Get openresty tcp socket's underlying fd

get_cosocket_fd.lua

local type = type
local error = error
local pcall = pcall
local getmetatable = getmetatable

local ffi = require "ffi"
do
    --if not pcall(ffi.typeof, "tmp_ngx_http_lua_socket_tcp_upstream_t") then
    --
    --end
    -- based on nginx 1.27.1
    -- @see ngx_connection_t, ngx_peer_connection_t, ngx_http_lua_socket_tcp_upstream_t
    ffi.cdef [[
        typedef struct {
            intptr_t    prefix[3];
            int         fd;
            char        rest[0];
        } tmp_ngx_connection_t;

        typedef struct {
            tmp_ngx_connection_t    *connection;
            char                    rest[0];
        } tmp_ngx_peer_connection_t;

        typedef struct {
            intptr_t                    prefix[9];

            tmp_ngx_peer_connection_t   peer;
            char                        rest[0];
        } tmp_ngx_http_lua_socket_tcp_upstream_t;
    ]]
end

local ffi_cast                             = ffi.cast
local C                                    = ffi.C

local lua_registry                         = debug.getregistry()
-- C types
local ngx_http_lua_socket_tcp_upstream_pct = ffi.typeof("tmp_ngx_http_lua_socket_tcp_upstream_t*")

-- constants
local SOCKET_CTX_INDEX                     = 1

local _M                                   = {
    _VERSION = '0.0.1'
}

-- @see lua-resty-core/lib/resty/core/socket.lua
local function get_tcp_socket(cosocket)
    local tcp_socket = cosocket[SOCKET_CTX_INDEX]
    if not tcp_socket then
        error("socket is never created nor connected")
    end

    return tcp_socket
end


local function get_socket_fd(sock)
    local ud = get_tcp_socket(sock)
    local cdata_ref = ffi_cast(ngx_http_lua_socket_tcp_upstream_pct, ud)
    return cdata_ref.peer.connection.fd
end

_M.get_fd = function(cosock)
    -- @note this may not nes
    if type(cosock) ~= "table" then
        return nil, "valid cosocket object expected"
    end
    return pcall(get_socket_fd, cosock)
end

_M.verify_cosocket = function(cosock)
    if type(cosock) ~= "table" or getmetatable(cosock) ~= lua_registry.__tcp_cosocket_mt then
        return nil, "valid cosocket object expected"
    end
    return cosock
end

return _M