
            content_by_lua '
                -- Construct HTTP request
                local http_req = {"GET /foo HTTP/1.1", "Host: localhost:81", "", ""}
                http_req = table.concat(http_req, "\\r\\n")

                -- Connect the socket
                local sock = ngx.socket.tcp()
                local ok,err = sock:connect("127.0.0.1", 81)
                if not ok then
                    ngx.log(ngx.ERR, err)
                end

                -- Send the request
                local ok,err = sock:send(http_req)

                -- Get Headers
                repeat
                    local line, err = sock:receive("*l")
                until string.find(line, "^%s*$")

                function foo()
                    repeat
                        -- Get and read chunk
                        local line, err = sock:receive("*l")
                        local len = tonumber(line)
                        if len > 0 then
                            local chunk, err = sock:receive(len)
                            coroutine.yield(chunk)
                            sock:receive(2)
                        else
                            -- Read last newline
                            sock:receive(2)
                        end
                    until len == 0
                end

                co = coroutine.create(foo)
                repeat
                    local chunk = select(2,coroutine.resume(co))
                until chunk == nil

                -- Breaks the timer
                sock:setkeepalive()

                -- Timer works as expected
                --sock:close()

            ';

            log_by_lua '

                local background_thread
                background_thread = function(premature)
                    ngx.log(ngx.DEBUG, premature)
                    if premature then
                        ngx.shared["test_dict"]:delete("background_flag")
                        return
                    end
                    local ok, err = ngx.timer.at(1, background_thread)
                end
                local dict = ngx.shared["test_dict"]

                if dict:get("background_flag") == nil then
                    local ok, err = ngx.timer.at(1, background_thread)
                    if ok then
                        dict:set("test_dict", 1)
                    end
                end

            ';
        }
    }

    server {
        listen 81;
        location / {
            echo 'foo';
        }
    }

}