csrf, encoding: make luaossl optional at runtime

[a-schaefers]

Related to #802

lapis.util.encoding and lapis.csrf required luaossl as soon as they were loaded. Every app loads lapis.util.encoding (lapis.application -> lapis.request -> lapis.session), so an OpenResty app could not start without luaossl, even though OpenResty signs sessions with ngx.hmac_sha1 and ships a CSPRNG in resty.random. Every request failed with:

./lapis/util/encoding.lua:3: module 'openssl.hmac' not found

This is the runtime half of leafo/lapis issue 802, where luaossl fails to compile with GCC 15+ (C23). Each module now picks a backend when it loads, the way pgmoon does in pgmoon/crypto.moon. luaossl is first in every chain, so nothing changes for installs that have it.

lapis.util.encoding

• HMACs are created with luaossl's openssl.hmac, falling back to lua-resty-openssl's resty.openssl.hmac. Both expose new(key, digest) and final(str).
• Inside OpenResty hmac_sha1 is still ngx.hmac_sha1, so the default hmac_digest ("sha1") needs neither library there. hmac_digest = "sha256" needs luaossl or lua-resty-openssl.

lapis.csrf

• The 32 random bytes behind the token cookie come from luaossl's openssl.rand, falling back to resty.random.bytes(n, true), which calls RAND_bytes in the OpenSSL that nginx links against.
• resty.random is only used inside a real nginx process, detected by ngx.config. In plain LuaJIT it loads fine but crashes when called ("undefined symbol: RAND_bytes"), and the fake ngx of lapis.spec.request has no ngx.config. This checks ngx.config rather than lapis.nginx.is_simulate because specs also install fake ngx tables that lack the simulate marker (stub_queries uses { null: nil }).

Errors

• Without a backend both modules still load. The function that needs the backend raises an error that includes why luaossl failed to load, so a broken luaossl install isn't hidden:

  lapis.util.encoding: hmac_sha256 requires luaossl or
  lua-resty-openssl, but luaossl failed to load: module
  'openssl.hmac' not found: ...

• luaossl is probed once per module. On Lua 5.1 and LuaJIT, when a module's loader raises an error, require leaves a sentinel in package.loaded, and a second require only reports "loop or previous error loading module" instead of the real reason.

Fixed from the prototype patch in the issue 802 plan: random_bytes returned assert's message as a second value. ngx.encode_base64 rejects it ("bad no_padding: boolean expected, got string", so CSRF tokens failed on OpenResty without luaossl) and LuaSocket's mime.b64 appends it to the key. random_bytes now returns only the bytes, and the specs check the exact key.

Not changed

• luaossl stays a dependency in the rockspec. Outside OpenResty, lapis migrate (pgmoon's password auth), sessions and CSRF still need it.
• The simulated ngx.md5 in lapis.spec.request still loads luaossl lazily when it's called.

Specs

• spec/helpers.moon: require_with_stubs loads a fresh copy of a module with chosen modules replaced by tables, or made to fail as if not installed, then restores package.loaded and package.preload.
• encoding: RFC 2202 and RFC 4231 HMAC test vectors, backend order, the error without a backend, and nginx with and without lua-resty-openssl, including hmac_digest = "sha256".
• csrf: backend order, strong bytes requested from resty.random, a failing resty.random, no resty.random outside nginx or in a simulated request, and validating tokens without a backend.

Docs: utilities.md now says which library backs the HMAC functions and the CSRF random bytes.

Tested

• busted spec: 1026 successes, 5 pending on Lua 5.1; 1027 successes, 4 pending on LuaJIT 2.1 (OpenResty's branch) and Lua 5.4. Before this change: 1013 successes, 5 pending on Lua 5.1.
• busted spec_cqueues: 13/13 on Lua 5.1 with lua-http 0.4.
• busted spec_openresty/server_spec.moon against Ubuntu 24.04's nginx 1.24 with ngx_lua 0.10.26, lua-resty-core and lua-resty-string 0.16: 10/10 with luaossl, and 10/10 with luaossl hidden from nginx. Without this change the second run is 0/10.
• A throwaway app on that nginx with luaossl hidden: the app loads, sha1 sessions round trip, CSRF tokens come from resty.random (32 bytes, padded base64) and are accepted, and hmac_sha256 raises the error above. With lua-resty-openssl 1.9.0 added, hmac_digest = "sha256" sessions round trip and hmac_sha256 matches RFC 4231. With luaossl visible, luaossl is used and no resty module is loaded.