ngx_lua: support ngx_lua custom coroutine's fun

[detailyang]

Hello

@mpeterv

Now the newest luacheck 0.19.0 redefines the ngx_lua standard and it will make the CI failed because of accessing undefined field _yield of global coroutine in lua-resty-core

OpenResty inject the custom API to coroutine table as the following:

    lua_getglobal(L, "coroutine");

    /* set running to the old one */
    lua_getfield(L, -1, "running");
    lua_setfield(L, -3, "running");

    lua_getfield(L, -1, "create");
    lua_setfield(L, -3, "_create");

    lua_getfield(L, -1, "resume");
    lua_setfield(L, -3, "_resume");

    lua_getfield(L, -1, "yield");
    lua_setfield(L, -3, "_yield");

    lua_getfield(L, -1, "status");
    lua_setfield(L, -3, "_status");

    /* pop the old coroutine */
    lua_pop(L, 1);

    lua_pushcfunction(L, ngx_http_lua_coroutine_create);
    lua_setfield(L, -2, "__create");

    lua_pushcfunction(L, ngx_http_lua_coroutine_resume);
    lua_setfield(L, -2, "__resume");

    lua_pushcfunction(L, ngx_http_lua_coroutine_yield);
    lua_setfield(L, -2, "__yield");

    lua_pushcfunction(L, ngx_http_lua_coroutine_status);
    lua_setfield(L, -2, "__status");

    lua_setglobal(L, "coroutine");

So add the coroutine fields to pass the CI.

[mpeterv]

Hello @detailyang, are these extensions documented? The field list could be added to the standard so that typos like coroutine._yieldd are detected. But if there are not documented maybe the field list is not stable.

[detailyang]

@mpeterv

As far as I know, these are not official document in OpenResty :(. But you know the code maybe is the best document although it could not stable.

Wish @agentzh can help us:)

[agentzh]

@detailyang Those were internal API functions and are undocumented by intention :) Use of those API functions by normal users are discouraged. I think maybe you should just whitelist those specifically in your own luacheck configuration file, if user whitelisting is supported in luacheck?

[detailyang]

gotcha