feature: ngx_http_lua_ffi_ssl_ciphers

[bjne]

Add ngx_http_lua_ffi_ssl_ciphers that returns a uint16_t array of
tls_protocol_id supported (and enabled) by both server and client

consider this pull request a request for comment
example usage from resty-core:

ffi.cdef[[
int ngx_http_lua_ffi_ssl_ciphers(ngx_http_request_t *r, uint16_t *ciphers,
        uint16_t *nciphers, char **err);

typedef struct {
    uint16_t nciphers;
    uint16_t ciphers[?];
} ngx_lua_ssl_ciphers;
]]

do
    local tls_proto_id = {
        [0x1305] = {
            iana_name = "TLS_AES_128_CCM_8_SHA256",
            tls_version = 1.3,
            kex = "none",
            auth = "none",
            enc = "AES 128 CCM 8",
            hash = "SHA256"
        },
        ...
   }

      local unknown_cipher = {
            iana_name = "UNKNOWN",
            tls_version = 0,
            kex = "UNKNOWN",
            auth = "UNKNOWN",
            enc = "UNKNOWN",
            hash = "UNKNOWN"
    }

    setmetatable(tls_proto_id, {
        __index = function(t,k)
            t[k] = unknown_cipher

            return unknown_cipher
        end
    })

    local iterate_ciphers = function(ciphers, n)
        if ciphers.nciphers > n then
            return n+1, tls_proto_id[ciphers.ciphers[n]]
        end
    end

    local ciphers_t = {}

    ffi.metatype('ngx_lua_ssl_ciphers', {
        __ipairs = function(ciphers)
            return iterate_ciphers, ciphers, 0
        end,
        __tostring = function(ciphers)
            for n,c in ipairs(ciphers) do
                ciphers_t[n] = type(c) == "table" and c.iana_name or format("0x%.4x", c)
            end

            return concat(ciphers_t, ":", 1, ciphers.nciphers)
        end
    })
end

local ciphers_typ = ffi.typeof("ngx_lua_ssl_ciphers")


local ciphers_buf = ffi.new("uint16_t [?]", 256)

_M.ciphers = function()
    local r = get_request()
    if not r then
        ngx.log(ngx.ERR, "no request found")
    end

    ciphers_buf[0] = 255
    local rc = C.ngx_http_lua_ffi_ssl_ciphers(r, ciphers_buf+1, ciphers_buf, errmsg)
    if rc ~= FFI_OK then
        return nil, ffi_str(errmsg[0])
    end
   
    local ciphers = ciphers_typ(ciphers_buf[0])

    ffi.copy(ciphers, ciphers_buf, (ciphers_buf[0] + 1) * ffi.sizeof('uint16_t'))

    return ciphers
end

I hereby granted the copyright of the changes in this pull request
to the authors of this lua-nginx-module project.

[zhuizhuhaomeng]

need to add some test cases. @bjne

[bjne]

need to add some test cases. @bjne

Yeah absolutely, just wanted to know if this is a way you would accept this implemented first

[zhuizhuhaomeng]

@bjne this implementation is OK.
But this only works in openssl, boringssl does not have the APIs in your PR.
we need to add macro to guard the code.