"_G write guard" warns that race conditions may occur on APIcast

Issue

• Apicast logs these warn messages. What do the messages mean?

2020/08/01 00:00:00 [warn] 15#15: *2 [lua] _G write guard:12: __newindex(): writing a global Lua variable ('sanitize_replace') which may lead to race conditions between concurrent requests, so prefer the use of 'local' variables
stack traceback:
        /usr/local/share/lua/5.1/liquid.lua:132: in main chunk
        [C]: in function 'require'
        /opt/app-root/src/src/apicast/cli/template.lua:13: in main chunk
        [C]: in function 'require'
        /opt/app-root/src/src/apicast/cli/command/start.lua:17: in main chunk
        [C]: in function 'require'
        /opt/app-root/src/src/apicast/cli.lua:18: in function 'load_commands'
        /opt/app-root/src/src/apicast/cli.lua:23: in main chunk
        [C]: in function 'require'
        /tmp/xxxxxxxx:60: in function 'file_gen'
        init_worker_by_lua:49: in function <init_worker_by_lua:47>
        [C]: in function 'xpcall'
        init_worker_by_lua:56: in function <init_worker_by_lua:54>, context: ngx.timer

2020/08/01 00:00:00 [warn] 15#15: *2 [lua] _G write guard:12: writing a global Lua variable ('yaml') which may lead to race conditions between concurrent requests, so prefer the use of 'local' variables
stack traceback:
        [C]: at 0x7fafc162f140
        [C]: in function 'require'
        /usr/local/share/lua/5.1/lyaml/init.lua:34: in main chunk
        [C]: in function 'require'
        /opt/app-root/src/src/resty/yaml.lua:6: in main chunk
        [C]: in function 'require'
        ...p-root/src/src/apicast/configuration_loader/data_url.lua:9: in main chunk
        [C]: in function 'require'
        /opt/app-root/src/src/apicast/configuration_loader.lua:5: in main chunk
        [C]: in function 'root_require'
        ...
        [C]: in function 'require'
        /opt/app-root/src/src/apicast/cli/command/start.lua:18: in main chunk
        [C]: in function 'require'
        /opt/app-root/src/src/apicast/cli.lua:18: in function 'load_commands'
        /opt/app-root/src/src/apicast/cli.lua:23: in main chunk
        [C]: in function 'require'
        /tmp/xxxxxxxx:60: in function 'file_gen'
        init_worker_by_lua:49: in function <init_worker_by_lua:47>
        [C]: in function 'xpcall'
        init_worker_by_lua:56: in function <init_worker_by_lua:54>, context: ngx.timer

2020/08/01 00:00:00 [warn] 15#15: *2 [lua] _G write guard:12: __newindex(): writing a global Lua variable ('result') which may lead to race conditions between concurrent requests, so prefer the use of 'local' variables
stack traceback:
        /usr/local/share/lua/5.1/liquid.lua:134: in function 'sanitize_replace'
        /usr/local/share/lua/5.1/liquid.lua:2998: in function </usr/local/share/lua/5.1/liquid.lua:2996>
        [C]: in function 'pcall'
        /usr/local/share/lua/5.1/liquid.lua:2423: in function 'visit'
        /usr/local/share/lua/5.1/liquid.lua:2042: in function 'visit'
        /usr/local/share/lua/5.1/liquid.lua:2018: in function 'visit'
        /usr/local/share/lua/5.1/liquid.lua:2501: in function 'visit'
        /usr/local/share/lua/5.1/liquid.lua:2018: in function 'visit'
        /usr/local/share/lua/5.1/liquid.lua:2018: in function 'interpret'
        /usr/local/share/lua/5.1/liquid.lua:2453: in function 'visit'
        ...
        /usr/local/share/lua/5.1/liquid.lua:2018: in function 'visit'
        /usr/local/share/lua/5.1/liquid.lua:2269: in function 'visit'
        /usr/local/share/lua/5.1/liquid.lua:2018: in function 'render'
        /opt/app-root/src/src/apicast/cli/command/start.lua:57: in function 'nginx_config'
        /opt/app-root/src/src/apicast/cli/command/start.lua:171: in function 'start'
        /opt/app-root/src/src/apicast/cli.lua:50: in function </opt/app-root/src/src/apicast/cli.lua:30>
        /tmp/xxxxxxxx:60: in function 'file_gen'
        init_worker_by_lua:49: in function <init_worker_by_lua:47>
        [C]: in function 'xpcall'
        init_worker_by_lua:56: in function <init_worker_by_lua:54>, context: ngx.timer